Biotic Diseases

(cont.)

Fungal Diseases (cont.)

Fungal Diseases (cont.)

Ergotism Smuts Rusts

Ergotism

                                                                                                        

                        

Claviceps purpurea on Sea Couch

Syptoms

Claviceps purpurea is commonly found on grains of rye or

sometimes on other grasses such as quackgrass. The fungus

infects the flowers when they're young. It induces the cells to

divide (hyperplasia) and to enlarge (hypertrophy), creating the

relatively large brown sclerotia. These sclerotia are hard resting

structures that allow the fungus to survive adverse conditions,

such as winter and desiccation.

In the life cycle of this organism, the sclerotia fall to the

ground and overwinter, germinating in the spring to produce a

stroma that contains perithecia, which produces spores.

It's really not a devastating parasite to the plant. You might

think its main detriment is that it replaces one of the grains of the

plant, thus reducing yield. However it also draws nutrients away

from the other uninfected grains so that they become stunted, thus

reducing yield quite a bit more. But its worst problem is when the

sclerotia inadvertently get mixed in with the grains and are

incorporated into foods, thus causing a devastating and sometimes

deadly syndrome called ergotism in humans and other animals.

Ergotism is caused by the chemicals in the fungus called ergot.

Consumption of foods contaminated with ergot and ergot

derivatives may cause vomiting, diarrhea, hallucinations, and may

lead to gangrene in serious cases.

Germinating sclerotium

Perithecial head

StalkStroma

L.S. in Perithecial head

L.S. in perithecial head showing perithecia containing asci

Causative organism: Claviceps purpurea

perithecium

asci

Covered Smut

Barley heads infected with covered smut

Symptoms:

Losses from the Covered smut disease are rare because the

environments are not conducive. In addition, seed treatments have

reduced incidence of the disease. However, because heads

infected with covered smut are harvested with healthy grain,

losses can arise from lowered grade due to smutty grain.

Causative organism: Ustilago hordei

Chlamidospores of Ustilago hordei

Loose Smut

Loose smut of wheat caused by Ustilago tritici

Barley Loose Smut

Loose smut is most obvious just after the plant has headed. Diseased plants produce blackened heads among a field of green heads. The spikelets of colonized heads become a mass of olive-black spores that have a characteristic "dead fish" odor. The fungus that causes loose smut survives as dormant mycelia (fungal threads) within the embryo of an infested seed. When the seed germinates, the fungus resumes growth along with the shoot apex. As the juvenile head develops within the stem the fungus colonizes the seed primordia (tissue that would become a seed within a head). When the head emerges from the boot, instead of flowering and releasing pollen, it releases the olive-black spores that can be wind-blown to "healthy" flowering heads. The spore germinates on the stigma (female receptive portion of flower) of a healthy head and colonizes the developing seed embryo. The colonized seed appears healthy but carries the dormant smut fungus within to start the cycle over again with the planting of the seed.

Beginning of the sporulation of Ustilago syntherismae on the rachis of Digitaria sanguinalis racemes.

Digitaria sanguinalis loose smut

Spikelets are replaced by a blackish-brown spore mass

Digitaria sanguinalis loose smut

Spores of Ustilago syntherismae

Scanning electron micrograph of Ustilago syntherismaeon Digitaria sanguinalis showing globose echinulate spores

Wheat plant with severe flag smut infection

Flag smut

Dark striping of leaves caused by flag smut

'Cracking' of stem caused by flag smut

Symptoms

Affected plants are severely stunted. The ears fail to emerge,

remaining within the boot. Plants show long dark grey to black

streaks on the leaf blades and leaf sheaths. The streaks eventually

erupt, giving the leaves a ragged appearance and exposing the

black teliospores which are then dispersed, giving the plants the

appearance of being covered in soot.

Causal organism:  Urocystis agropyri

Chlamidospores forming spore balls

Sterile cells

Fertile Chlamidospores (1 to 4)

Wheat stem rust

Berberis leaf infected with Puccinia graminis

SYMPTOMS: Uredinia generally appear as oval lesions on leaf sheaths, true stem, and spike. Uredinia can appear on the leaves if other diseases have not killed them. Uredinia are brick red in color and can be seen to rupture the host epidermis, on the leaves uredinia generally penetrate to sporulate on both surfaces. Infected areas are rough to the touch. Stem rust is favored by hot days 25-30 C, mild nights 15-20 C with adequate moisture for night time dews. Wind can effectively disperse urediniospores over great distances. Rain is necessary for effective deposition of uredinospore involved in regional spore transport. Disease dispersal through aeciospores from Berberis vulgaris is currently rare, but historically it was an important source of inoculum. Mycelium or uredinia on volunteer wheat, are the most important source of inoculum in tropical and subtropical climates.

Urediniospores and aeciospore germinate when in contact with

free water. Infection is by penetration through the stoma.

Penetration requires at least a low light intensity.

Stem rust can survive as teliospores during winter when

aeciospores are a major source of inoculum. It generally survives

as mycelium or uredinia on volunteer wheat during the non-wheat

growing season. Uredinospore can be spread by wind into disease-

free areas. Sporulating uredinia are active in tropical and some

subtropical areas throughout the winter. Occasional dormant

mycelium may survive beneath the snow pack in more northern

temperate regions.

Urediniospores and aeciospores are wind borne. Teliospores

remain with the straw.

T. S. in infected wheat stem showing uredosorus with uredospores of Puccinia graminis

T. S. in infected wheat stem showing teleutosorus with teleutospores of Puccinia graminis

T.S. in infected Berberis leaf showing pycnidium with pycniospores of Puccinia graminis

T.S. in infected Berberis leaf showing aecidial cup with aeciospores of Puccinia graminis