EFFECT OF TRIADIMENOL SEED TREATMENT AND TIMING OF FOLIAR FUNGICIDE APPLICATIONS ON ONSET AND EXTENT OF POWDERY MILDEW AND LEAF RUST EPIDEMICS

Powdery mildew and leaf rust occur yearly in wheat-growing areas of th e southeastern United States. However, the time of onset and peak seve rity differ between diseases, and consequently, wheat plants are subje ct to disease stress throughout a long period of the growing season. T hree cultivars of winter wheat varying in their level of disease resis tance and either seed treated with triadimenol or untreated were plant ed at Kinston and Plymouth, NC, during the fall of 1989 and 1990. Plot s received foliar fungicide applications at different times during the two growing seasons to determine how cultivar, seed treatment, and ti me of foliar fungicide application affected development and severity o f powdery mildew and leaf rust, yield components, and yield. The exten t of mildew and leaf rust and their impact on yield components and yie ld varied between environments. At Kinston, where both powdery mildew and leaf rust were severe during 1989-1990 and 1990-1991, mildew posit ively influenced tiller number, and rust influenced 500-kernel weight and number of seeds per head. Powdery mildew was present only at low l evels at Plymouth; leaf rust reduced 500-kernel weight and yield. Seed treatment reduced the extent of powdery mildew in all environments an d reduced leaf rust at Kinston during 1989-1990 and at Plymouth during 1990-1991. When seed was treated with triadimenol, the optimum time o f foliar fungicide application for maximum yield response was delayed in six of nine cultivar X environment combinations.