Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
We had over one inch of rainfall around the Ellendale area of eastern Sussex County. Again, conditions for the development of downy mildew are favorable in higher rainfall areas. We are finding white mold in a high percentage of canopied fields with a history of lima beans in the past and some Phytophthora capsici in a smaller percentage of fields at this time.
A note on temperature sensitivity of downy mildew: Races E and F (F is the prevalent race now) are tolerant of much higher temperatures than past races. The following is a comment from Dr. Tom Evans, plant pathologist at the University of Delaware “the temperature to affect P. phaseoli is now much higher than 90°F and we have not determined an exact number but 95°F didn’t stop it in one experiment with Race E. Lab studies with Race E and F indicated that even 96°F didn’t negatively affect their growth.”
This means that we are not likely to see any help from high temperatures slowing downy mildew as we go into September even though we have few days in the 90s.