Yesterday, thunderstorms passed by Frederick county from around 4 pm to 5pm. There were other rain events afterwards, and the relative humidity was high until 7:30 am or so. It accounted for >15 hours of wetness event with average temperature of 68F or so. Total amount of precipitation was 0.47 inches. It was warm and long enough for Phomopsis infection, powdery mildew ascospore discharge, black rot infection, and downy mildew infection.

Also, now we are having 5 consecutive nights with temperature above 55F with the relative humidity in 80-100% range which can promote downy mildew sporulation.

Speaking of downy mildew, I started to see potential downy mildew lesions on some of our untreated vines. Typically, after infection, “oily spot” will show up on the upper surface of a leaf, then you will see “downy” fungal body on the underside of the leaf. This downy fungal body mainly consists of many sporangiophores which look like a tree of sporangium. Sporangium contains spores called zoospore which causes next round of infection. It will take 4 to 5 days after infection to symptom development, thus, I’m seeing either one from rains during May 17th to 18th or the current series of rain events from may 25th. It will take 1 to 2 weeks after infection for production of spores. I was rained out after taking this picture, so, I’ll go back in the field later today to look for more.