Virginia Grape Disease Updates

As with other regions in VA, Winchester area observed a series of thunderstorms this morning.  It started from 2:40AM and lasted until 7:30AM or so.  The RH was high (>90%) until 8:40AM, thus, it accounted for 6 hours of estimated wetness with 68F as the average temperature.  It was a light infection risk for Phomopsis, and a borderline case for black rot.  Also, since the average temperature was high, it was a risk event for downy mildew as well. The other concern is the wash-off of the fungicide(s) on the leaf surface  We had about 1 inch of rain with this event.  A typical rule of thumb for fungicide application timing is  "2 weeks or 1 inch of rain whichever comes first", thus, this and a series of rains we had since last Sunday might washed materials off from your vines.  However, please keep in your mind that the risk of disease development also depends on a history of disease(s) in your vineyard, and also, you need to take the weather ...

We will have this year's first vineyard meeting tomorrow at De Vault Vineyard in Concord, VA.  If you have a time, please stop by.  The meeting will start from 11AM. I was posting handout for each meeting in the past, but I decided to post a template this year to save some time.  This handout covers fungicide updates, disease notes (disease biology of major fungal diseases and management tips), and some discussions on how to manage grape diseases.  If you are not able to come to the meeting, but want to have the handout, or if you are simply interested in, please click here to download.   As season goes, I will add more information to make it up-to-date for the meeting, but the basic information on disease biology and management tips are probably not going to be changed.

Here's an updated version of 2011 spray guide .  It is essentially the same as the one went out with the Viticulture Notes.  Typos, some minor items, and grammatical mistakes were fixed on this version.

Winchester area received a series of rain event since the morning of 4/22/11.  The first event started at 9:40AM on 22nd, and lasted until 12:40PM on 23rd.  Since the relative humidity stayed above 90% until 2PM on 23rd, it was a very long event (>28 hours), but the average temperature was low (48F).  Thus, there was a low to moderate risk of Phomopsis and a very low risk of Black rot infection.  This condition was a borderline case for black rot because the majority of wet hours are in lower 40's. Then, we had rain events on 9:20PM on 23rd, 3:40 PM and 10:20PM on 24th.  All of them are in higher temperature ranges (mid to high 60's), but lengths of wetness were short (2-3 hours).  The only disease that can cause under these conditions are downy mildew. The optimum temperature range for downy mildew infection is mid-60's to mid-70's, and under that condition, it takes only 90 min or so to cause infection.  On the other hand, it is relatively ear...

Winchester area received a light rain (0.02 inches) during early hours today.  It started around 1AM and even though the rain stopped soon, the relative humidity was above 90% until 9AM.  Thus, it accounted for 8 hours of estimated wetness with the average temperature of 62F.  It was a moderate risk for Phomopsis, a borderline risk for black rot, and a risk for downy mildew.

At Winchester, we received 0.25 inches of rain on 4/19/11.  It started from 9:20AM and lasted until 4:20 PM or so, and the relative humidity was above 90% until 5:00 PM.  Thus, we had about 7 hours 40 min of wetness with the average temperature of 55F.  It was long enough for light infection of Phomopsis (= low risk), but it was neither long nor warm enough for black rot.

During this winter , I received a few questions about a sanitation (= removal of infected tissues) as a part of disease management .  Our understanding is that most of the grape pathogens can survive on infected tissues over the winter to cause diseases on next spring .  Some survive on infected berry tissues (e.g., black rot ) or leaf tissues ( downy mildew ), and others survive on cane or wood tissues (e.g., Phomopsis , Botryosphaeria ).  Thus , my recommendation is a removal of any infected tissues ( berries , leaves , and canes ) from the vineyard .  Many major grape diseases we deal with such as powdery mildew ( which survives in the bark as a fruiting body over winter ) are polycyclic diseases , meaning they have a multiple generation of disease cycle within a season .  When we compared the risk of having an outbreak of the disease throughout a season on these...