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MOFGA's Pest Alert - July 5, 2012
Compiled by Eric Sideman, PhD - MOFGA's Organic Crop Specialist

Late Blight Found In Maine

Late blight has popped up this week in Maine. I have received pictures from a farm in Woolwich, and it has been confirmed in the lab at the Pest Management Office, University of Maine Cooperative Extention. It is on both potatoes and tomatoes, although the confirmed sample was potato.

It was found earlier this week in Massachusetts. Quickly after, here. Rainy, humid, cloudy conditions provided favorable conditions for the pathogen to successfully be dispersed long distances and for infection to take place. Clouds protect spores being dispersed in wind from the killing effect of UV radiation. The spores are wimps and on a sunny day like today will die quickly in the wind.

All growers, including gardeners, should thoroughly inspect their potato and tomato plantings because this can be a very destructive disease when not managed, quickly killing foliage and rotting tomato fruit and potato tubers, AND be a source of spores spreading the disease to other growers.

Classic symptoms are large (at least nickel sized) olive green to brown spots on leaves with slightly fuzzy white fungal growth on the underside when conditions have been humid (early morning or after rain, or if it is dry you can induce it for identification by putting an infected leaf in a plastic bag over night). Sometimes the lesion border is yellow, but not usually. Often it has a water-soaked appearance. Leaf lesions begin as tiny, irregularly-shaped brown spots. Brown to blackish lesions also develop on upper stems. Firm, brown spots develop on tomato fruit. There are great photographs at:


If you believe you have the disease, get a sample to your local Extension office, or send me a picture. Still, most of the suspects are proving to be early blight or Botrytis, so don’t jump to conclusions, but it is important to be vigilant.

If you find late blight in a localized spot in a field or garden, promptly destroy all symptomatic plants plus a border of surrounding plants to eliminate this source of inoculum. Physically pull, bag and get rid of affected plants. (This is done to prevent the spores from spreading to other farms.) Do not just drop in field as they will still be a source of spores. If disking is used to destroy a whole field, the crop should first be sprayed with fungicide because of the potential to move spores on equipment especially while driving out of the field, and the equipment should be pressure washed afterwards.

If you feel you have found late blight in only a localized spot, and most of your field is still clean of the disease, then you may want to spray a copper based fungicide. Copper products have proven to give good control, if sprayed before infection has occurred. DO NOT USE AT RATES OR FREQUENCY BEYOND LABEL INSTRUCTIONS.

[The legal Champ WP rate in the greenhouse is the same rate as in the field, that being 1 to 1.5 pounds (up to 4 pounds in severe cases) per acre for potatoes and 2 to 4 pounds per acre for tomatoes.

Greenhouse tomato coverage is about 1 ounce per square foot (the 1 ounce per square foot rate is equivalent to 340 gallons per acre in a field rate). Two level tablespoons of Champ WG per 1000 square feet of greenhouse is equivalent to 1 pound per acre. One level tablespoon of Champ WG per gallon of water is equivalent to 1 pound per 100 gallons.]

REMEMBER, only certain formulations of copper fungicides are allowed in organic production. Be sure to check with your certifier if you are not sure. Common formulations include Champion WG, Cueva Fungicide Concentrate, and COC WP.

Some of this information on management was provided by Dr. Steve Johnson, University of Maine Cooperative Extension, and Drs. Meg McGrath and Tom Zitter, Cornell University.

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