Top dieback caused by anthracnose. Diseases typically target weak plants. Fields that are deficient in nitrogen or fertilizer and stressed by oversaturation and drought are the first victims. Also, watch your older varieties. Newer products tend to have much better disease tolerance.
Black lesions on the stems is a clear indicator of anthracnose.
Split the stem and crown to look for rotten and discolored tissue. These plants will not stand late into the season.