Potato Late Blight Advisory – 5/8/09

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

This is the first report for 2009. If you would like a fax or email report please call (302) 831-4865 or email bobmul@udel.edu.

We are using the E-WEATHER SERVICE from SkyBit, Inc. as we have in the past. The service determines specific requested weather parameters (temperature, relative humidity and rainfall) based on calculations of data from the nearest National Weather Service stations. This weather data is used in the WISDOM software program for predicting late blight and making spray recommendations. Our location this year is:

Shadybrook Farms, Little Creek, Kent County.
Greenrow: May 1

Disease severity values have been accumulating very quickly since the beginning of this rainy period. The threat of late blight from seed infection is low to moderate, but there was some in Maine last season. Be vigilant, given this recent weather pattern. The Wisdom program that is used for calculating the disease severity values will not generate values until one week after greenrow. The way DSVs have been accumulating based on the forecasts, when Friday comes we will have accumulated the necessary numbers to initiate the first spray. The late blight advisory from Skybit has calculated that we have accumulated 21 DSVs to date. My suggestion would be to make a fungicide application when you can get on the field. This is a little early but the weather warrants an application if you reached greenrow on or about May 1. Apply sprays of mancozeb (Dithane, Manzate, Pencozeb, Manex II, Gavel) or Bravo (chlorothalonil) now, and then again before plants canopy down the row.

Fortunately late blight has not been a problem here in Delaware for many years and unless you have seed from an unknown source the risk of late blight is low. However, the weather has been perfect for it if it is present on seed.

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