Pythium Root Rot in Soybeans

Life Cycle of Pythium

• Pythium is a family of approximately five species of fungi. These pathogens live in the soil, and are considered a type of water mold, because they need wet soil to grow and reproduce. They can spend several years as oospores, which are rounded, dormant cells with double walls, but the oospores germinate and form a sporangium when moisture levels accumulate in the soil. The sporangium is a reproductive structure that allows zoospores to develop. Zoospores, in turn, are a mobile form of the fungus. They have small tails that allow them to swim through the water in the soil and enter soybean roots.

Climatic Conditions

• Pythium root rot usually appears during periods of heavy rainfall. This disease can be particularly devastating when soybean crops are cultivated in soil that is prone to flooding or does not have adequate soil drainage. Some species of Pythium live primarily in the south, and they infect soybean plants when temperatures range between 85 and 95 degrees Fahrenheit. Other species live in the north and infect plants when temperatures reach 50 to 60 degrees Fahhrenheit.

Symptoms

• Pythium root rot occurs in maturing or older plants. Infected stems initially look water-soaked, especially near the soil line, but soon the damaged areas of tissue turn brown and peel away from the plant. Diseased soybean plants have decaying or rotten roots, and can be pulled easily from the soil. The leaves turn yellow and wilt, and the plant dies soon after becoming infected. Although Pythium root rot occurs in mature plants, the fungus can attack soybean plants at any stage of development. Contaminated seeds may rot in the soil or fail to develop, while seedlings that emerge from the soil will wilt quickly and die, resulting in a condition called damping-off.

Controlling Pythium

• Crop rotation is not especially effective in controlling Pythium root rot, and there are no resistant soybean strains, according to the West Virginia University Extension Service. Treating seeds with a chemical, called mefenoxam, offers some protection against seed or seedling decay, but does little to help older plants. Plant soybean crops in fields with adequate soil drainage, and avoid performing heavy irrigation for at least 10 or 15 days after planting the crops. Only plant high-quality seeds that do not have excessive numbers of cracked or damaged seed coats, and avoid planting seeds that are over two years old.