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.