Downy mildew and Alterneria were confirmed on collard greens this past week. Current weather conditions are ideal for the development of both diseases on all cole crops as well as other crops.
Symptoms of downy mildew on cole crops include the production of white spore masses only on the undersides of infected leaves. The tops of leaves will develop slightly chlorotic (i.e., yellow) spots on the upper leaf surface that become papery in texture.
Alternaria typically starts out as small pinpoint spots on infected leaves that enlarge up to 2 inches. Lesions will become tan, brown or black and may have concentric rings and leave ‘shot holes’ in leaves.
Control of both diseases begins with regular scouting and preventative fungicide programs. Control of downy mildew is best with downy mildew specific fungicide such as Presidio (fluopicolide, 43) at 3.0 to 4.0 fl. oz 4SC/A, or Revus (mandipropamid, 40) at 8.0 fl. oz 2.08SC/A, or Zampro (ametoctradin + dimethomorph, 45 +40) at 14.0 fl. oz 4.38SC/A included in tank-mix with chlorothalonil (M5) at 1.5 pt/A. A phosphite fungicide (FRAC group 33) such as Prophyt, Rampart, or K-Phite can be included to also help control downy mildew. For control of Alternaria, apply chlorothalonil (M5) at 1.5 pt/A alone, or tank-mixed with a FRAC code 11 fungicide such as Quadris (azoxystrobin, 11) at 6.0 to 15.5 fl. oz 2.08F/A or Cabrio (pyraclostrobin, 11) at 12.0 to 16.0 fl. oz 20EG/A, or Fontelis (penthiopyrad, 7) at 14.0 to 30.0 fl. oz 1.67SC/A.