Alyssa K. Betts, Extension Field Crops Pathologist; akoehler@udel.edu
Too dry, too wet – a year filled with extremes. Over the past week you may have noticed field areas starting to look yellow. In most cases this is driven by the heavy rainfall and areas that may have been saturated for extended periods. If plants do not recover after the soil has dried, you may want to check for Phytophthora root rot (PRR). Phytophthora in soybean is one that we don’t consistently see, but the weather conditions this year have been conducive, so it is one to keep an eye on. In mature plants yellow leaves and wilting are the first symptoms. When dug up roots will be brown and rotting, these plants will easily pull from the ground. You may also see a dark or purple discoloration on the stem (Figure 1). With this disease leaves remain attached, and you may see dead plants in a row or as patches, particularly in those low field areas. The Phytophthora that impacts soybean (P. sojae) is a different species than the ones we see in vegetable production so crop rotation is a good management option. Other management approaches include soybean variety selection for genetic resistance and seed treatments.
Figure 1. Stem lesion from Phytophthora root rot
In addition to diseases like PRR, nematodes are also in many of our soils. Soybean cyst nematode (SCN) and root-knot nematode (RKN) may act alone or could be in combination with PRR or other diseases like soybean sudden death syndrome, Diaporthe diseases, brown stem rot, or others. It is thought that SCN can survive in wet conditions but may not thrive. Root-knot often has more visual symptoms than SCN and any time the plant has been stressed, these tend to be more pronounced. With root knot nematode, when you dig up the roots you will be able to see galling (Figure 2).
Figure 2. Root galling on soybean caused by root knot nematode