Potato Disease Advisory #3 – May 20, 2010

Bob Mulrooney, Extension Plant Pathologist; 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 and early blight and making spray recommendations.

Location: Art and Keith Wicks Farm, Rt 9, Little Creek, Kent County.
Greenrow: May 6

Date DSV Total DSV Spray Recommendation
5/1 – 5/10 0 0 none
5/11 – 5/12 3 3 none
5/12 – 5/13 1 4 none
5/14 – 5/17 0 4 none
5/17 – 5/19 7 11 none


The threat of late blight from seed infection is low, but there was some in Maine last season. Be vigilant anyway, given this recent weather pattern. The first late blight fungicide application is recommended once 18 Disease Severity Values (DSVs) accumulate from green row. Green row occurred approximately on May 6, 2010. 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), chlorothalonil or Polyram are still very effective early season protective fungicides to use.

The recent wet, cool weather produced 7 DSVs from late Monday night through Wednesday night. This leaves us with only 7 DSVs before the first late blight spray should be applied if it has not already been applied.

The Plant Management Network has just posted an excellent webinar about late blight on potato by Dr. Steve Johnson, Extension Potato Specialist, Maine Cooperative Extension. Steve was an invited speaker at several of our Delaware Vegetable Growers Conference potato grower sessions many years ago. You can find it at: http://www.plantmanagementnetwork.org/edcenter/seminars/potato/PotatoLateBlight

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