Phytophthora is a well-known, aggressive fungal disease that can occur in soybean at any growth stage from seedling to maturity. Most soybean varieties marketed in Iowa have some form of resistance to Phytophthora.
When Phytophthora infects early, it causes damping off or seedling blight that can result in stand reductions sometimes mistakenly attributed to environmental conditions or missed seeds during planting. Later in the season it causes stem and root rot. In Iowa, symptoms of this disease become most noticeable after July when temperatures start to rise.
Rotted roots and stem lesions leading from the soil line are diagnostic for Phytophthora.
Diseased seedlings are wilted and have rotted roots. Leaves look gray, then turn yellow, but remain attached to the plant. Brown lesions often develop leading from the soil line up the stem of the plant. The lesion will girdle the stem and kill the plant, and if not killed, the plant will be stunted (Figure 1).
Later in the season in early August, stem canker symptoms can look similar to the stem rot symptoms of Phytophthora. One way to separate stem canker and Phytophthora is to check whether diseased plants have root rot because stem canker does not cause root rot.
Periodic rainfall patterns such as one week wet weather followed by one week of dry weather, and warm soil temperatures are optimum for this disease.
Phytophthora development is very dependent on soil moisture. The fungus requires a period of saturated soils to form infective spores, and another period of saturation in which infection occurs. Phytophthora diseases are most common in fields or parts of fields with poor drainage. But they can occur in well-drained fields that are saturated for 7-14 days due to excessive rain or irrigation.
Management of Phytophthora
Many soybean varieties marketed in Iowa have race-specific resistance genes (Rps genes), which confer complete resistance to a specific race of P. sojae. This pathogen does adapt to the Rps genes, but it is a slow process. Growers should monitor the performance of the soybean varieties they choose. If Phytophthora damping-off occurs on a Phytophthora -resistant variety, the resistance has been defeated by the fungus. Consider using a variety with a better resistance gene for the next soybean crop.
Most soybean varieties are also rated for their level of partial resistance, also called field resistance or tolerance to Phytophthora. Partial resistance is most effective against Phytophthora root and stem rot in later growth stages and not very effective in the seedling stage.
If optimum conditions for Phytophthora occur later in the growing season, scout for Phytophthora stem rot. If a large number of plants with Phytophthora stem rot are found, choose varieties with a different Rps gene and higher levels of partial resistance.
Where Phytophthora is a problem, seed treatments with mefenoxam (Apron formulation) as an active ingredient can provide some protection, especially in the case when a resistant variety is not available, such as in the case of specialty soybeans.
Read more about Fungicide Seed Treatments