Virginia Grape Disease Updates

I think many of you are either harvesting or getting ready for it.  We harvested our Chardonnay and Merlot yesterday. (Of course most of them are damaged due to our fungicide trial, but there are a few left...) It seems like we are expecting some rain over this weekend.  The question is what to do in terms of the last minute disease management.  If you have clean vineyard without major rot issues, and you only have only a few days to go, you probably not need to take action.  If you have clean vineyard, but you still have more than 10 days to go, it may depends on the condition of the berries.  If you notice any wounding on berries (by birds, hornets, sun damage, etc), then you may want to consider captan before this rain because many of late season rots take advantage of these wounds.  If you have nice clean berries, then, you may able to wait and see. If you have some rot issues, again captan is a nice tool.  It is effective against varie...

After yesterday's post, Winchester area received measurable rain.  ;)  The estimated wetness duration was 14 hours with 72F.  It was long and warm enough for Botrytis and Downy mildew.   It looks like we are expecting a few more rain events in next few days due to the effect of a tropical depression Issac.  We will see...

We are having a very humid day today in Winchester area.  There have been several rains passed since early morning, but only a few of them were enough to be recorded at weather station (it need to be more than 0.25 cm/0.01 inches).  Plus, even though the RH has been high, it was not higher than 70% in most of the day.  Thus, if anything, there were a low risk for downy mildew infection event. I imagine many of you are preparing for harvesting now.  I am hoping not seeing too many rains!

There were two wetness hours recorded yesterday through today.  One happened around 6AM with 6 hours of estimated wetness with 66F, and the other happened around 5PM with 22 hours of estimated wetness with 69F.  Thus, we had infection events for downy mildew and Botrytis. Once again, our recent conditions (warm and humid nights) are favoring downy mildew pathogens to produce spores.  Please keep eye on younger leaves (upper part of the canopy) to catch their early development.  Severe downy mildew infection can cause defoliation, and you do not want to that to happen at this time of the season. Also, these extensive humid conditions also call for Botrytis and other late season rots.  Canopy management to promote air circulation and bird management can be a very useful tool to deal with late season rots.  That also reminds me to mention about Spotted wing Drosophila (SWD) .  We do have some isolated cases of SWD in VA vineyards this year.  Plea...

We received a showed yesterday afternoon, and it resulted in about 7.5 hours of estimated wetness duration with an average temperature of 65.5F around Winchester area.  It was an infection event for downy mildew and Botrytis, and it was a potential infection event for black rot and Phomopsis (Note: these two diseases are of relatively minor importance at this point of the season).  Once again, I see the risk of downy mildew remains high.  Please scout your vineyards (especially for younger leaves) often to catch early development of downy mildew.   (note: the East of us received a considerable rain over the weekend, but we did not. i.e., other Northern VA area might have one more risk events.)  

Yesterday's shower brought about 7.5 hours of estimated wetness duration with an average temperature of 71F around Winchester area.  It was an infection event for downy mildew and Botrytis, and it was a potential infection event for black rot and Phomopsis (although these two diseases are of relatively minor importance at this point of the season).  Once again, I see the risk of downy mildew remains high.  Please scout your vineyards (especially for younger leaves) often to catch early development of downy mildew. Around this time of the season, I am often asked about the potential impact of a fungicide application on wine quality.  There were several studies where authors detected chemical residues on the must.  It is often practiced to avoid application of sulfur and copper 3-4 weeks prior to the harvest.  It would be relatively easy to do when we have a dry condition, but sometimes (like in 2011), we may just need to do whatever we can to secure yield....

Since last week, we are having several nights where relative humidity is very high (>90%) (e.g., 8/9-10, 8/13-15).  This condition favors downy mildew fungus to produce spores.  Unlike others, this pathogen can cause disease after a short rain (90 min at optimum conditions).  Thus, some of short showers we are having here and there this week may be long enough as an infection event for downy mildew.  For example, yesterday's short shower event was probably long enough for downy mildew to spread.  Also, please keep in mind that these showers are often geographically nonuniform and you might have had a longer wetness event. At this point, a phosphonate (aka phosphorous acid, such as Prophyt) is a good option to control downy mildew.  Some people prefer to include captan to add more forward protection.  The target of this late season application is to keep foliage healthy in order to help berries to mature, and also to accumulate carbohydrates for wi...