Molecular Plant Pathology – Pathogen Profiles

Gaeumannomyces graminis, the take-all fungus and its relatives

Jacqeline Freeman and Elaine Ward

Wheat Pathogenesis Programme, Plant-Pathogen Interactions Division, Rothamsted Research, Harpenden, Herts. AL5 2JQ, UK

Summary:Take-all, caused by the fungus Gaeumannomyces graminis var. tritici, is the most important root disease of wheat worldwide. Many years of intensive research, reflected by the large volume of literature on take-all, has led to a considerable degree of understanding of many aspects of the disease. However, effective and economic control of the disease remains difficult. The application of molecular techniques to study G. graminis and related fungi has resulted in some significant advances, particularly in the development of improved methods for identification and in elucidating the role of the enzyme avenacinase as a pathogenicity determinant in the closely related oat take-all fungus (G. graminis var. avenae). Some progress in identifying other factors that may be involved in determining host range and pathogenicity has been made, despite the difficulties of performing genetic analyses and the lack of a reliable transformation system.

Infection of wheat by Gaeumannomyces graminis and Phialophora graminicola. (a) G. graminis var. tritici: perithecia produced on wheat roots in a rotting test, surrounded by released asci; (b) G. graminis var. tritici: ascospores in a single ascus (arrowed, a), with one ascospore separated from the ascus (arrowed, as); (c) G. graminis var. tritici: dark runner hyphae on the surface of a wheat root; (d) a patch of stunted and yellowed wheat plants (foreground), typical of take-all, in the summer; (e) whiteheads of infected plants in a wheat crop with take-all, in late summer; (f) similarities in discoloration of wheat roots, and differences in stem-base blackening of wheat, caused by infection with G. graminis var. tritici (Ggt) (left), G. graminis var. graminis (Ggg) (centre) and P. graminicola (right), in a pathogenicity test; (g) Phialophora sp. (lobed-hyphopodia): lobed hyphopodia on the leaf sheath at the stem base of a wheat plant; (h) Phialophora sp. (lobed-hyphopodia): swollen cells with pores (known as growth cessation structures or vesicles) in a wheat root; (i) P. graminicola: swollen cells with pores (known as growth cessation structures or vesicles) in a wheat root. Images (a), (b), (d), (f), (h) and (i) were kindly provided by Richard Gutteridge (Rothamsted Research).