Advances in the Characterization of the <i>Pyrenophora tritici-repentis</i>—Wheat Interaction

ABSTRACT Tan spot of wheat, caused by the fungus Pyrenophora tritici-repentis, is a destructive disease found in wheat-growing regions worldwide that can lead to serious yield losses. Changes in cultural practices have led to an increase in the severity and incidence of tan spot. Following infection, compatible races of the fungus elicit two distinct symptoms in differential wheat lines: tan necrosis and (extensive) chlorosis. Tan necrosis has been clearly demonstrated by several groups to result from the action of a protein toxin, Ptr ToxA. Wheat sensitivity to this toxin is conditioned by a single dominant gene. The chlorosis response may be more complex and appears to involve at least two other toxins, Ptr ToxB and Ptr ToxC, produced by different races of the fungus. Distinct genes apparently condition the reaction of wheat lines to each of these chlorosis-inducing toxins. This review concentrates on significant advances that have occurred during the past decade in the characterization of this disease interaction, ranging from the epidemiology and management of tan spot to molecular host-parasite interactions. Particular emphasis is placed on work describing fungal race differentiation, production of toxins and their importance in pathogenicity, and the genetics and physiology of host response to infection.