Virginia Grape Disease Updates

It seems that we have more rains on the horizon. I hope you had a chance to protect your vines, it it was not done recently. At Winchester area, yesterday's rain events were sporadic and it was not significant disease event except for downy mildew, but some of nearby area probably had more continuous rains, thus you need to check your local weather. We have having 13 consecutive nights that favors downy mildew sporulation, thus even though the threat on fruits are gone, you may want to consider downy mildew materials to protect your foliages.

We had a series of thunderstorms passed by the area. In terms of the duration of rain, all are short; however, it was wet afterwards because many of these rains happened after 5pm and the relative humidity stayed high until following morning. On 25th, we had rain started from 9:40 pm and RH was high until 10:00 am next morning (>12 h of wetness). On 26th, the strong thunderstorms passed starting 5:00 pm, and RH was high until 8:00 am this morning (>15 h of wetness). Average temperature during these events were mid-60F, thus, infection of downy mildew, black rot, Phomopsis, and Botrytis could happen. But as I mentioned several times here, if you passed the critical period (4-5 weeks after bloom), your berries should be resistant to downy mildew and black rot infections. Since we had such a strong wind with the thunderstorms, some of you may have experienced damages on berries (and/or even shoots and trunks). Hopefully they will dried out, but these wounds can be a course ...

We had a series of thunder storms on Wednesday and last night. The condition of air were dry to begin with, so, the relative humidity did not stay high after the rains (stayed in 70% range). Thus, these rains probably did not account for disease event at Winchester; however, your vineyard might had more rain or longer rain. Please check your local weather . Also, we are still having 7 consecutive nights with conditions for downy mildew sporulation. Even with a short rain (>90 min), it can cause an infection, thus, if your berries are still in a critical time (4-5 weeks after bloom), and you see leaves with downy mildew in your vineyards, then you may need to react to these series of infection event with Phosphorus acid or Ridomil products. However, if your berries passed the critical time, the risk of infection is very low. Once again, the fungus you may need to concern at this point is Botrytis. Make sure to protect your bunches with Botrytis materials (please read previo...

Rain on this Monday did not last long, but since it started later in the day, the relative humidity was high (>90%) until next morning. Thus, we had >15 hours of wetness event with an average temperature of 68F or so. This accounts for infection event for Phomopsis, downy mildew, black rot, and Botrytis, and ascospore discharge for powdery mildew. And I'm aware that areas in the south of us received more rain last night. Please check your local weather record . In addition, we are having five consecutive nights with high relative humidity that favors downy mildew sporulation. If you have issues with downy mildew this year, you may need to take some action before or after next round of rain. ( but again, it depends on growth stage of your berries. If it is past 4-5 weeks after bloom, the risk of downy mildew infection on berries is very low. ) Unfortunately, there is no curative fungicide for Botrytis, thus you need to protect your berries against infection. As I me...

We had two wetness event due to lines of rain passed by yesterday (7/17/09). The first one started around 11:30 am, and wetness lasted until 3:30 pm or so (4 hours) with an average temperature of 71F. The second one started around 8:40 pm and wetness lasted until 2:00 am (6 hours) with an average temperature of 66F. These events accounted for downy mildew infection, potential powdery mildew ascospore discharge, and light infection for Botrytis. Both are little shy for black rot infection event. As I mentioned in the previous post, your vines should be very close to the end of the critical period. After it is done, you can relax a bit in terms of downy, powdery, and black rot infection on berries. More rain events are in the forecast on Monday to Wednesday.

There is a vineyard meeting tomorrow at Pollak Vineyards from 11 am. I won't be there, but here is my handout for tomorrow , which talks about risks of diseases at this point and a disease update at Winchester. Tony will be there to discuss about viticulture topics. There are chances of rain in next few days in the forecast. However, so far, we haven't had a night which favors downy mildew sporulation for a while. In addition, your berries should be developing resistance to downy mildew and powdery mildew at this point (4-5 weeks after bloom). For black rot, it may take little longer (up to 8 weeks after bloom, depends on variety), thus, please scout your vineyards for black rot. Black rot fungus takes about 2 weeks from infection to sporulation. Thus, you do not see it right now, your risk is low because there is not enough time to have an outbreak. If you have some degree of infections on berries and leaves at this point, and if you are concerned, Rally (myclobutan...

We haven't experienced major disease event for a while. On 7/9-7/10/09, we had two consecutive nights that were favorable for downy mildew sporulation, and on 7/11/09, there was a short thunderstorm (~ 1 hr, 75F, 0.42 inches). It dried up quickly, thus, it may or may not be long enough for downy mildew infection (~ 90 min is required). For some location, it might been an infection event, so, please check your local weather record. Most of your vines are getting close to the end of the critical period (bloom to 4-5 weeks after bloom) for berry infection by powdery mildew, downy mildew, and black rot. After this period, you can relax a bit. The next disease to be considered is probably Botrytis. There is no formal study done yet, but many growers experienced that if you apply fungicide before bunch close and varison, the risk of Botrytis outbreak decreases. I think the logic behind it is that the application before bunch close provide a good spray coverage for both outside...