Pythium is primarily a seedling disease. Early planting dates increase the risk of disease in Iowa. Diseased plants often occur singly or in small patches in
low-lying areas of the field that are prone to flooding.
Pythium species cause pre- or post-emergence damping off. Infected seed appear rotted and soil sticks to them. Infected seedlings have water-soaked lesions on the hypocotyl or cotyledons that develop into a brown soft rot. Diseased plants are easily pulled from the soil because of rotted roots. Older plants become resistant to soft rot, but root rot may cause plants to become yellow, stunted or wilted if infection is severe.
The pathogen survives either in plant residue or in soil as oospores. Severity of disease depends on the amount of the pathogen in the soil, plant age and environmental conditions at the time of infection. Saturated soil is critical for infection for all Pythium species. Like Phytophthora, Pythium produces zoospores that swim in free water and infect the roots of plants. In general, Pythium species that are prevalent in the north infect plants at lower temperatures (50 to 60ºF), and Pythium species in the south infect plants at warmer temperatures (85 to 95ºF), although there are exceptions.
The best time to scout is VE through V2 in areas where soil temperatures have been cold and soils are very wet. This diseases occurs in low and wet areas of the field. Pythium- like symptoms in late-planted soybeans may actually be caused by Phytophthora.
Planting date: Planting in cold, wet soils should be avoided to reduce infection by Pythium species that infect at low temperatures.
Seed treatment: Where Pythium is a problem, seed treatments with mefenoxam (Apron formulation), metalaxyl or strobilurins as active ingredients can provide some protection. Resistance to metalaxyl/mefenoxam has been documented; however, they are generally considered more effective than strobilurins.
Tillage: No-till soils often have higher soil moisture and lower soil temperatures, factors that increase the risk of Pythium infection. If tillage is considered to improve drainage, use conservation tillage practices to maintain soil quality.
Photo by Tristan Mueller