|Scientific Name||Claviceps purpurea (Fr.) Tul. [teleom.]
Sphacelia segetum Lév. [anam.]
Cordyceps purpurea (Fr.) Berk. [syn.]
Sphaeria purpurea Fr.
|Common Names||English: Ergot of cereals; French: Ergot du seigle; German: Mutterkorn; Spanish: Cornezuelo del centeno|
|Description||Claviceps purpurea is a member of the Ascomycota, order Hypocreales (Clavicipitaceae). The fungus produces three dissimilar morphological stages; a conidial and a sclerotial stage occur on wheat, an ascogenous stage develops from sclerotia in soil.|
The most characteristic symptoms of ergot are purple to black, horn-like sclerotia (ergots) protruding from the glumes on maturing heads. Sclerotia replace one to several seeds in cereal ears; the size of sclerotia increases with the size of the host seed, decreases with the number of sclerotia per head and is up to 10 times larger than seeds. Sclerotia of C. purpurea are elongated (2-40 mm), cylindrical with rounded ends, straight to sharply curved. The rind is hard and wrinkled or with longitudinal ridges (on rye); the interior is gray to white and parenchymatous.
The sclerotial stage of the fungus is preceded by a commonly ignored honeydew stage at flowering. Infected inflorescences exude a sugary sap that accumulates in sticky, yellowish droplets. Before sclerotia develop, infected ovaries swell and become stroma-like. Their surface is often covered with a layer of dense conidiophores.
Sclerotia of C. purpurea contain a number of ergoline alkaloids (derivatives of lysergic acid) toxic to humans and animals. Ergotism may occur through consumption of ergot-contaminated feed or grain products or during grazing of infested pastures. Ergolines are stable during processing, cooking or baking.
C. purpurea sclerotia are blue-black, 2 - 40 mm long, and require a period of low temperature (0 - 10 °C; 32 - 50 °F ) for several weeks (vernalization) before germinating in response to moisture conditions. In spring, germinating sclerotia produce one or more stalked, spherical stroma (optimum 18 - 22 °C; 64.4 - 71.6 °F, favored by high RH); repeated stroma formation is possible and 1-60 stroma may develop from each sclerotium. Numerous flask-shaped perithecia (200 - 250 x 150 - 175 µm) are embedded in spherical heads formed by the flesh-colored stalks (5 - 25 mm). Club-shaped, hyaline asci (100 - 125 x 4 µm) contain eight filiform septate ascospores (60 - 100 x 0.6 - 1.5 µm) which are actively ejected from ascomata and dispersed by wind.
Infection of grasses is restricted to open flowers. Within 24 hours, ascospores as primary inoculum germinate and invade the ovaries directly or by way of the stigma. A host-parasite interface at the base of the ovary supplies host nutrients to the pathogen. Host colonization proceeds upward, within 5 days a hymenium is produced and the conidial stage develops within 7-10 days. Instead of forming starch for kernels, host sugars are used by the pathogen to produce honeydew droplets. They attract insects generally regarded as vectors for spreading hyaline, ellipsoidal, aseptate conidia (4-6 x 2-3 µm) produced on simple phialides; conidial size varies depending on the host. The high osmotic potential of honeydew prevents conidial germination. Conidia are also spread to flowers by rain splashes.
A single sclerotium develops in each infected flower; the sclerotial stage occurs within 12 days after infection and matures about the same time as the healthy seeds. The sclerotia are viable in soil for about one year and for longer periods with grain in storage. Isolates of C. purpurea differ in the amount of alkaloid produced in the sclerotia.
At maturity, sclerotia drop to the ground or are among the harvested seed.
Ergot is favored by wet, cool weather that accompanies and prolongs the flowering period.
Additional Crop Information
C. purpurea has a broad host range infecting rye, barley, and wheat, corn as well as several weed grasses like Lolium, Festuca, Arrhenatherum, Dactylis, Phleum, Alopecurus, and Agrostis. .
Ergot causes yield losses up to 10% through seed replacement, sterility of neighbouring spikelets, and reduced kernel weight due to diversion of host nutrients at the expense of adjacent florets.
Integrated Crop Management
Sclerotia buried deeper than 5 cm by plowing can not extend their stromata above the soil surface. Burning of stubble and residues reduces ergot viability.
Various mechanical methods for removing sclerotia of C. purpurea from seed are used.
Resistance to ergot has been reported for barley, triticale, and wheat.
Biological control agents tested so far had no effect on sclerotial germination and ascocarp formation.
Seed cleaning: Ergot bodies are relatively easy to clean from the seed lot if professional cleaning equipments are available but cleaning can be expensive, especially if gravity or color separators are used.
Seed treatment: Some fungicidal seed treatments have been shown to reduce germination and emergence of ascocarps above the soil surface. For example, systemic fungicidal seed treatments ( containing triazoles such as prothioconazole and strobilurins such as fluoxastrobin) have been shown to have a good activity in ergot control.
However, the reduction of inoculum by cereal seed treatment remains always incomplete because parts of the ergot sclerotia may drop to the ground well before harvest and are, therefore, a potential inoculum for the next cereal crop during spring.
An integrated crop management using different tools such as crop rotation, seed treatment and seed cleaning is therefore needed to solve Ergot problems.
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