Potato Disease Advisory #6 – June 3, 2010

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

Disease Severity Value (DSV) Accumulation as of June 2, 2010 is as follows:
Location: Art and Keith Wicks Farm, Rt 9, Little Creek, Kent County.
Green row: May 6

  LATE BLIGHT EARLY BLIGHT
Date Daily DSV Total DSV Spray Recs Accumulated P- days*
5/20 0 11 none -
5/21-5/23 0 11 none -
5/23-5/25 18 29 5-7 days -
5/26 0 29 7-days -
5/27-5/28 1 30 10-days -
5/29-5/30 1 31 10- days -
5/31 0 31 10-days 199
6/1 1 32 10-days 206
6/2 0 32 10-days 214

Maintain the recommended spray interval. At 300 P-days fungicide sprays will be needed to control early blight. 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 down the row. At this point weekly fungicide applications would be suggested. There have been no reports of late blight on potatoes in the region.

* P days- We use the predictive model WISDOM to determine the first fungicide application for prevention of early blight as well. The model predicts the first seasonal rise in the number of spores of the early blight fungus based on the accumulation of 300 physiological days (a type of degree-day unit, referred to as P-days) from green row. To date, 214 P-days have accumulated at the site. Once 300 P-days have accumulated, the first fungicide for early blight control should be applied. This usually occurs when rows are touching.

The Spray Recs column in the table is also generated by the WISDOM software program. This recommendation combines the DSV accumulation for late blight as well as the P-day accumulations for early blight and computes a spray recommendation. This is presented as a guide only. Spray decisions should be made with local conditions in mind and this information can help to determine if disease conditions are favorable.

For specific fungicide recommendations, see the 2010 Delaware Commercial Vegetable Production Recommendations Book.

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