I have looked at several fields recently that have started to show late season symptoms of Phytophthora root rot. Early Phytophthora causes a seed decay before emergence or a damping off symptom at emergence or slightly after. However late season Phytophthora root rot often shows up at early pod set. See the picture below to see the classic symptoms one of our Redfield agronomists and I found in a local field.
The symptom seen from a distance is wilting and dying plants, usually in field entrances, compacted areas and of course low areas of fields. Plants can usually be found in various stages of damage, ranging from green to fully wilted to plant death (as seen in the photo). Often unnoticed until plants start to wilt, the disease has slowly progressed until now.
On close view, a brown discoloration of the stem leading upward from the root is clearly seen. The line between brown and green stem is very clear. Wilted plants may continue to die, but at this growth stage, the disease will not widely continue to spread in the field and affect neighboring plants. Withered leaves remain attached even after the plant dies. Preventive measures are the main means for managing Phytophthora root rot. Select varieties with either race-specific resistance (Rps genes), high field tolerance, or a combination of the two, plant in good seedbed conditions, tile to improve drainage, take steps to reduce compaction, rotate crops and use an appropriate complete fungicide seed treatment. If Phytophthora is a known issue in a field, use the Phytophthora rates of the metalaxyl or mefenoxam portion of the full seed treatment package. This is a higher rate than the “standard” rate automatically included in the multi-component seed treatment packages.