Monday, July 13, 2009

Another dry weekend :)

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 and inside of the bunch, and the application before varison adds protection when more Botrytis spores tend to be present in the air. There is no fungicide with a curative activity against Botrytis, thus, preventative application is important.

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