TEMPERATURE-INDUCED RESISTANCE TO TAN SPOT [PYRENOPHORA-TRITICI-REPENTIS] OF WHEAT

Pyrenophora tritici-repentis (Ptr) induces tan necrosis and chlorosis in wheat. These symptoms are interaction phenotypes between individual isolates and specific wheat genotypes. The induction of tan necrosis is associated with the production of the Ptr-necrosis toxin by nec+ is olates of Ptr and the presence of a dominant gene for toxin sensitivit y in the host. The effect of temperature on the development of tan nec rosis and chlorosis was investigated using eight wheat genotypes diffe ring in reaction to tan spot and three isolates from the three current ly available virulent pathotypes of Ptr. The following constant temper ature regimes were tested: 10, 15, 20, 22, 25, 27, and 30 degrees C. A t 27 and 30 degrees C, a clear change towards resistance, both necroti c and chlorotic, was observed in the susceptible genotypes. Reaction t ypes of the same lines/cultivars were not altered at temperatures of 2 5 degrees C or below. Throughout this study, no breakdown of resistanc e was observed. Temperature-induced resistance to the fungus was paral leled by temperature-induced insensitivity to the Ptr-necrosis toxin. No symptoms were observed in toxin-infiltrated sensitive Glenlea wheat plants kept at 30 degrees C. The data of this study suggest that, in the necrotic system, the induced resistance observed at temperatures a bove 27 degrees C may have resulted from the failure of the toxin to i nteract with its putative receptor.