Potato Disease Advisory #7 – June 9, 2011

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

Late Blight Advisory

Location: Art and Keith Wicks Farm, Rt 9, Little Creek, Kent County.
Greenrow: May 3

Date Late Blight Early Blight Spray Interval Recommendation
DSV Total DSV Accumulated P-days*
5/27-5/28 2 54 175 5-days
5/28-5/29 2 56 183 7-days
5/29-5/30 1 57 191 5-days
5/30 0 57 197 7-days
5/31-6/1 0 57 207 10-days
6/1-6/5 0 57 232 10-days
6/6 1 58 251 10-days
6/6-6/8 0 58 263 10-days


The forecasted hot weather will reduce the risk and slow any development of late blight. Continue to scout fields for symptoms of late blight. Conditions will continue to favor early blight with the increased temperatures and high humidity. Heat stress will make the oldest leaves more susceptible. We should reach 300 P-days this weekend.

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, 263 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.

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) 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. Presidio was also labeled this spring for pink rot control. The label uses for pink rot control include an in-furrow application at planting with a follow-up application between hilling and tuber initiation. There is no labeled use for a foliage application for pink rot control. It is labeled for late blight control at 4.0 fl oz/A.