Disease risks from yesterday’s rain

After a short dry period, we received rains from around 8:15 PM last night (18 May), and although the rain stopped within a few hours, the relative humidity was high (>90%) until 11 AM this morning (+ we had a short shower in the morning).  Thus, we experienced about 14.5 hours of the estimated wetness with an average temperature of 57F.  It was long and warm enough for Phomopsis, black rot, and downy mildew infection.

We had a nice sunny afternoon; however, I see several thunderstorms coming in this way.  We will see…

Speaking of black rot, we observed the first black rot symptom today.  (the picture is fresh out of the vineyard!)  Since it is already producing fruiting body,  infection probably happened much earlier, but I did not have much time to spend in the vineyard due to rains.

Having sporulating black rot colony before bloom means that we need to be concern about berry infection.  Please scout your vineyard for disease symptoms so that you can adjust your spray program accordingly.

To my surprise, I have not seen much of downy mildew in our vineyards, yet.  (I have seen some suspicious leaves, though.)  It maybe due to the dry hot summer we had in 2010.  (i.e., the inoculum is low in the vineyard.) However, having a series of wet events with the average temperature in 60’s make me concern about downy.

So what I would do for next spray?  What I am thinking is either a Phosphorous acid material (e.g. Prophyt) or a metalaxyl material (Ridomil) for downy, mancozeb for black rot, downy and Phomopsis, and sulfur for powdery mildew.  I will watch for th weather and growth stage, and I will throw in mycrobutanil (Rally) or QoI’s for black rot in the pre-bloom/bloom spray (but not the next one), if needed.

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