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[Type text] Page 19
REVIEW OF LITERATURE
The literature has been reviewed on the host- poplar, nursery diseases on poplars in
general and Bipolaris leaf blight in particular. Due to lack of direct information on the
disease, parallels from similar pathogen (other species of Bipolaris), host (other clones,
species of P. deltoides and related genus) and production system (agriculture) has been
drawn.
2.1. The Host: Poplar
The prime objective of forestry is to develop and protect forests, which are one of the
world’s renewable natural resources for maximum productive and protective functions.
Poplar is amongst world’s fastest growing multipurpose tree species. It is widely
distributed in many parts of the world and India. Poplars are known to naturally occur
in subtropical broadleaved hill forests, wet temperate, moist temperate, deciduous
forests and dry temperate forests. Poplars grow well on low lying and moist areas
preferring loamy soils but may be planted on riverbeds with sandy soils and in areas
with clayey loams in forest soils (Tiwari, 1993).
Poplars, by virtue of their fast growth, offer a great potential for meeting the
requirements of the farmers and wood-based industry in the country. Besides, they are
also widely accepted as industrial raw material, compatible with agriculture crops,
environmental ameliorating properties and short rotations of 6-12 years (Tiwari, 1993).
In India, poplars have become integral to agriculture, which is demonstrated by their
ubiquitous presence in the farms throughout the north. Poplars are usually raised as
single clone monocultures and, thus, prone to epidemic spread of pests. Poplars are
propagated vegetatively by use of cuttings in order to maintain their genetic purity.
Such cuttings may carry pests and diseases on them from one region to another and,
thus, may contribute to the spread of pests to new localities (www.fao.org, 17/8/2008).
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2.1.1 Taxonomic position P. deltoides
Kingdom Plantae
Sub Kingdom Tracheobionta
Super-division Spermatophyta
Division Magnoliopsida
Class Magnoliopsida
Sub class Dilleniidae
Order Salicales
Family Saliaceace
Genus Populus
Species deltoides
Source: Natural Resources Conservation Service (United States Department of Agriculture), 12/2/2009. 2.1.2. Distribution
Important countries, where poplar planting is initiated to meet the challenge of
increasing demand and shortage of wood supply are Belgium, France, Germany, Hungry,
Romania, Spain, Yugoslavia and Korea. In the east region, Afghanistan, Iran, Iraq, Israel,
Lebanon, Syria and Turkey also took up poplar rising. Australia and New Zealand too
attempted introduction of poplars successfully (Dalal & Trigotra, 1983).
Poplars are mainly restricted to the north Indian states. The researchers, after
trials of different species of poplars in hills as well as plains, were convinced that only P.
yunnanensis and P. deltoides were the most promising and successful species (Seth,
1969; Lohani, 1976; Chaturvedi, 1982). Forest resources in Punjab and Haryana are
almost negligible. Agriculture is the main land use. Therefore, poplar has a possibility of
growing as agroforestry species. Plantations of poplar (Populus spp.) are becoming
popular in the plains of northern India particularly in Haryana, Punjab, Uttarakhand and
Uttar Pradesh (U.P.) due to its fast growing habit, compatibility with crops, eco-friendly
and multipurpose uses in different industries.
[Type text] Page 21
Under irrigated conditions of Punjab, Punjab Agriculture University (PAU) has
conducted a series of nursery (Sidhu, 1996) and field trials (Sidhu & Dhillon 2007; Singh
et al., 2008; Dhillon et al 2010; Dhillon & Sidhu 2010). In 1996, PAU released seven
clones i.e. PL-1(L-39/84), PL-2 (L-71/84), PL-3 (L-154/84), PL-4 (L-313/85), PL-5 (G-
48), PL-6 (L-188/84) and PL-7 (113324) on basis of field trials conducted at two
sites of the state from 1992 to 1996. The trials showed 26 to 35 per cent volume
production superiority than control (G-3). On the basis of four clonal trials in different
climatic zones, two clones (L-47/88 and L-48/89) were recommended during 2009. In
Haryana, promising clones reported are IC and G-3 (Singh et al., 1983; Dalal & Trigotra,
1983). Toky et al. (1996) reported CP-82-4-1 and CP-82-4-2 clones of P. deltoides better
than G-3.
Although, almost all clones of P. deltoides growing successfully in particular area
are suitable for agro forestry. However, Karanatak et al. (1994) have suggested some
clones (A-26, A-343, T-75, T-94, T-185, 73/53-3, 3276, 3294 and 3297) which are more
favorable for this purpose as they shed their leaves before the starting of sowing period
of agriculture crop and flush after harvesting. However, they cautioned that the clones
were suitable to Dehradun or similar climate. Dhiman and Gandhi (2006) analyzed the
match splint quality of wood of 8 clones and concluded that wood of clones G-48 and
Bahar produced match splints with low wood defect parameters i.e. crookedness and
cross grain splints. Total splint defects were higher in S7C15, Wimco-22, L-34, L-49 and
Udai clones.
For North West plains of U.P., suitable clones are G-3, G-48, D-121, D-61, D-67,
S7C8 and S7C15 (all from Americans origins; Piare Lal, 1991). Rawat et al (2001)
screened 75 clones at nursery stage in eastern U.P. and selected eleven best clones on
basis of independent culling method of selection. Another study (Singh et al., 2001)
conducted in eastern U.P. (Sultanpur) found significant variation in all growth traits,
number of branches and crown width among 50 clones with high expected genetic gain
of 30.28 per cent for volume by selecting 5 best clones (40-N, UD 9116, 25-N, 63-N and
UDH 1002).
Under sub-humid conditions of eastern Madhya Pradesh (now Chhattisgarh)
Puri et al. (2002) identified 19 promising clones out of 106 on basis of survival and
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growth performance in nursery trials. After four year of field evaluation they selected
five clones (65/27, S7C1, G-48, G-3 & D-121).
2.1.3. Characteristics of P. deltoides
It is also known as eastern cottonwood. It retains height up 18-32m and trunk diameter
0.5-1.0m. Trunk is broadly rounded or irregular, spreading and with some drooping
branches. The trunk of open grown trees often splits into more stems. Bark thin, smooth
and gray on young trees, becoming thick, rough and furrowed with age. Twigs stout,
yellowish green, gray or tan somewhat angled. Buds are yellowish brown, 1.25cm long,
slender, three-sided long pointed and resinous. Terminal buds are 1-2cm long. Leafs are
alternate on 5-10cm long flattened yellowish leaf stalks green and shiny above paler
beneath 6-12.5cm long nearly as wide broadly triangular with a straight or slightly
heart shaped base and abruptly pointed tip, coarsely toothed with gland tipped teeth,
thick and firm. In autumn, the leaves turn yellow. Flowers either male or female, borne
on separate trees in hairy bract catkins. Male catkins short stalked, reddish 8-10cm long
and densely flowered. Female catkins short stalked, greenish yellow 15-20cm long and
few flowered. Fruit are two- to four-valved capsule, borne in 15 - 25 cm long drooping
catkins, short-stalked, greenish brown, and elliptic. Seeds light brown with cottony hairs
attached. (www.vplants.org, 12/2/2009).
2.1.4. Cultivation
P. deltoides is the only species of polar that is planted on a significant scale in India. P.
deltoides, hereafter called poplar, constitutes the backbone of agro forestry in irrigated
plains of Northern India (Kishwan & Kumar, 2003). It has been estimated that 60,000
hectares equivalent plantations of poplar exists in India.
When poplar is planted on the field boundaries, kharif as well as rabi crops can
be grown in field throughout the rotation of poplar. In block plantation of poplar, the
usual kharif crops can be grown for two years only; thereafter shade-bearing crops like
ginger, turmeric, etc. are planted. However, rabi crops can be grown as usual. Poplars
also serve as a windbreak. The earliest contribution of poplar trees to the farmers is fuel
wood from pruning. Intercropping is almost always preferred as it provides agricultural
returns on the one hand and results in increased growth rate of poplar on the other due
to frequent irrigation and hoeing operations of agricultural crops. Pure poplar is seldom
raised. lf raised pure, the spacing is kept about 3m x 3m and the stems remain thin
[Type text] Page 23
which fetch low price in the market. The rotation suggested for poplar cultivation in
India is 10 to 12 years but farmers prefer to cut it at 6 to 8 years rotation. Unlike
developed countries, coppicing of poplar at cutting cycles of 3 to 4 years is not practiced
in India. The felled areas are replanted with fresh nursery stock (Kishwan & Kumar,
2003).
Irrigation should be provided as soon as the planting of cuttings in any bed is
completed. Undue delay in irrigating the beds can cause dehydration of the planted
cuttings and result in poor sprouting. The first irrigation should be medium heavy, so
that, about 5 to 7 cm water is uniformly above ground level at the time of irrigation.
Subsequent irrigation should be light and the interval may vary between 7 to 10 days
depending upon the type of soil. Light sandy soils will need frequent irrigations whereas
soils with clay need irrigation at longer intervals. Proper and effective drainage of
excess water during rainy reason is essential to prevent lodging and collar rot. After the
rainy season, one to two irrigations per month will be adequate
(www.eucalyptusclones.com/downloads/agro_pdf9, 17/8/2011).
Well decomposed farmyard manure which is rich in macro- as well as micro-
nutrients essential for the plants should be applied to the total area under poplars while
preparing the land for inter-cultivation of rabi and kharif crops. Application of
nitrogenous, phosphatic and potassic fertilizers as well as micro nutrients will depend
on the fertility status of the land. At least, during the first year of the plantation
micronutrient may be arranged to guard against any possible deficiency
(www.eucalyptusclones.com/downloads/agro.pdf, 17/8/2011).
2.1.5. Utilization
2.1.5.1. As a tree
P. deltoides has been found suitable as line support for overhead power and
telecommunication lines. It is suitable for the manufacture of match splints and
plywood. Young plant of P. deltoides was evaluated for making fibers hardboards
(Tewari, 1993). The wood of poplar is used in papermaking, plywood, matchsticks,
packing cases, sport goods, light construction and furniture.
Poplar was introduced in tarai region of U.P. to meet the need for the plywood
and matchwood industries (Chaturvedi & Rawat, 1994). However, large scale plantation
of poplar has started only during the last decade and thousands of hectares of fertile
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land in north India have been put under plantation of poplar in the state of U.P.,
Uttarakhand, Haryana, Punjab and Himachal Pradesh. As a result of increased poplar
plantation, plywood industries have been mushroomed in this region of India.
2.1.5.2. As Intercrop
Poplar is deciduous tree and very suitable for agro forestry system. It has no shading
effect on crop, rather adds to soil fertility through its leaf litter. Environmental
amelioration is another intangible benefit from poplar planting. It has been estimated
that poplar grows 36 times faster than other Indian forest trees and, therefore, utilizes
36 times more carbon dioxide (Chandra, 1998).
Growing auxiliary crops with poplar is highly beneficial to the tree crop. Its
growth has been enhanced up to 40 percent. This is because of regular irrigation
weeding, hoeing and fertilizer application during cultivation of the secondary crops (Jha
& Gupta, 1991). Intercropping of cereals, cash crops forage, etc. add to the income
through agriculture. Wheat, oat, sorghum, maize, sugarcane, berseem, turmeric, zinger,
potato, dhanicha, etc. can easily be grown as inter-crops. Intercropping with poplar is
remunerative during the rabi as well as kharif season.
Farmers grow poplar both in block as well as boundary plantation along with
agricultural crops. The latest trends are to plants them on agriculture field
simultaneously with companion crops. This is a sustainable land management system,
which increases the yield of the land by combined production of crops and forest plants.
There are evidences to prove that tree planting improves the agro climatic condition
and mitigates their adverse effect by changing the microclimatic in area. Some
progressive farmers have obtained three times higher income from poplar and
agricultural crop combination than pure agriculture (Mandal et al., 2005). Therefore,
poplar agro forestry is undoubtedly very lucrative business.
This is considered to be the best agro-forestry species for inter-cropping, having
high rate of growth, short rotation, good economic returns and has less effect on
intercrops but proper management of intercropping is necessary. Farmer’s plant this
species on their cropland and within 6-7 years harvest and market it to the industry.
During the kharif season soybean, maize and grain legume can be grown while during
the Rabi season wheat, potato, peas, etc., can grow successfully. Poplar is winter
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decideous and adds tremendous amount of leaf litter to the soil (www.tribuneindia.com,
2010).
2.1.6. Marketing and Economics
Poplar wood is sold by weight. Profits to the tune of 38.8 and 100.9 percent of
investments are reported from rising of nursery stock within one year (Singh &
Vashista, 2001). Benefit:cost ratio of 1.92:1 and 2.13:1 have been estimated with pure
poplar and with poplar + intercropping in a pay-back period of 7 years (Dillon et al.,
2001). Owing to very little risks and high profits in poplar cultivation, large farmers and
absentee land-lords prefer to put their lands under poplar-based agro forestry rather
than other agriculture/agro forestry options (Kishwan & Kumar, 2003).
Poplar (P. deltoides) is extensively grown agroforestry tree in selected district of
Punjab, Haryana, Uttar Pradesh and Uttarakhand. Approximately 2 core sapling planted
per annum throughout the poplar growing region has created verdure sylvan landscape
in agricultural land. These saplings quickly grow, form thick forest and attain 10 percent
crown cover for both block (400 trees/ha) and boundary plantation (one ha base) in the
first year itself (Dhiman, 2009) and qualify as forest based on the crown parameter used
by the Forest Survey of India (FSI, 2009) much faster than other tree species grown
both inside and outside farms than other forests.
2.2. Nursery Diseases of Poplars in India
Many fungi, bacteria, viruses, etc. are capable of causing damage to leaves, branches,
boles and roots of the poplars. The Table2.2.1. details about the diseases of poplar in the
country.
Table2.2.1. Nursery diseases of poplars in India
Disease Pathogen Distribution Reference
Nursery disease
Set rot
Botryodiplodia.
palmarum
Lasiodiplodia
palmarum
U.P. Singh and Khan (1979)
B. palmarum Solan (H.P.) and U.P. Singh and Khan (1983)
B. palmarum H.P. Khan (1988)
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B. palmarum Lalkua Forest Research
Nursery, Lalkua,
Nainital
Harsh and Kumar (1992)
B. palmarum and
Macrophoma sp.
Northern India Pandey and Khan (1992)
B. palmarum U.P. and H.P. Khan (1999)
Leaf rust Melampsora
rostrupii
Oidium sp. and
Myrothecium
roridum
H.P. and Kashmir valley Singh and Khan (1979)
Leaf spot Cercospora
populina,
Sphaceloma
populina,
H.P. and Kashmir valley Singh and Khan (1979)
Leaf rust
and Powdery
mildew
Melampsora sp.
and
Uncinula sp.
J. & K. and H.P. Rehill and Puri (1980)
Leaf spot Cercospora sp.
S. populina,
M. roridum,
Septoria populi
and Phyllosticta
adjuncta
J. & K. and H.P. Rehill and Puri (1980)
Leaf rust Melampsora sp. J. & K. Singh and Khan (1983)
Exotic Rust M. ciliata Himalayas Singh and Khan (1983)
Alternaria tip
blight and
Cladosporium
leaf spot
Alternaria stage of
Pleospora
infectoria and
Cladosporium
humlie
J. & K. and nurseries of
U.P.
Singh and Khan (1983)
Melampsora rust Melampsora sp. Kashmir valley Singh et al. (1983)
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Melampsora rust M. ciliata H.P., U.P. and J&K Khan et al. (1988)
Leaf rust and
Powdery mildew
M. populina and
U. salacis
H.P. Khan (1988)
Leaf spots and
Asteroma leaf
bilght
S. populi, C. humile
and Asteroma
frondicola
H.P. Khan (1988)
Alternaria tip
blight and
Phyllosticta leaf
spot
Alternaria stage of
P. infectoria and P.
adjuncta
H.P. Khan (1988)
Pollaccia blight,
Melampsora rust
and Powdery
mildew
Pollacia elegans,
M. cilata,
M. populina and
U. salacis
Kashmir valley Rehill et al. (1988)
Septoria leaf
spot,
Cladosporium
leaf spot and
Sphaceloma leaf
spot
S. populi,
C. humile and
S. populina
H.P. and J. & K. valley Rehill et al. (1988) and
Sharma and Sharma
(2000)
Pollacia blights P. nigra
P. elegans
Potusai, Kamraj Forest
Division, J & K State
Khan and Mishra (1989)
Cladosporium
leaf spot
C. humile H.P. Khan (1990)
Leaf rust M. ciliata H.P. Khan (1990)
Leaf spot C. humile,
Alternaria sp. and
Phyllosticta sp.
H.P. Khan (1990)
Sclerotium leaf
spot
Sclerotium rolfsii New Forest, Dehradun Mishra and Khan (1991)
Leaf blight Bipolaris maydis U.P. (Tarai), Punjab and
Haryana
Chauhan and Pandey
(1992)
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Leaf spot Alternaria
alternata, P.
adjuncta,
S. populi and S.
rolfsii
U.P., Punjab and
Haryana
Singh et al. (1991)
Leaf rust M. ciliata and
M. larici- populina
U.P. , H.P. and J.&K. Pandey and Khan (1992)
Leaf spot A. alternata, P.
adjuncta, S. populi,
S. rolfsii M.
roridium and
Dreshclera maydis
U.P. , H.P. and J.&K. Pandey and Khan (1992)
Bipolaris leaf
blight
B. maydis U.P., Punjab and
Haryana
Chauhan and Pandey
(1995)
Leaf rust Phomopsis sp.,
R. solani,
M. ciliata and
M. larici-populina
U.P. and H.P.
U.P. and H.P.
U.P. , H.P. and J.&K.
Khan (1999)
Leaf disease C. humile,
D. maydis,
Alternaria sp. and
S. rolfsii
Eastern Himalayas,
Payal, Haryana and
Central Tarai Forest
Division, U.P.
Khan (1999)
Alternaria leaf
bilght
A. alternata H.P. Sharma et al. (1999)
Leaf rust M. ciliata H.P. Sharma and Sharma
(2000)
Leaf spot Cladosporium sp. Solan, Kullu, Lahaul &
Spiti and Kinnaur
Sharma and Sharma
(2000)
Leaf spot Alternaria sp.,
Phyllosticta sp.
FRI campus, Dehradun Pandey (2002)
[Type text] Page 29
and Bipolaris sp.
Leaf rust M. populina
Europe, China, India and
Chile
Pai and Mc.Cracken
(2005)
Alternaria leaf
spot and tip
blight
Curvularia leaf
spot
A. alternata and
Curvularia lunata
H.P., J. & K., Uttarakhand
and Punjab
Sharma ( 2009)
Stem rot S. rolfsii New Forest, Dehradun Mishra and Khan (1991)
Stem rot Phellinus
phephyphloccus,
Phellinus noxius
Tarai plains of U.P. Khan (1999)
Root rot Ganoderma
lucidum
Lacchiwala Range,
Dehradun, East Forest
Division, U.P.
Singh and Khan (1979)
Root rot G. lucidium Phillaur, Punjab Rehill and Puri (1980)
Root rot
G. lucidum Tarai Central Forest
Division, Srinagar
Singh and Khan (1983)
Root rot
G. lucidum Tarai plains, U.P. Khan (1999)
White root rot Rosselinia
nectrarix
H.P. Khan (1988)
White root rot Macrophoma sp. U.P. and H.P. Khan (1999)
White root rot
Dematophora
necatrix
H. P. Sharma (2009)
Plantation diseases
Pink disease and
Canker
Corticum
salmonicolor and
B. palmarum
Haldwani and Lalkuan
plantations, U.P
Singh and Khan (1979)
[Type text] Page 30
Pink disease,
Canker,
Dieback and
Heart rot
C. salmonicolar,
B. palmarum,
Phomopsis sp. and
Trametes sp.
Pipalparao, Belachaur
and Gangapur Patia
plantations, U.P.
Tarai & Bhabar
Plantation Division, U.P.
Rehill and Puri (1980)
Poria root rot Poria vinta Tanda Block, T. & B.
Plantation Division,
Dehradun, U.P.
Rehill and Puri (1980)
Canker and
Die back
C. salmonicolor
and
Cytospora
chrysosperma
U.P. and H.P. Khan (1999)
2.3. Poplar Blight
2.3.1. Historical perspective
The blight pathogen has been identified as a major parasite of poplars and clones like G-
3, G-48, D-153 and 6244 showed moderate to high susceptibility in nurseries (Jones &
Lal, 1989). It has been detected in its conidial stage as Bipolaris maydis on a number of
clones of P. deltoides causing serious foliage blight (Tewari, 1993). Later, a leaf blight of
P. deltoides caused by B. maydis was reported by Chauhan & Pandey (1992).
In 1995, Chauhan and Pandey (1995) identified isolates of B. maydis to be race T
on the basis of their virulence to certain male cultivars of G-3 of host from Texas
provenance raised in agro forestry system by M/S Wimco Limited in India particularly
in the states of Uttar Pradesh (north–west region), Punjab and Haryana. Besides, toxin
production, sclerotia formation, light pigmentation and poor sporulating potency on the
culture media under controlled condition were also studied. Among diseases, leaf blight
caused by B. maydis was the most serious disease. However, it affects only G-3 clone and
cultivation of this clone in the affected areas has been stopped (Kishwan & Kumar,
2003). B. maydis leaf blight, a serious disease on P. deltoides, has been reported by
Sharma (2009) to be prevalent in Uttarakhand and Punjab.
Bipolaris maydis (Nisikado) Subram. et Jain [= Bipolaris maydis (Nisikado et
Miyake) Shoem.], the anamorph of Cochliobolus heterostrophus (Drechsler) Drechsler
[Type text] Page 31
(family-Pleosporaceae, order- Dothideales, phylum- Ascomycota), long known as an
important leaf parasite on maize, recently observed in India (isolates designated T race)
on some clones - mainly male of Texan origin - of P. deltoides, where it is responsible for
a leaf blight of very high incidence; on the contrary, tests showed that P. nigra and P.
ciliata are totally resistant (www.fao.org, 2/2/2008.)
2.3.2. Blight in nursery trials
Reliable, time- series information is available in the annual reports of Wimco Seedlings
Ltd., Baghwala, Rudrapur regarding disease status of Bipolaris blight vis-a- vis genotype
susceptibility. Mainly two fungal diseases were noticed in poplar germplasm nursery.
The leaf blight caused by B. maydis and leaf blotch caused by C. populina usually
appeared after onset of monsoon. Poplar clone, G-3 was heavily infected by these
diseases;, highly susceptible to leaf blight, clone G-3 is becoming unsuitable for planting
out in humid location (Annual Report, 1997). Approximately 36,000 half- sib seedlings
of 1996 population poplar were produced. Of these, initially, 188 individuals were
selected for multiplication based on their high growth rate and resistance to B. maydis
(Annual Report, 1998). It was also observed that clone G-3 was highly susceptible
whereas other clones were resistant against B. maydis.
In the Annual Report of 1999, it was mentioned that germplasm bank of 284
poplar clones consisting of 100 developed by Wimco Seedlings at R & D centre
Baghwala, Rudrapur, 73 of University of Forestry & Horticulture, Solan, 12 of U. P.
Forest Department and 97 exotics were maintained in Baghwala. Only for 10 cuttings of
each clone, diameter and height/diameter ratio were recorded at 12 month age. Only 2
clones were recorded growing above population mean. Clone WSL- 54 attained
maximum average height of 481cm and average collar diameter of 3.54cm. A poplar
germplasm bank consisting of 379 clones (226 of Wimco seedlings Limited, 64 of
University of Forestry & Horticulture, Solan, 8 of U. P. Forest Department and 81
exotics) was maintained in nursery trials at spacing of 70cm x 60cm at Research &
Development Centre, Baghwala, Rudrapur during 2006-07 (Annual Report, 2007).
Three hundred forty seven clones were retained out of 2006 poplar germplasm by
culling 3 clones on account of their susceptibility to leaf blight diseases and poor growth
whereas, 32 new clones were added to the germplasm bank during the year. It was
[Type text] Page 32
found that only clone G-3 was highly susceptible for B. maydis while, remaining clones
planted in germplasm were found resistance to the disease.
2.3.3. Symtomatology
Chauhan and Pandey (1992) defined the symptoms on poplar in detail i.e. diseases
appear after onset of the monsoon usually during July. Pink to light brown lesion of
pinhead size develops on leaves. Lesions gradually convert into dark brown rounded
spot, often surrounded by a chlorotic zone having concentric ring towards center. B.
maydis was the causal agent of the disease. In rainy or humid weather, blight often
appears by collapsing together of several lesions or when lesions involve midrib and/or
large veins of the leaves. Chlorosis frequently extends well beyond the necrotic lesions
and subsequently turns into big blighted patches. Sometimes, light infected leaves get
complete chlorosis. Severely infected/chlorotic leaves curl in dry weather and
ultimately fall off prematurely. Necrotic lesions, sometimes developing into canker, also
appear on green shoots of the susceptible clones. Premature defoliation of transplant,
trees of the susceptible clones predisposes them to weak parasites/saprophytes and
environmental stresses resulting extensive dieback. Tewari (1993) reported the disease
in late September as irregular blotches starting from the margin of leaves. They turned
grayish brown in the centre surrounded, sometimes, with a pale halo. Under cool and
humid conditions, the blotches covered the larger area of the leaf bearing grayish black
specks consisting of conidiophores and conidia all over the dried portion. In advance
stage, the leaves of the entire plant up to crown region exhibit as if they were scorched
by fire.
Young lesions of maydis leaf blight on maize are small and diamond shaped. As
they mature, they elongate. Lesions may coalesce, producing a complete ‘burning’ of the
leaves. They vary in size and shape among inbreeds and hybrids with different genetic
background. Race ‘O’ produced tan, elongated (2-6 x 3-22mm) lesion between the vein
with limited margins, with buff to brown borders. The exact leaf blight symptoms on
southern corn depend upon the race of the agent and the strain of corn affected. As a
general rule, tan lesions are seen on leaves with the number and size depending upon
the fungal race and the strain of corn. In the worst case the lesions are numerous and
can be several centimeters long and have dark red or purple edges. Ears are also
infected with a black substance that is actually masses of conidia (asexual spores) on
[Type text] Page 33
kernels that can lead to ear and cob rot. Stalks may also be damaged
(www.cbwinfo.com, 9/12/2011).
First report of leaf spot caused by B. seteriae was on the leaves of cassava
(Manihot esculenta) in China. Initially elliptical, chlorotic, and water-immersion lesions
of 2 to 4 mm in diameter appeared. These lesions became dry and yellow due to the
progress of the disease. A brown halo was around the lesions, and in wet conditions, a
dark gray mildew often appeared in the middle of the lesion. Diseased leaves turned
yellow and the plants eventually became defoliated (Shi et al., 2010).
2.3.4. Taxonomic position
Kingdom Fungi
Phylum Ascomycota
Class Euascomycetes
Order Pleosporales
Family Pleosporaceae
Genus Bipolaris
2.3.5. Causal agent Bipolaris is a dematiaceous, filamentous fungus. It is cosmopolitan in nature and is
isolated from plant debris and soil. The pathogenic species have known teleomorphic
states in the genus Cochliobolus and produce ascospores. The genus Bipolaris contains
[Type text] Page 34
several species. Among these, three well-known pathogenic species are B. spicifera, B.
australiensis and B. hawaiiensis.
2.3.6. Cultural and morphological features
Table2.3.6.1. Fig.2.3.6.1. to 2.3.6.3. detail colony and spore characters of those Bipolaris
species that have been/are being reported from P. deltoides.
2.3.7. Pathogenicity
The pathogenicity of B. spicifera was tested on the leaves of two cultivars of watermelon
(Peacock 124 and Mabrouka) which was widely cultivated in Morocco. The infection
coefficients (incidence × severity index) of the cv. Mabrouka and Peacock 124 after
inoculation with B. spicifera conidial suspensions were 53.3 and 22.4, respectively.
Calculated disease development rates were greater for cv. Peacock 124 than for cv.
Mabrouka. Conidium production of B. spicifera on inoculated leaves was very abundant
on the two cultivars, and the fungus was re-isolated from lesions on inoculated plants.
This is the first record of B. spicifera on watermelon in Morocco (Mhadri et al., 2009).
The artificial inoculation of the healthy leaves of three plants of Punica granatum
L. ’Nana’ by conidial suspension of the pathogen induced the same lesions to that
observed in nature by Kadri (2011). The diseased foliar surface and the total number of
diseased leaves 30 days after inoculation with B. spicifera conidial suspension were 37.5
and 78 percent. Conidia production of B. spicifera on inoculated leaves was 0.81 x 105
spore cm-2 and the fungus was re-isolated from lesions on inoculated plants. This is the
first report of B. spicifera on pomegranate in Morocco.
The pathogen B. seteriae was isolated and pathogenicity was established by
following Koch's postulates on Cassava (Manihot esculenta Crantz). This is an important
food crop in tropical regions of China (Shi et al., 2010). Young, healthy and fully
expanded green leaves of Cassava cv. HuaNan205 were surface sterilized and then
inoculated by spraying them with a suspension of conidia (1 × 105 conidia per ml) of the
isolate. Sterile water was used as a control. The leaves were kept in a humid chamber at
28°C for 4 days, in the mean time similar symptoms were observed on the leaves. The
pathogen was re isolated from inoculated leaves
[Type text] Page 35
2.3.6.1. Features of Bipolaris species
Causal organism and
host
Hyphae/
Culture type
and color
Conidiophore Conidium Reference
B. maydis
Graminecous family
Populus deltoides
Septate and
brown
Initally white to
grayish brown
and become
olive green to
black;
Velvetty to
wooly
Conidiophores (4.5-6 µm wide) are brown,
simple or branched, geniculate and sympodial,
bending at the points where each conidium
arises from. This property leads to the zigzag
appearance of the conidiophore.
The conidia, which are also called poroconidia, are
3- to 6-celled, fusoid to cylindrical in shape, light
to dark brown in color and have sympodial
geniculate growth pattern. The poroconidium (30-
35 x 11-13.5 µm) is distoseptate and has a
scarcely protuberant, darkly pigmented hilum.
This basal scar indicates the point of attachment
to the conidiophore. From the terminal cell of the
conidium, germ tubes may develop and elongate
in the direction of longitudinal axis of the
conidium
www.doctorfungus.org,
11/10/2010
B. spicifera
Vigna aconitifolia
Graminicolous species
common on plant
material particularly
grasses
Septate and dark
Initally white
soon become
dark gray and
olivaceous black
texture;
Wooly to
cottony
Conidiophores may be up to 300 µm in length,
are sympodial, geniculate, simple or branched,
bearing conidia through pores or openings
(poroconidia).
Conidia contain predominately 3 transverse
distosepta or pseudosepta (septa that do not
extend to the cell wall with cells enclosed within
sacs) and 4 cells. They measure approximately 20-
40 x 9-14 µm. A flattened hilum or point of
attachment is seen on the basal cell. Conidia
germinate from both poles (bipolar)
www.cybertruffles retrieved
on 9/12/2011
www.doctorfungus.org,
retrieved on 11/2/2011
B. setariae
Brachiaria reptans ,
Panicum fasciculatum,
Pennisetum typhoides
Setaria italica , and
Septate and dark
brown
Initally brown to
grayish black;
Flocosse to
Conidiophores solitary or in small groups,
straight or flexous, sometimes geniculate, pale
to mid brown or olivaceous brown, up to 200
µm long, 5-9 µm wide, sometimes swollen at
the base to 11 µm wide.
Conidia slightly curved or sometimes straight,
fusiform or navicular, pale to mid golden brown,
smooth, with 5-10 pseudosepta, (45-) 50-70 (-
100) x (10-) 12-14(-15) µm
(Sawada) Shoemaker, 1959
www.cybertruffles,
9/12/2011
www.mycobank.com,
[Type text] Page 36
other cereals. It also
isolated from soil and
leguminous seeds
cottony 11/2/2011
B. setariae.
Desmostachya
bipinnata, Oryza sativa
Sorghum sp.
Triticum sp. (India)
lawn grass and
gramineous crops
Conidiophores were fasciculate and brown,
septate and straight and the basal cell was
enlarged and hemispheric
al.
Well-developed conidia were long- obclavate,
obtuse at both ends, straight, brown, with five to
eight transverse septa, and measured 49.7 to
117.1 × 13.3 to 17.2 μm
Sivanesan, (1987)
www.cybertruffles,
9/12/2011
Shi et al. (2010)
[Type text] Page 37
Fig.2.3.6.1. Detailed structures of conidiophores and
condia of B. maydis.,
Source: www.cybertruffle.com.
[Type text] Page 38
Fig.2.3.6.2. Detailed structures of conidiophores and conidia of B. spicifera. Original in: Hoog, G.S. de. 2000, Atlas of clinical fungi, ed. 2: 1-1126.
[Type text] Page 39
Fig.2.3.6.3. Detailed structure of conidia and
bipolar germination of B. seteriae.
Source:www.cybertruffle.com.
[Type text] Page 40
2.3.8. Host Range
The hosts of different Bipolaris spp. are as below:
Causal organism Host Location Reference
B. spicifera Seeds of bermuda
grass
Seoul Branch Station
of National Plant
Quarantine Services
of Korea located at
Kimpo airport
Koo (2004).
B. spicifera Isolated from
necrotic leaves of
watermelon
Taroudant area of
southern Morocco
Mhadri et al. (2009).
B. spicifera Leaves of 100 plants
of pomegranate
(Punica granatum L.
’Nana’)
Public gardens and
the University of
Sciences of Kenitra
city (Morocco)
Kadri (2011)
D. seteriae
(B. seteriae)
Pearl millet seed India Shetty et al. (1982)
and Ahmed and
Reddy (1993)
B. seteriae
and B. incurvata
Diseased orchids,
bromeliads, proteas,
and other plants.
Heliconia in Hawaii Sewake and Uchida
(1995).
B. seteriae
and B. incurvata
Grasses Heliconia fields in
Hawaii
Uchida and Aragaki
(1995).
B. seteriae Cassava
(Manihot esculenta
Crantz)
Danzhou, Hainan
Province and
tropical regions of
China
Shi et al. (2010)
2.3.9. Nutrition
Fungi have quite simple nutritional requirements. They need a source of organic
nutrients to supply their energy and to supply carbon skeletons for cellular synthesis.
But, given a simple energy source such as glucose, many fungi can synthesize all their
other cellular components from inorganic sources – ammonium or nitrate ions,
phosphate ions and trace levels of other minerals such as calcium, potassium,
[Type text] Page 41
magnesium and iron. Fungi that normally grow in a host environment or in other
nutrient-rich substrates might require additional components, but still, the nutrient
requirements of most fungi are quite simple. Having said this, fungi need to capture
nutrients from their surroundings. The cell wall prevents fungi from engulfing food
particles, so fungi absorb simple, soluble nutrients through the wall and plasma membrane. In
many cases, this is achieved by releasing enzymes to degrade complex polymers and,
then, absorbing the nutrients released by these depolymerase enzymes. Fungi produce a
huge range of these enzymes, to degrade different types of polymer. In fact, there is
hardly any naturally occurring organic compound that cannot be utilized as a nutrient
source by one fungus or another (Deacon, 2006).
Five strains of H. maydis Nisik. et Miyake and two strains of H. carbonum were
grown on 14 carbohydrates and 13 fatty acids each as the sole carbon source. All
carbohydrates except L (-) -sorbose supported excellent growth. Fatty acids having 12
or more carbon atoms supported growth of all strains except H. carbonum Race II,
which failed to grow on linolenic acid. Optimum growth of strain NRRL 5128 H. maydis
Race T in shaken culture occurred within 7 days at 280C. Under these conditions, this
strain utilized 45-55 percent of corn oil when incorporated into the medium at 5
percent by volume as the sole carbon source (Ellis, 1973).
Silver scurf, caused by Helminthosporium solani, is an important storage disease
of potatoes. Experiments designed to evaluate control alternatives are limited by
difficulty in producing conidial inoculum. In an effort to better understand how this
difficulty could be overcome this study evaluated the influence of various carbon-to-
nitrogen (C: N) ratios, carbon concentrations, and amino acids on conidial germination,
colony diameter and conidiation of H. solani grown on solid-phase basal salts media.
Under the conditions tested, the highest concentrations of conidia were produced with
1.25 to 2.5 g of carbon/L at a C: N ratio of 10: 1. Higher C: N ratios or higher carbon
concentrations reduced conidiation. Total conidia production was improved by use of
tyrosine or arginine as the sole nitrogen source. Use of leucine, lysine, methionine,
phenylalanine, or threonine severely inhibited H. solani conidia production. Use of a
nitrogen source containing a mixture of amino acids resulted in a defined medium that
permitted conidiation and growth of H. solani that was similar to or better than that
obtained with standard V8 Juice medium (Elson, 1998).
[Type text] Page 42
Bipolaris Shoemaker, Drechslera Ito and Exserohilum Leonard and Suggs are
known to be pathogenic to food crops, foliage and turf grasses (Yamaguchi, 2010). A
selective culture medium for fungal isolation will be useful for studies of diseases
caused by the fungi. The sensitivity of Bipolaris, Drechslera and Exserohilum to 9 kinds of
carbon sources were tested in vitro. While D-mannose, a carbon source, enhanced the
growth of those fungi, especially it was effective in growing Bipolaris and Drechslera
that grew a little bit slowly on potato sucrose agar. A selective medium has been
developed based on potato extract broth containing D-mannose (1%), agar (1.5%),
thiophanatemethyl (100mgL-1), and chloramphenicol (100mgL-1) as an antibiotic, and
the pH was adjusted to 4.8. Plant pathogenic fungi, Bipolaris, Drechslera and Exserohilum
were consistently isolated from diseased rice, oat and red sprangletop, respectively by
using the selective medium.
The EF-37 isolate, one of DSE fungi (Dark -septate endophytes), is beneficial to
the growth and development of its host plant, Saussurea involucrata Kar. et Kir. The
cultivation requirements including basic culture medium, temperature, light, pH, carbon
source and nitrogen compounds were studied for their effects on mycelial growth of a
dark-septate endophytic (DSE) fungus EF-37 by using one-factor-at-a-time method.
Potato dextrose agar (PDA) was the best medium for the growth of endophyte EF-37.
Our studies showed that 20°C, 24 h dark cultivation and pH 7 significantly influenced
the growth of endophyte EF-37 on PDA medium. Moreover, glucose and calcium nitrate
were found to be the best nutrients for EF-37 growth. Under the optimal cultivation
conditions, DSE fungus EF-37 isolate could grow actively. This is the first study about
the effect of cultivation conditions on the growth of this strain, which provides the
preparatory knowledge for the biological characteristics of DSE fungus EF-37. (Ya-li Lv
et al., 2010)
Sulphur is also one of the important constituents of a suitable nutrient medium
for fungi, but it is required in much smaller quantity in comparison to other essential
elements. It plays a vital role in their metabolism; it enters into the composition of the
mycelium and the spores of many fungi. It also takes part in protein synthesis,
respiration and other biochemical processes. Volkonsky (1933) classified fungi into two
categories on the basis of their sulphur requirements (I) Parathiotrophs- organisms
utilizing only reduced form of sulphur (II) Euthiotrophs- organisms utilizing sulphate
[Type text] Page 43
and other oxidised sulphur. The importance of sulphur compounds with special
reference to their role in biological methylation by fungi has been reviewed by
Challenger (1953). In spite of its importance there are certain fungi that grow without
sulphur (Steinberg, 1941; Srivastava, 1951; Agarwal, 1955). There is considerable
variation as regards the response of various fungi to sulphur compounds (Armstrong,
1921; Saksena et al., 1952; Agarwal, 1955; Agnihotri , 1962; Kumar,1962). Rastrick and
Vincent (1948) have made an extensive study on the utilization of sulphur compounds
and they reported that fungi converted essentially all the sulphate sulphur into organic
compounds. Among sulphur compounds, sulphates have mostly been reported to be
good sources for the growth of various fungi by several workers (Armstrong, 1921;
Mosheretal, 1936; Tandon, 1950). On the other hand, Volkonsky (1933 & 34) reported,
that members of Saprolegniaceae were unable to utilize sulphur as sulphate
(www.biologiezentrum, 7/10/2011).
2.3.10. Management
2.3.10.1. Chemical management
The genetic basis of susceptibility to southern corn leaf blight is well known and plants
resistant to it are widely available and constitute the first line of defense. If necessary,
fungicides can also be used against B. maydis. A large number are approved for use and
these include: chlorothalonil, mancozeb and propiconazole (www.cbwinfo.com,
21/10/2011). The best chemical management against maydis leaf blight of maize was
spraying of mancozeb@2g/l of water (www.agritech.tnau.ac.in, 9/12/2011).
Seed treatment was effective for controlling mortality due to seedling blight (B.
sorokiniana and Fusarium sp.). In growth stage trial of 1999-2000, seed treatment was
also effective for seedling mortality due to S. rolfsii. Percentage of black seed (carrier of
spot blotch for next season) was reduced with a single early spray (45 days after
sowing) for Tilt-250EC only and for combined treatment also, which suggests the
reduction of inoculum level of soil and seed borne B. sorokinana. Significant yield
increase of 31-48% was obtained for Tilt-250EC combined with Vitavax-200 in a single
early spray (35-45 days after sowing). This increase of yield for Tilt-250EC was only 22-
25% and for Vitavax-200 only 14-29%. The result suggests the reduction of inoculum
level of B. sorokiniana for seed, foliar, root and soil with combined treatment (Mehta &
Igarashi, 1985; Loughman et al., 1998). Dithane M-45 and Bayleton have been able to
[Type text] Page 44
inhibit growth of the Drechslera maydis in laboratory bioassay test in low concentration
(Khan, 1999).
Twelve seed sample of rice were tested by Ahmed et al. (2002) and all were
found infected with brown spot diseases caused by B. oryzae. Highest (5.5%) and lowest
(1.5%) incidence was found in sample Bhabokhali and Mahozompur, respectively. Four
fungicides viz. Bavistin, Hinosan, Tilt-250EC and Dithane M-45 were evaluated against
B. oryzae. Dithane M-45 was the best with 100% reduction of the prevalence of the
pathogen at 0.3% of the seed weight as the seed treatment and inhibited the mycelial
growth about 62.83% at 500ppm followed by Tilt-250EC (54.67%), Hinosan(29.60%)
and Bavistin (28.00%), respectively. All test fungicides were effective against B. oryzae
at higher concentration.
Overall, Medallion + Daconil provide good control as soon as 2 WAFT compared
to the other fungicide treatments. In addition, 2 WAFT all treatments showed significant
reductions in the number of Helminthosporium leaf spots (Drechslera sp. and Bipolaris
sp.) per unit area compared to the control, but it wasn’t until 6 WAFT that the other
fungicide treatments attained the same level of control as Medallion + Daconil. In
addition, Medallion + Daconil provided the best control throughout the 12 weeks of the
study at Washington State University Golf Course at Pullman,WA. Increased rates of
Medallion did not result in increased disease control. Daconil alone provided as good
control as any of the Medallion treatments alone (Golob & Johnston, 2004).
The sensitivity of Bipolaris, Drechslera and Exserohilum to 11 different fungicides
was tested in vitro by Yamaguchi (2010). Thiophanate-methyl, a chemical fungicide, had
little impact on the growth of B. oryzae (Breda de Haan) Shoemaker, D. avenacea
(Curtisex Cooke) Shoemaker, and E. rostratum (Drechsler) Leonard and Suggs. Chemical
fungicides examined were copper oxychloride (Sun-Bordeaux, 73.5% active ingredient),
benomyl (Benlate, 50% active ingredient), captan (Osocide, 80% active ingredient),
chinomethionat (Morestan, 25% active ingredient), maneb (M-Dipher, 75% active
ingredient), manzeb (Ziman Dithane, 75% active ingredient), polycarbamate (Bis-
Dithane, 85% active ingredient), polyoxins (Polyoxin AL, 10% active ingredient),
thiophanate-methyl (Topsin M, 70% active ingredient), potassium bicarbonate (Kari-
Geen, 37% active ingredient) and triflumizole (Trifmine, 30% active ingredient). All
fungicide formulations used in this study were wettable powders. B. oryzae, D. avenacea
[Type text] Page 45
and E. rostratum did not grow on the media containing 100ppm of maneb, manzeb,
polycarbamate and triflumizole. The fungal growth was mostly suppressed by
polyoxins. It was suggested that the Bipolaris, Drechslera and Exserohilum species were
sensitive to those fungicides. On the media containing 100ppm of benomyl, captan,
chinomethionat, copper oxychloride and potassium bicarbonate, the fungi grew to some
extents; however, the growth was slower compared to those on control media without
fungicides. On the other hand, the fungal growth was vigorous on the medium
containing thiophanate-methyl. The results revealed that Bipolaris, Drechslera and
Exserohilum were resistant to thiophanate-methyl at a concentration of 100ppm.
Therefore, thiophanate-methyl was suggested to be useful for the selective medium.
Moreover, as thiophanate- methyl has been reported to be effective against a wide
range of fungi including Monilia, Gloeosporium, Botrytis, Sclerotinia, Corticiium, Fusarium
and Pyricularia, this chemical fungicide is expected to suppress the growth of
saprophytic fungi on Gramineae such as oat, red sprangle top and rice plants.
Leaf spot is a disease provoked by the fungus B. maydis, which causes great
damages for annual crops such as corn, wheat and oats. In vitro tests were accomplished
to evaluate the efficiency of the mycelial growth inhibition of the pathogenic fungus
through five fungicides: tetraconazol, tebuconazole, azoxystrobin + cyproconazole,
trifloxystrobin + propiconazole and trifloxystrobin + cyproconazole (Yamashita, 2010).
For the determination of the efficiency of the products, the daily growth of the fungus
was evaluated, being compared with the control. All of the treatments were found
effective for the inhibition of the growth of the fungus.
2.3.10.2. Biological Management and Biogenic Interactions
Biological control by the antagonistic microorganism is a potential, non- chemical and
ecofriendly tool for crop protection against phyto-pathogenic fungi and the
management of several plant diseases (Papavizas, 1985). Considering the cost of
chemical pesticides and hazards involved, biological control of plant diseases is now
increasingly being practiced all over world. The use of antagonitic plant pathogens is
risk free when it results in enhancement of resident antagonist (Monte, 2001).
Following mixed inoculation with Cochliobolus sativus, incitant of spot blotch,
and Pyrenophora tritici-repentis, incitant of tan spot, wheat leaves developed less
necrosis than average produced by the two pathogens alone at inoculum concentrations
[Type text] Page 46
equal to those used in the mixed inocula. Spot blotch predominated over tan spot
following simultaneous inoculation or sequential inoculation where P. tritici-repentis
preceded C. sativus by up to 6hr. Antagonism occurred even when inocula contained
only 20% C. sativus. Inoculation with C. sativus resulted in reduced conidial germination,
slowed germ tube development and reduced appresorium formation in P. tritici-
repentis. Tan spot development may be suppressed in the field where environmental
conditions favour spot blotch C. sativus or its metabolites could potentially be
manipulated to produce an effective biological control for tan spot of wheat (Da Luz &
Bergstrom, 1987).
Black point is a brownish or black discolouration of wheat kernels and biological
control is a complementary strategy to manage the disease. This work evaluated the
effect of five strains of Trichoderma harzianum and one strain of T. koningii on the
growth of B. sorokiniana and A. alternata and compared the results of screening tests
under controlled conditions and field evaluations on bread and durum wheat ears.
Disease incidence, infection percentage and seedling emergence percentage determined
in a greenhouse assay were evaluated. Dual cultures showed that Trichoderma spp.
inhibited significantly the mycelial growth of B. sorokiniana between 36 and 71 percent
and of A. alternata between 41 and 61 percent. Microscopic examination of B.
sorokiniana and A. alternata showed plasmolysis and vacuolization of hyphae of the
pathogens in the presence of the antagonists tested. With pre-inoculation of wheat ears
at anthesis under field conditions, disease incidence, infection percentage by blotter
tests and seedling emergence in the greenhouse did not show significant differences
between controls and treatments with Trichoderma spp. (Monaco et al., 2004).
Interactions of 10 Fusarium spp., namely F. equiseti, F. longipes, F. moniliforme, F.
oxysporum, F. proliferatum, F. scirpi, F. pallidoroseum, F. sporotrichioides, F. solani and F.
subglutinans with other fungi viz., Alternaria alternata, Aspergillus niger, A. flavus, A.
terreus, A. versicolor, Cladosporium herbarum, Drechslera hawaiiensis, Paecilomyces sp.,
Penicillium digitatum, P. funiculosum, Rhizoctonia solani and T. hamatum were studied in
vitro by Fakhrunnisa (2006). In dual culture plate assays, T. hamatum showed inhibition
in growth of Fusarium spp., by producing zones of inhibition.
In order to assess the potential of rhizopheric micro organism in biological
control of soil-borne diseases, 180 isolates of both Pseudomonas and Bacillus spp. from
[Type text] Page 47
rhizoplane and surrounding soil of healthy and infected wheat were collected by
Soleimani et al. (2005) in Hamadan province Iran. Only 9 isolates with the high
antagonist ability against the growth of the two pathogenic fungal spp., B. australiensis
and B. sacchari were selected by dual culture method and purified.
Pathogenic micro-organisms living on wheat foliages may interact with each
others. This study was conducted to reveal the interactive relationships existing
between spot blotch causing fungus, B. sorokiniana and selected pathogenic fungi
subsisting on wheat foliages. Four fungal pathogens of wheat were selected and the
intensity and severity of the selected pathogenic fungi on wheat leaves were assessed.
Pure cultures of the fungi were produced by isolating them from the spot blotch
infected and blighted wheat leaves. Separate in vitro dual culture studies in completely
randomized design with five replications were carried out to assess the interactions
between each pair of B. sorokiniana and selected rival pathogens of wheat foliages.
Percent inhibition in radial growth of either fungus was calculated. Viability test of the
mycelium at the interface zone and pathogenicity test of the isolates were carried out. B.
sorokiniana strongly inhibited the colony growth of Cercospora sp. and Phoma sp. under
in vitro conditions. Similarly, there was no effect on colony growth of either B.
sorokiniana or Bipolaris sp. due to dual culture with each other. The dual culture of B.
sorokiniana and A. triticina results in the suppression of colony growth of both fungi.
There were non- significant differences in percent growth inhibition between the first
and the second week of dual culture in some of the tested fungi. The viability of mycelia
of all the tested fungi was intact in dual culture. B. sorokiniana exerts antagonistic ability
against some minor pathogens of wheat foliages under in vitro conditions (Bhandari &
Ranamukharachchi, 2010).
2.3.11. Toxin
In the fungal kingdom, the ability to cause disease in plants appears to have arisen
multiple times during evolution (Does & Rep, 2007). In many cases, the ability to infect
particular plant species depends on specific genes that distinguish virulent fungi from
their sometimes closely related non- virulent relatives. These genes encode host-
determining ‘virulence factors’ including small, secreted proteins and enzymes involved
in the synthesis of toxins. These virulence factors typically are involved in evolutionary
arms races between plants and pathogens. Current knowledge of these virulence factors
[Type text] Page 48
from several fungal species in terms of function, phylogenetic distribution, sequence
variation and genomic location have been summarized. Also some issues that are
relevant to the evolution of virulence in fungi toward plants; in particular, horizontal
gene transfer and the genomic organization of virulence genes have been addressed.
A toxin can be defined as microbial metabolites excreted or released by lysed cell
which in very low concentration is directly toxic to cells of the suspect. In pathology the
term toxin is used for a product of the pathogen, its host and pathogen –host interaction
which even at very low concentration directly acts on livings host protoplasm to
influence the course of disease development or symptoms expression. Pathotoxin
produce all or most of the essential symptoms of the diseases in susceptible host as a
convincing evidence of their causal role and can be host specific such as victorin, or non
specific. Only victorin meets fully the stricts requirements of a pathotoxin. For others
evidence of pathotoxicity is indirect and independent confirmation is lacking (Singh,
2009). Therefore, their status as pathotoxin is only tentative. Phytotoxin are the
substances for which causal role in disease is merely suspected rather than established.
These are product of parasites which induce few or none of the symptoms caused by the
living pathogen. They are non specific and there is no relationship between the toxin
production and pathogenicity of the diseases causing agent.
A host specific toxin is a metabolic product of a pathogenic micro organism
which is selectively toxic only to the susceptible host of the pathogen. These substances
have a very high degree of toxicity to the suspect in low concentrations at which they do
not affect other organism and they produce essential symptoms of the diseases when
placed in the healthy susceptible host (Singh, 2009).
Alternaria alternata tobacco pathotype, the causal organism of brown spot
diseases of tobacco produced a host selective toxin (named AT-toxin). The toxin was
purified from culture filtrates and this purified toxin inhibited the seedling growth of
both susceptible and moderately resistant cultivars of tobacco at 0.2µg/ml. To
determine the toxin activity following three bioassay procedures were employed. In a
leaf spray assay, sample solution to be tested were sprayed with atomizers on detached
leaf pieces (3 x 3cm) which were kept in moist chamber at 260C at florescent light. The
toxin activity was accessed by the development of characteristics necrotic spots and
chlorosis of leaf pieces treated with a series of test solution 24 to 48h after the onset of
[Type text] Page 49
treatment. In a leaf drop assay, detached pieces 3x3 cm were slightly wounded at center
with a needle and kept on sponge mat in moist chamber. The leaf, thus, prepared was
immediately treated by a drop (30µl) of sample solution to the wounded portion.
Necrosis and chlorosis developed on the leaves were recorded 48h after the treatment.
In the root dip assay, a seed of tobacco and other plants were pre-incubated on a wet
filter paper in Petri dishes at 260C. The uniformly germinated seed were transferred on
to filter paper 2cm in diameter in a small vial containing 0.5ml of sample solution. Root
length seedling was measured after 5 days of incubation at 260C. The average length
was compared to control (Kodama, 1990). Among several plants tested, only species
belonging to the genus, Nicotiana were sensitive to the toxin. These results show AT-
toxin has host recognition factor in the genus, Nicotiana - A. alternata pathosystem.
Culture filtrates of A. sonchi, its organic extract, the chromatographic fractions
and pure compounds 24-27 were assayed by leaf disc-puncture bioassay on S. arvensis
and a number of non-host plants (Punzo, 2009). The plants were produced from pieces
of underground shoots or seeds and grown in a greenhouse. The discs (10 mm diam.)
were cut off well-expanded leaves with cork borer, placed on moistened filter paper and
punctured by sharp needle in the centre. Crude organic extract, chromatographic
fractions and pure compounds were dissolved in a small amount of Et OH and then
brought up to desirable concentration with distilled H2O. The final concentration of Et
OH in test solutions was 5% v/v that is non toxic to leaves of all plants in the control.
Droplets (10μl) of the test solution were applied on the discs and, then, incubated in
transparent plastic boxes at 24°C under 12h photoperiod. After 2 days of incubation, the
diameter of the necrotic lesions (mm) was measured. Alternethanoxins A and B showed
a significant phytotoxin activity against host plants and several other weeds.
Alternethanoxins A and B didn’t show antimicrobial activity. Application of both
alternethanoxins on leaves of host plant did not showed synergistic effect.
Pathogenic reactions of total eleven isolates of B. maydis obtained from infected
leaves of poplar as well as maize were studied by Chauhan and Pandey (1995). Both
group of isolates caused leaf blight to their natural hosts- poplar or maize within 10-
15days of incubation. P-isolates caused flecking to the maize cultivar whereas, Z isolates
were non-pathogenic to the poplar cultivars. It was observed that P-isolates infected
young shoot of poplar cultivars while Z-isolates did not infect stalk tissues of the maize
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cultivars during the study period. Toxin metabolites were invariably isolated by
chloroform extraction from the culture filtrate of P-isolates, but not from Z-isolates.
Responses of the cultivars of the both poplar and maize were tested to the different
concentrations of the toxin dissolved in distilled water. It is obvious that sensitive
reaction to the toxin was exhibited by only poplar but not maize cultivars. Moreover,
toxin concentration was recorded as an important factor determining rate of necrosis.
Leaf discs floated on 500-1000ppm concentration exhibited about cent percent necrosis
within 12-15h of inoculation. At 100ppm concentration, necrosis was very slow
compared to the higher concentration. But at 10ppm only flecking was recorded in
compared to control (absolutely healthy).
Screening for resistant barley genotypes in response to fungal toxin of B.
sorokiniana was assessed on standing barley plants as well as selected callus lines
(Chand, 2008). For the standing lines, manifestation chlorosis in response to toxin
infiltration showed a significantly slower diseases progress as compared to the necrotic
lines. Also necrosis in the callus tissues of the susceptible cultivar in MS medium
supplemented with different concentrations of the crude toxin was significantly higher
than the callus tissues of the chlorotic lines. Similar host response to the toxin in in vitro
and field situation open up the possibility of screening barley cultivars for resistance to
spot blotch using callus culture as against classical methods of screening in order to
increase accuracy and save time and space.
Recommended