Evidence for a Pythium sp as a chronic yield reducer in a continuous grainsorghum field

Pythium spp. have been reported to reduce stands and cause stalk rot of gra in sorghum. Evidence is presented that it also can cause a serious seed and root rot in the field under a continuous grain sorghum production system. Experiments were conducted for 4 years in a field that had been cropped con tinuously to grain sorghum for at least 10 years. Effects of seed treatment s with captan and metalaxyl on plant stands, early to mid-season plant vigo r, and grain yields were evaluated. In five field experiments, seed treatme nt with metalaxyl (73 g a.i./100 kg) increased grain yields by an average o f 24.0% compared with nontreated seed. In three out of four field experimen ts, seed treatment with metalaxyl increased grain yields by an average of 1 3.1% above seed treated with captan (73 g a.i./100 kg). The yield increases could not always be explained in terms of differences among treatments in plant stands or in visual estimates of the amount of top growth 26 to 72 da ys after sowing. Apparently, the Pythium sp. causes a chronic root and seed rot that has a significant negative effect on grain production without nec essarily affecting stands or early to mid-season growth. P. ultimum var. ul timum was the fungus most commonly isolated from roots and seeds collected from the field. Tests for Koch's postulates conducted in a greenhouse verif ied it as the causal organism. In the greenhouse, treatment with metalaxyl protected seeds and roots from attack by P. ultimum var, ultimum for at lea st 28 days after planting.