Description and Symptoms
Many pathogens cause seed decay and seedling blight. The first symptom may be an area of the field with poor or no stand. Decayed seeds are very soft and may be covered with fuzzy fungal growth; seeds can be difficult to find if badly rotted. Postemergence damping off appears as yellowing and wilting; seedlings soon collapse leaving only dead leaves above the soil. Nodal roots may show the same decay symptoms as seminal roots and the mesocotyl. If the nodal root system escapes decay, the plant will survive; it may be stunted or recover fully. Favored by cool, wet soils, these diseases are more common in low-lying or poorly drained areas or in fields planted too early in spring. Standard seed treatment fungicides on corn reduce risk of seed decay and seedling blights. Planting corn when the soil temperature is above 50°F and soil moisture is not excessive may also reduce risk.
The best time to scout for seedling diseases is VE through V6. Seedling diseases are more prevalent in cool and wet soils, particularly in fields with reduced tillage and greater residue cover. Look for uneven plants or parts of a field with poor emergence.
Damping off is favored by cool, wet soils, so it is more common in low-lying or poorly drained areas or in fields planted too early in the spring.
If the stand reduction is due to Phytophthora sojae, resistant varieties are available. Many soybean varieties marketed in Iowa have race-specific resistance, which means a complete resistance to a specific race of P. sojae. Soybean varieties are also generally rated for their level of partial resistance to Phytophthora (also called field resistance or tolerance ). However, partial resistance is most effective against Phytophthora root and stem rot in later growth stages and less effective in the seedling stage.
If damping-off is caused by Phytophthora or Pythium, seed treatments with a fungicide containing mefenoxam (trade name Apron) will be effective when a resistant variety is not available, such as in the case of specialty soybeans. However, mefenoxam fungicides will not be effective against other damping-off fungi. Some formulations have a combination of active ingredients and therefore are effective against a spectrum of fungi. In all cases, good coverage of the fungicide on the seed coat is critical.
Photo by Alison Robertson