Epidemiology and crop loss assessment for disease management in winter wheat (Triticum aestivum L.)

The epidemiology of Mycosphaerella graminicola (Fuckel) Schroeter was studied for two seasons and the role of ascospores as primary inoculum of the disease was examined as a basis for the development of control measures. Ascospores were released following rain and there were two peaks of ascospore release. The first peak occurred during April and May and it was concluded from spore trapping and bait plant exposures that these ascospores provided the inoculum source for primary infection. The second peak occurred during the middle of November. Since secondary spread by means of rain-splashed pycnidiospores occurs during crop growth after primary infection, the significance of this second peak for epidemic development is not fully understood. Speckled leaf blotch infection in the field was detected during late autumn and sporulation first occurred during tillering. The disease was prevalent during late autumn and winter, and spread rapidly in rainy periods. The effect of disease on leaf area was again demonstrated in this research study. The non-random distribution of disease on leaves of unequal size necessitated the use of a weighted mean to fully describe disease. Furthermore, a carry-over effect of early disease on leaf area could be better accounted for by expressing disease as the total green leaf area per shoot. Absolute green leaf area may be more appropriate for all studies in which the mechanism of yield reduction is to be analysed. Several protectant fungicides strongly inhibited pycnidiospore germination in laboratory tests. Many systemic fungicides tested did not prevent spore germination. Once germination had taken place, systemic fungicides were highly effective in inhibiting mycelial growth. In the 1982-83 efficacy trial, foliar applications with the systemic fungicides triadimefon, PP450 and propiconazole controlled disease and resulted in significant yield increases in Kopara wheat crops. Two wheat trials were conducted in 1981-82 and 1982-83 to study the effects of natural and fungicide modified epidemics of speckled leaf blotch and stripe rust on yield and yield potential. In these trials, seed treatment with triadimenol and a triadimefon foliar spray applied at the 6 or 7 leaf stage proved to be highly effective in controlling both diseases, resulting in significant yield increases: Detailed analysis of epidemic development aided the identification of the duration of fungicide protection and the necessity for additional sprays. From knowledge of the development of yield potential, it was shown that the early phase of the epidemic was the most damaging, with reductions to the grain number per ear and ear number per unit area as well as to the individual grain weight. The results can serve as a basis for optimal timing of fungicide applications rather than routine spraying where applications may sometimes be unwarranted.