RED LEAF BLOTCH (DACTULIOCHAETA GLYCINES) OF SOYBEANS (GLYCINE-MAX) AND ITS RELATIONSHIP TO YIELD

Red leaf blotch of soybeans, caused by Dactuliochaeta glycines, was ev aluated on soybean plants in field plots located in Zambia. Two experi ments were conducted in each of two seasons. Experiment 1 had four cul tivars that were either fungicide-sprayed or not sprayed. Disease seve rity was greatest on leaves at the lowest nodes from early vegetative through the reproductive growth stages. Area under the disease progres s curve (AUDPC) values and percentage of nodes defoliated at growth st age R5 were significantly (P = 0.05) greater in unsprayed plots for al l cultivars in both seasons. Yield losses ranged from 8 to 37% while r educed seed size ranged from 21 to 29% for the four cultivars. Number of pods per plant in fungicide-sprayed plots did not differ from those in unsprayed plots. However, the number of seeds per plant and seeds per pod were significantly (P = 0.05) greater in sprayed than unspraye d plots for some cultivars. In experiment 2, cultivar Tunia was either fungicide-sprayed at different times or not sprayed. The lowest attac hed leaf had the most variation in the amount of disease while ratings of the most median leaf in the canopy were generally less variable. T he AUDPC values calculated from the lowest attached leaf, the mean of all attached leaves, and the median attached leaf differed significant ly (P = 0.05) the number of times plants were sprayed with fungicide. Defoliation and vertical incidence of red leaf blotch from lower to hi gher nodes were significantly (P = 0.05) reduced in fungicide-sprayed plots in one season, but not the other. One thousand-seed weight and y ield differed significantly (P = 0.05) with treatment as one applicati on of triphenyltin acetate increased yields by 18% over unsprayed plot s in season 1. One thousand-seed weight and yield, regressed on the AU DPC for the median leaf in the canopy, explained 92. and 72% of the va riation, respectively.