Plant Health Initiative

Seedling Diseases - Risk Assessment

Saturated soil promotes spore germination and infection by the water-molds Pythium and Phytophthora. Photo credit: X.B. Yang, Iowa State University

Most seed and seedling-infecting fungi are common to soybean fields. The fungi are usually associated with soil debris and can survive for long periods in the soil.

Root and seed exudates stimulate the germination of the fungal spores, which grow and infect the seed or seedling when conditions are favorable. Conditions that favor fungal growth and infection are generally those that slow the germination and growth of the soybean seedling.

Conditions that favor Pythium and Phytopthora

• Poorly drained and compacted soils
• Very early planting
• Heavy periods of rainfall
• Minimal tillage
• Poor quality seed
• Plant stress

Conditions that favor Rhizoctonia

• Delayed emergence
• Moist soil but not saturated
• Herbicide injury
• Soil types with high amounts of organic matter
• Warm soils in late May and June