Erysiphe necator: Powdery Mildew

Sadly, we are learning at the school of hard knocks that we cannot keep our vineyard chemical free and in fact if we want to maintain a healthy vineyard we must judiciously choose and use some chemicals appropriately. To recap this growing season for us, the spring was cool with almost no rain, which is a little unusual for the Northeast. Our vineyard was planted in the third week in May and a deluge of rain followed in the month of June. To top this off, July was the hottest on record in the state of Connecticut. July was so hot and humid that our vines grew aerial roots. We heard from some of our Long Island friends that they needed to irrigate in July. So far we have had only two rain events in the month of August, on the 9th and on the 27th, which is to say, we haven't had much rain.

I mention the weather because, although we heard from other vineyardists that typically, the inoculum for powdery and downy mildew are not present in a newly planted vineyard, we found our plants infested with downy. We were well aware of our problem with downy mildew but we were unprepared to hear the next bit of news. We sent Fritz Westover the photo shown on the left and he identified for us that this photo shows a shoot infected with

powdery mildew (PM). Powdery mildew or Erysiphe necator is also known as Uncinula necator and is commonly called oidium.
I found an excellent article written by Michelle Moyer, Extension Specialist and Gary Grove, Plant Pathologist at Washington State University titled Grapevine Powdery Mildew illustrated with some photos that showed that our shoot (and much of our vineyard) does in fact have symptoms of PM as explained in the article.
Upper: Shoots infected by powdery mildew exhibit classic, grey web-like scarring.
Middle: Fall periderm formation and
Lower: Spring infections are noticeable as brown to red web-like discolorations on the cane.
                         Photos by Michelle Moyer.¹

Powdery mildew prefers mild temperatures between 68 and 85°F, relative humidity levels above 75%, and low levels of solar radiation. We had those conditions in spades in July. In the article from Washington State University, Moyer and Grove mention that the disease should be properly managed in the current year because that will influence how powdery mildew develops in the following year. If incompletely managed in Year 1, the fungus will mate and form overwintering structures called chasmothecia on infected fruit and foliage.
We are now faced with trying to beat down the inoculum before the onset of winter.
Reference:
1. Michelle Moyer, Viticulture Extension Specialist and Gary Grove, Plant Pathologist, WSU-IAREC, Grapevine Powdery Mildew.