Friday, June 25, 2010

Disease risks from yesterday's rain

We had thunderstorms went though yesterday afternoon in Winchester area.  It started around 2 PM, lasted about 20 min, then another one hit around 3:30 PM to 3:50PM.  However; the air was very dry for last few days, and it seems that leaves dried up very quickly after the storm.  Thus, I think it was not good enough for any pathogens to cause disease.

On the other hand, the information is just for our area and your vineyards might have experienced a longer wet event.  Downy mildew pathogen needs only about 90 min to cause infection under the optimal condition (about 68F).  Please check with your local weather service.

In addition, if the night time relative humidity is high, say above 85% or so, the condition will favor the pathogen to produce spores.  Based on the weather record, I see such conditions 4-5 days per week on average in the past few weeks.  In the other word, even though we have not received a rain event that can be an infection event, the environmental conditions are favoring spore production so that once we receive a rain event that gives a long enough wet event, downy mildew can be a problem.  It will really depends on the current downy mildew situation in your vineyards, thus, please spend some time to scout for diseases.

Also, some of varieties are getting close to the stage of bunch closure.  It is earlier than the normal year, but we are having such a hot weather.  This will be a critical time for Botrytis control.  The past study has shown that the number of Botrytis spores in the air tends to be high during veraison; however, if we wait until veraison, the cluster will be tight and a fungicide cannot penetrate into inside of the clusters.  Thus, if your varieties are susceptible to Botrytis, bunch closure is the timing to apply a fungicide against Botrytis.  If needed, you may need to repeat the application at veraison.

We are expecting 20-40% chance of rain during this Sunday and Monday.

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