Virginia Grape Disease Updates

Boy, it was some strong wind we experienced during a thunderstorm yesterday!  I saw a few trees fell down in the downtown Winchester.  That storm brought about 0.38 inches of water, the rain lasted about 80 min, then everything dried up very quickly.  This rain was a borderline case for downy mildew infection event.  This fungus (or oomycete, if you wan to be precise), can infect grape tissues in 90 min.  At the same time, it has been very dry for us, so, the overall risk may be low.  Even though the critical time has passed for berry infection of downy and powdery mildew, you need to maintain your leaves reasonably healthy for your crop and also for next year (accumulation of carbohydrate to trunks).  Please keep scouting for powdery and downy by checking younger susceptible leaves on the top of canopies. The weather forecast is showing a chance of rain tomorrow and Thursday.  I think we need little more rain for our vines... FYI: I will be out for a meeting until this Thursday

There is a chance of rain this afternoon (50%).  More importantly, there is a heat advisory issued throughout northern VA.  Dulles airport recorded historical high (99F) yesterday.  Please be careful if you are working outside.  We only received less than one hour of rain during early part of the week.  It was a little sprinkles here and there and not a disease event.  (I was so disappointed that I forgot to post on here.)

There are some chance of rain (30-40%) predicted tomorrow and then Tuesday.  At this point the main concern will be downy mildew on foliages and also late season rots .

Finally, we received a decent amount of precipitation.  Although the central VA received a much needed rain on Monday, Winchester and surrounding area received only about 0.02 inches of rain.  Then on Tuesday, Winchester area received a series of thunderstorms from 5 PM until little after midnight on Wednesday (~ 1 inch).  The relative humidity stayed high (>90%) until 8 AM this morning.  It accounted for an estimated 15 hours of wetness with an average temperature of 77F.  It was long and warm enough for infection process of downy mildew, black rot, Botrytis, and Phomopsis. We are also expecting around 30% chance of rain on both Friday and Saturday.

We received a series of thunderstorms from Friday night to Saturday morning.  At Winchester, the rain started around 8:00 PM Friday night and lasted until around 9:40 AM Saturday morning.  Soon after the last drops of rain, the sun came through the clouds and things were dried up again. Thus, it accounted for about 13:40 hours of wetness with an average temperature of about 75F.  It was warm and long enough for downy mildew, black rot, and Phomopsis. We will have a vineyard meeting tomorrow from 11 AM at Blenheim Vineyards .  If you have a time, please stop by.

There are chance of rain (40-70% across the state) predicted during tonight to Saturday. We really need some rains, so, I'm hoping that we all will get a decent rain.  We will see... Now probably many of you are considering disease management at or after veraison.  One of diseases you may be concerned at this time of the season are late season rots (sour rot and/or Botrytis gray mold, caused by Aspergillus niger, Alternaria tenuis, Botrytis cinerea, Cladosporium herbarum, Rhizopus arrhizus, Penicillium sp. , and others).  These late season rots come in when fruit sugar content accumulates around 8%, and often times, white varieties with tight clusters such as Vignoles and Chardonnay, tend to get this disease more than other varieties because of the structure of the cluster which can hold water inside and also because of the fair skin. The sour rot pathogens, including Botrytis, are opportunistic in nature, and the damage and the risk will increase if Botytis rot is involved.  T

As you know, we are expecting to have a high temperature for next few days.  Please protect yourself from heat related illnesses.   Having this much of heat is not really a fun, but there is a good aspect as well.  Powdery mildew pathogen cannot survive very well under high temperature.  At 33C (92F) or more, this fungus will suffer, and colonies can get killed.  If there is an extensive exposure to the sunlight and high temperature (as we have right now), they will have a difficulty surviving.  A study has shown that the fungus colonies will be completely killed after 12 hours at 35C.  Thus, this heat wave should reduce the risk of disease development, especially if you had a good canopy management.

I wish you all a happy 4th of July!! We haven't have rains lately = no risks for downy mildew nor black rot.  If you are happened to be in your vineyards, please scout for powdery mildew.