Potato Disease Advisory #8 – June 12, 2008

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

Disease Severity Value (DSV) Accumulation as of June 11, 2008 is as follows:
Location: Broad Acres, Zimmerman Farm, Rt. 9, Kent County
Greenrow: April 27

Date Daily DSV Total DSV Spray Recs Accumulated P days*
5/16 – 5/17 6 27 7-day interval 141
5/18 – 5/21 5 32 5-day interval 166
5/22 – 5/26 0 32 7-day interval 203
5/30 – 6/1 3 35 10-day interval 252
6/2 – 6/4 1 36 10-day interval 279
6/4 – 6/5 4 40 5-days 279
6/5 – 6/6 2 42 5-days 288
6/6 – 6/7 1 43 5-day 298
6/7 – 6/11 0 43 10-day interval 321

* 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, 307 P-days have accumulated at the site.  The 300 P-day threshold has been exceeded so fungicides for early blight should now be applied if not already done.

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.

If pink rot or leak is a concern and no pink rot fungicide was applied at planting consider applying one of the following when potatoes are nickel-sized and repeating 14 days later. Apply in as much water as possible (20-30 gal/A): Mefanoxam/chlorothalonil (Ridomil/Bravo or Flouranil) 2 lb/A, or Ridomil Gold/MZ 2.5 lb/A, or Ridomil Gold/Copper 2 lb/A. If Platinum/Ridomil Gold was applied at planting the label allows one foliar application of one of those products at tuber initiation if conditions warrant.

For specific fungicide recommendations, see the 2008 Delaware Commercial Vegetable Production Recommendations Book.