Potato Disease Advisory – May 7, 2008

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

Late Blight Advisory
This is the first report for 2008. 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, early blight and making spray recommendations.

Disease Severity Value (DSV) Accumulation as of May 6, 2008 is as follows:
Location: Broad Acres, Zimmerman Farm, Rt. 9, Kent County
Greenrow: April 27
Remember that 18 DSVs is the threshold to begin a spray program.

Date Total DSV Spray Recommendation
4/27- 5/6 7 none

Disease severity values have been accumulating slowly so far this season. The threat of late blight from seed infection is low, but more of a potential problem than last year.

Remember that these values are for potatoes that would have had about 50% emergence and made a row that you can see on or before April 27. If you had potatoes that made a row earlier, there were no severity values calculated for the week prior to April 27. Growers who do not want to rely only on the DSV calculations for scheduling fungicide applications should apply at least 1-2 sprays of mancozeb (Dithane, Manzate, Pencozeb, Manex II) or Bravo (chlorothalonil) before plants canopy or touch down the row. 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. For more fungicide recommendations see the 2008 Commercial Vegetable Production Recommendations.

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