Late Blight Management in Organic Potatoes and Tomatoes

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland;

Currently, late light has been reported in a conventionally managed tomato planting in Maryland and in a conventionally managed potato field in Delaware. Interested potato growers in Maryland and Delaware can receive regular Potato Late Forecasts, which are provided by both states. To receive these reports in Delaware contact Phillip Sylvester at 302-730-4000, and in Maryland contact Kate Everts at 410-742-8788 or Because the information in these forecasts is more focused on conventionally managed fields and I have received several questions from organic potato and tomato growers on how to manage late light in their fields.

Two excellent resources are available for organic growers. One was written by Dr. Amanda Gevens at the University of Wisconsin and can be viewed here The other article is from the extension web site eOrganic, Both provide researched-based information on late light management in organic production. The articles are very comprehensive, but a few of the highlights are:

● In field trials of organically managed crops, copper products have provided the best control of late blight, although preliminary lab studies indicate some other products may also be effective.

Be Proactive. Preventive treatments are necessary to manage late blight on an organic farm. It is always better to apply the products before late blight onset than to wait to treat after late blight is present.

● If late blight gets out of hand, the potato or tomato crop should be destroyed to limit risk for other fields on your farm as well as fields on adjacent farms. This is a community disease and management by all growers is necessary to reduce damage within a region.

● When deciding if it is worthwhile to try to manage the disease (vs. destroying the crop), consider how close the crop is to harvest. One study estimated that applications of copper treatments for late blight will prolong potato plant productivity for two to four weeks. Remember also that the length of time a crop will survive is dependent on the weather; cool temperatures and lots of rain will make the disease progress very rapidly.