Biotic Diseases (cont.). Fungal Diseases (cont.). Ergotism. Claviceps purpurea on Sea Couch . Syptoms
Fungal Diseases (cont.)
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.
L.S. in perithecial head showing perithecia containing asci
Barley heads infected with covered smut
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.
Chlamidospores of Ustilago hordei
Loose smut of wheat caused by Ustilago tritici
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 rachisof Digitaria sanguinalis racemes.
Digitaria sanguinalis loose smut
Digitaria sanguinalis loose smut
Scanning electron micrograph of Ustilago syntherismaeon Digitaria sanguinalis showing globose echinulate spores
Wheat plant with severe flag smut infection
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.
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Chlamidospores forming spore balls
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.
with uredospores of Puccinia graminis
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