Zentralbl. Mikrobiol. 146 (1991), 209-212

Gustav Fischer Verlag lena

[Department of Plant Pathology, University of Agricultural Sciences, Bangalore, India]

Effect of Cowpea Phylloplane Fungi on Fungicide Resistant Strains ofColletotrichum truncatum (SCHW.) ANDRUS & MOORE 1

)

K. RAvI and T. B. ANILKUMAR

Key words: Fungicide resistance, CoLLetotrichum truncatum, phylloplane fungi, cowpea

Summary

The effect of three phyllosphere fungi from cowpea (Vigna unguiculata ssp. unguiculata) on conidialgermination of wild sensitive as well as carbendazim and thiophanate resistant strains of CoLLetotrichum truncatumwas studied. All the three phyllosphere fungi viz., Penicillium sp., Phoma sp. and Pestalotiopsis sp. inhibitedconidial germination, maximum being with Phoma sp. (38.5%) followed by Penicillium sp. (6.9%) andPestalotiopsis sp. (4.1 %). The inhibitory effect of these fungi on either carbendazim or thiophanate resistant strainswere significantly greater.

Zusammeofassung

Es wurde der EinfluB von 3 Phyllospharen-Pilzen der Kuherbse (Vigna unguiculata ssp. unguiculata) auf dieKonidienkeimung von anfalligen und Carbendazim und Thiophanat resistenten Stammen von Colletotrichumtruncatum untersucht. All diese 3 Phyllosphiiren-Pilze hemmten die Konidienkeimung mit 38,5% bei Penicilliumsp. 6,9% bei Phoma sp. und 4.1 % bei Pestalotiopsis sp. Der hemmende Effekt dieser Pilze auf beide resistenteColletotrichum Stiimme war signifikant groBer als auf die anfalligen.

Fungicide resistance is becoming increasingly common among plant pathogenic fungi espe­cially with the introduction of systemic fungicides (SCHROEDER and PROVVIDENTI 1969; DELP

1980; DAVIDSE and DE WAARD 1984). When one takes into consideration the extent of time andmonetary inputs in bringing out a successful fungicide and the threat of resistance problem to thedurability of fungicides, one wonders whether we can cope up and overcome this threat. Infactseveral models have been proposed to manage such resistant strains (KABLE and JEFFERY 1980;LEVY et al. 1983; SKYLAKAKIS 1981). The success or failure of a systemic fungicide is not justbased on the emergence of resistant strains but more on the fitness of the resistant strains. No onlythere is lack of any regular pattern with regard to fitness levels among pathogens but also there isvariation in fitness levels within a pathogen to different fungicides. It is in this context the effectof associate leaf surface mycotlora on wild sensitive and fungicide resistant strains of CoLLetotri­chum truncatum was studied.

Material and Methods

A virulent culture of C. truncatum (Ctd from cowpea (Vigna unguiculata ssp. unguiculata) and its twocarbendazim resistant UV mutants (Ct2, Ct3) and three thiophanate resistant mutants (C4 , Cts and C4;) (RAVI 1988)were used in this study. Three species of cowpea leaf resident fungi viz., Phoma sp., Pestalotiopsis sp. and

1) Part of the thesis submitted to the University of Agricultural Sciences, Bangalore by the Senior author for theaward of Master's degree.

14 Zentralbl. Mikrobiol .. Bd. 146.3

210 K. RAVI and T. B. ANILKUMAR

Table I. Effect of Penicillium sp. * on spore gennination in fungicide sensitive and resistant isolates of Colletotri­

chum truncatum.

Isolates

Ctl ( I x 104)

Ctz ( 4 x 103)

Ct3 (18 x 103)

Ct4 ( 3.32 x 103)

Cts (13 x 103)

Ct6 (13 x 103)

% gennination

Pure culture

62.8522.20

33.3058.33

58.3334.61

Mixed withPenicillium sp.

58.5020.5028.90

52.6548.0025.25

% inhibition

6.927.65

13.219.73

17.70

27.04

* Penicillium sp. was mixed with isolate of Colletotrichum truncatum at I x 104 concentration in equal volume.

Table 2. Effect of Phoma sp. * on spore germination in fungicide sensitive and resistant isolates of Colletotrichum

truncatum.

Isolates

Ct l ( I x 104)

Ct2 ( 4 x 103)

Ct3 (18 x 103)

Ct4 ( 3.32 x 103)

Cts (13 X 103)

Ct6 (13 X 103)

% gennination

Pure culture

62.8522.2033.3058.3358.3334.61

Mixed withPhoma sp.

38.6513.7019.8039.5030.5512.60

% inhibition

38.5039.2840.5432.2847.62

63.59

* Phoma sp. was mixed with isolates of Colletotrichum truncatum at 3 x 104 concentration in equal volume.

Table 3. Effect of Pestalotiopsis sp. * on spore germination in fungicide sensitive and resistant isolates ofColletotrichum truncatum.

Isolates

Ct l ( I X 104)

Ct2 ( 4 X \03)

Ct3 (18 x 103)

Ct4 ( 3.32 x 103)

Cts (13 x Iv')

C~ (13 X 103)

% gennination

Pure culture

62.8522.2033.3058.3358.33

34.61

Mixed withPestalotiopsis sp.

60.2518.0030.5040.5056.0030.95

% inhibition

4.1318.908.40

30.563.99

10.57

* Pestalotiopsis sp. was mixed with isolates of Colletotrichum truncatum at 3 X lO3 concentration in equalvolume.

Penicillium sp. were used to study their effect on condidial germination of C. truncatum isolates. A known quantityof spore suspension of required leaf surface fungus was mixed to spore suspension of C. truncatum at theconcentrations indicated in Table 1 to 3 and the extent of conidial gennination was assessed by slide genninationtechnique after eight hours. The spore suspension was prepared in sterile distilled water and the cavity slides wereincubated at 28 ± 2°C in diffused day light conditions.

Effect of Cowpera Phylloplane Fungi 211

Results

Penicillium sp. was found to inhibit conidial germination ofwild sensitive isolate Ct, to an extentof 6.9 per cent. The fungicide resistant strains were relatively more affected by Penicillium sp. Theinhibition in conidial germination of isolate Cit; was as much as 27 per cent. This was folIowed byisolates Ctj (17.7%), Ct3 (13.2%) etc. (Table 1).

Inhibitory effect of Phoma sp. on germination of conidia of C. truncatum isolates was muchgreater than Penicillium sp. The germination in wild sensitive isolate was inhibited to an extent of38.5 %. Here again isolate Cit; was most sensitive wherein the extent of inhibition in conidialgermination was as high as 63.S per cent. Only isolate Ct4showed inhibition slightly lower than that ofsensitive isolate Ct. (Table 2).

Pestalotiopsis sp. was least effective of the three phyllosphere fungi in inhibiting conidialgermination of C. truncatum isolates. The extent of inhibition in conidial germination of sensitiveisolate was 4. I percent. However, it wasasmuchas30.5% in case of isolate Ct4 and 18.9% in case ofCt2' With isolate Ct6 it was 10.5% (Table 3).

Discussion

The fact that leaf resident microflora affect adversely the follicolus pathogens is welI established(HEuvEL 1969; WARREN 1972; FOKKEMA 1973; SOMASUNDARA and ANILKUMAR 1986). Suchadverse effect could be due to competition for nutrients (FOKKEMA 1971 ; WARREN 1972; BLAKEMAN1973), production of antibiotics (AGARWAL and GUPTA 1966; SINHA and BAHADUR 1974) etc. Thepathogen that is capable ofovercoming such suppressive effect from associate microflora can succeedbetter and thus such an ability would be an important component of fitness. In the present studies, ofthe phyllosphere fungi, Penicillium sp. and Phoma sp. showed strong inhibitory effects towardssensitive isolate of C. truncatum. Their inhibitory effect was much greater on fungicide resistantstrains. The third fungus from phyllosphere Pestalotiopsis sp., though relatively less inhibitory,nevertheless was capable of inhibiting conidial germination upto 30.5 per cent as in case of resistantisolate Ct4. It is likely that the leafsurface fungi compete fornutrition and the sensitive isolates are lesscompetitive or the fungicide resistance is associated with nutrient dependence for germination. Theassociation of fungicide resistance in this case with reduced sporulation and germination, and theirincreased sensitivity to several other fungicides (RAVI 1988), coupled with inhibition in germinationby associate mycoflora, should be helpful in formulating suitable management practices against suchresistant strains.

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Authors' address: K. RAvl and T. B. ANILKUMAR, Department of Plant Pathology, University of AgriculturalSciences, G. K. V. K. Campus, Bangalore-560 065, India.