Alert to Potato and Tomato Growers
[Reprinted and modified from a posting by Meg McGrath,
Department of Plant Pathology and Plant-Microbe Biology, Cornell University]
Late Blight Found in Potato in Suffolk County, on Long Island, NY and in PA
Late blight is spreading quickly north. Some large, nationwide retail stores are selling seedlings already infected, even in Maine. If you bought seedlings at one of these stores check you plants and keep on checking.
Symptoms of late blight were confirmed on 23 June in a commercial field of potatoes in Riverhead and on 24 June on tomato in a home garden in Wading River. Rainy, cloudy conditions have been providing favorable conditions for the pathogen to successfully be dispersed long distances and for infection. Clouds protect spores being dispersed in wind from the killing effect of uv radiation.
This is the fourth year late blight has occurred on LI over the past 21 years. The earliest it had ever previously been detected was 26 Aug.
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. Late blight was the cause of the Irish Potato Famine.
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 or 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. Photographs are posted on the web at www.mofga.org . If you believe you have the disease, get a sample to your local Extension office.
If late blight is found 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. 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 first because of the potential to move spores on equipment especially while driving out of the field, and the equipment should be pressure washed afterwards.
Management in organic crops. Apply fungicides preventatively, using a 5-day schedule when conditions are favorable. Copper is effective for protecting a crop, but copper has been found to be ineffective when used as the sole practice for controlling late blight once it has started to develop. Other OMRI-listed fungicides labeled for late blight include Sporatec, Sonata, Serenade Max, and Companion. It is important to scout regularly and promptly destroy affected plants when found to reduce the amount of inoculum in a field. It is recommended that plants with symptoms be physically pulled up plus a few border plants, preferably on a bright sunny day when possible, then tarp the plants; spores will be killed by sunlight and also heat under the tarp. Scout daily thereafter for a few days to see if more plants develop symptoms. Clean after working in infested fields to avoid moving spores on equipment and workers. As soon as harvest is complete disk down field. And next year scout for and destroy any volunteer potato plants.
Some of this information on management was provided by Dr. Steve Johnson, University of Maine Cooperative Extension, and Dr. Tom Zitter, Cornell University.