Symptoms

The disease symptoms appear when wheat plants are seven to eight weeks old. Symptoms gradually spread to the upper leaves and by the time the crop matures symptoms are severe. Infection is first evident as small, oval, tan-coloured lesions, scattered on the leaves. Lesions enlarge and turn dark-brown to grey and are irregular in shape. Some lesions are surround by bright-yellow margins. The lesions vary in size, reaching a diameter of one centimetre or more.

As the disease progresses, several lesions coalesce to cover large areas, resulting in the death of the entire leaf. In some cases the leaf starts drying up from the tip, prematurely, when lesions appear. Black, powdery spores (conidia) may cover the lesions at this stage under moist conditions. The lowermost leaves are the first to show the signs of infection; the fungus gradually spreads to the upper leaves. In severe cases, similar symptoms are produced on the leaf sheath and stem, as well as the awns and glumes if spikes are infected before flowering (pre-anthesis stage). If the spike is infected this early, seeds do not form. Infection at the dough stage of seed development results in glume infection, ear infection, and seed infection. Heavily infected fields appear ‘burnt’ and can be identified from a distance.

Organism

Alternaria triticina

Host range

Wheat (Tritcum aestivum).

Method of spread

The fungus is considered to be seed-borne. It can survive for two months on infected wheat debris on the soil surface and for four months on infected debris buried in the soil. The fungus multiplies in the soil and infects leaves either by rain splash of spores onto leaves or by leaves contacting the ground.

Conditions favouring disease

Susceptibility to the pathogen increases with plant age.

Infection requires at least 48 hours of 100 per cent relative humidity. Maximum disease development occurs on ten week old plants under high relative humidity and temperatures up to 25°C.

Confused with?

Other fungal diseases or insect damage.

Where?

Asia, Europe, Africa, Mexico and Argentina.