A few weeks ago we shared on Instagram a picture of a cluster of grapes that prompted many wine lovers to comment on the image. The photo was from a vine in our garden that we organically grow for fun and research. The grapes in the picture were at the early stage of veraison and were suffering from powdery mildew. Although we were aware of the mildew issue, we still wanted to share the veraison shot as it is one of our favorite stages of the grape growing process. To our surprise, we got many comments on the powdery mildew infection which inspired us to write this brief on the problematic grapevine pest.
Powdery Mildew is caused by the fungus Erysiphe necator, also known as Uncinula necator, and it is one of the most common grapevine diseases. To identify the disease look for white powdery spots on the leaves and grapes. As the disease advances the leaves might also show spots of different colors as if they are stained or even begin to curl.
The best way to prevent this disease is by making sure that your vines have good air circulation and are exposed to plenty of sunshine. These measures will expedite the drying of the leaves and berries after moisture exposure and will increase the flow of air during high humidity periods. To aide with these two important issues make sure that vines are planted in an area with direct sunlight and prune the canopy of the vines in a way that promotes good air flow. Also you might want to strategically remove some of the leaves around the grape clusters.
All vine varietals have different levels of susceptibility to this fungus but it is believed that Vitis vinifera varietals are more susceptible to it than their American counterparts. It is preferable that grapes affected by this disease are not used to make wine as research has shown that infection levels as low as 3%-5% of the grapes can taint the wine and give it a foul taste.
As far as our vine is concerned we will have to wait until next year to see how it rebounds from this years infection. To all you grape growing mavericks out there, with harvest upon us, we wish you happy pickings.
