Phytophthora Root Rot in Alfalfa

The grassy circular area is somewhat lower than the rest of the field, and the alfalfa has died because of Phytophthora root rot which occurs in  alfalfa when the soil temperature is higher than 60^oF (16^oC) and the soil is saturated for several days or weeks and the plants are susceptible.  Severe above ground symptoms include dead plant which are normally replaced by grassy weeds.