Potato Disease Advisory #2 – May 19, 2011

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

Late blight Advisory
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 early blight and making spray recommendations. Our location this year is:

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

Date DSV Total DSV Spray Recommendation
5/3–5/12 0 0 none
5/13-5/15 11 11 none
5/15-5/16 3 14 none
5/16–5/18 16 30 5-days

The threat of late blight from seed infection is low, but there was some in Wisconsin last season. Maine was reported to be late blight free in 2010. Be vigilant anyway, given this recent weather pattern, which is supposed to continue for several more days. The first late blight fungicide application is recommended once 18 Disease Severity Values (DSVs) accumulate from green row. Green row occurred on approximately May 3, 2011 at this location. Please be vigilant and keep a look out for suspect infections on young plants coming from infected seed pieces! Growers opting not to use the forecast system should put the first late blight fungicide application on when the plants are 6 inches tall, and repeat every 7 days. There are numerous fungicides now labeled for late blight control; however, use of mancozeb (Manzate, Penncozeb, or Dithane) or cholorthalonil (Bravo) is still a very effective early season protective fungicide to use. A recent addition to the list of late blight fungicides available for use in DE is Presidio from Valent. This registration came out after the 2011 Commercial Vegetable Production Recommendations was printed so it is not in the current book.

The threshold of 18 DSVs has been exceeded as of today. Thirty (30) DSVs have accumulated so far for any potatoes that established green row (approximately 50% emergence) prior to and since May 3. Spraying as soon as possible with a protectant fungicide would be advised.