The wheat blast disease threat

The recent detection of wheat blast in Zambia raised alarms in the wheat industry in the SADC region. Once established, the disease is difficult to control, say researchers Dr Tarekegn Terefe of the Agricultural Research Council’s Small Grain Institute and Dr Willem Boshoff of the Department of Plant Sciences at the University of the Free State.

- Dr Tarekegn Terefe and Dr Willem Boshoff

Wheat blast, caused by the fungus Magnaporthe oryzae pathotype Triticum (MoT) (syn Pyricularia oryzae), was reported for the first time on wheat in 1985 in Brazil and has subsequently spread across most wheat-producing areas of that country. The disease has the potential to cause yield losses of up to 100% on susceptible cultivars.

SYMPTOMS

Wheat blast infects the leaves and heads of the crop, but the canopy often remains green. Infection starts as brown to black spots, and eventually the entire spike above the infection points dries out and has a bleached or straw-like appearance.

Infected wheat heads produce shrivelled, poor-quality grain or fail to produce any grain at all. Wheat blast symptoms on the head resemble those of Fusarium head blight, but wheat blast lacks the characteristic pinkish discolouration that often develops on wheat heads infected by Fusarium. Instead, greyish fungal growth may be visible on the rachis between spikelets on the head. Some grass species may also serve as an alternative host for the pathogen, creating an additional source of inoculum and helping ensure the survival of the fungus.

Wheat blast poses a serious threat to global wheat production and food security because the fungus produces airborne spores that can be dispersed by wind within and between wheat fields in a short period, resulting in severe yield loss. Moreover, the fungus is seed-borne and can be transmitted from infected seed to seedlings after planting. This may facilitate long-distance dispersal across the world’s major wheat-producing countries through infected seeds.