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Dr. M.T. McGrath, Department of Plant Pathology and Plant-Microbe Biology, Cornell University is our guest writer this week.  Dr. McGrath is raising awareness of a killer in our gardens - a spore that decimates tomatoes, killing the whole plant in a matter of days.  This disease, called late blight, also attacks potatoes and is the organism responsible for the Irish Potato Famine in the 1840's.  It is imperative that home gardeners recognize the symptoms, get a correct diagnosis, and responsibly remove the infected plants so that the spores cannot travel.  They are wind borne.  The photos included in this article were all taken by Dr. McGrath.  Visit her Blog at

---Anne K Moore July 10, 2009---

Irish Potato Famine Disease affecting Gardens
and Farmers throughout the Greater Northeast

Revised by A. Wyenandt, NJAES, Rutgers University and M.T McGrath, Cornell University

Original article by Thomas A. Zitter, Cornell University, Ithaca, NY

Updated July 8, 2009 - Late blight, caused by Phytophthora infestans, is a serious disease that affects tomatoes and potatoes around the world. Late blight can become a serious problem because it can quickly kill affected plants.  Its spores are easily carried in wind currents to infect other susceptible plants in even the most remote areas in our region.

Late blight occurs sporadically in the Northeast in any given year because farmers diligently use methods to prevent the pathogen from surviving overwinter. Since our summer thus far has been cool with frequent rains, weather conditions have been very conducive for late blight development.

late_blight_tomato_plant-coCurrently, all tomato and potato plants grown in home gardens and in commercial fields are susceptible to late blight! Commercial growers are able to respond to reports of the disease by spraying fungicides to prevent its spread, which otherwise would mean certain death of their entire crops.

Unfortunately, many homeowners may not be as aware of this important disease, and if they take no corrective actions in a timely manner, home gardens can provide a source of inoculums (i.e. spores) for their neighbor's gardens and for commercial interests.

The occurrence of late blight in 2009 is different compared to most seasons.  This is the earliest the disease has been reported over such a broad region of the country.  More tragic for the Northeast is that infected plants have been found in large retail stores throughout the region (Ohio to Maine). 

Never before has this been known to have occurred. The inoculum is exceptionally contagious and can easily spread on air currents among tomato plants on garden center shelves. Thus, a gardener purchasing a healthy appearing plant may actually have one that just became infected.

late_blight_tomato_leaf5.jpgVegetable pathologists from throughout the Northeast have spread the word of this impending disaster, and within a day of the first sighting, a major supplier, working with the Department of Agriculture in the affected states, had begun to remove affected plants from the stores.

What to do now? Many families have started vegetable gardening, given the tough economic times, and tomato is the most important crop in gardens. The organism is not seed borne (however, it is tuber borne in potatoes), so that tomato plants started from seed locally in an area where late blight was not present likely are free of the disease, at least for now.  Given the scenario, we must assume that many infected tomato plants have been planted across the entire NE region.

Identification: The symptoms that develop on tomato leaves, stems and fruit are quite dramatic, and are very obvious to the naked eye. The leaf lesions are water-soaked, varying late_blight_tomato_leaf1in size from a nickel up to a quarter. They are water-soaked when the foliage has been exposed to watering or heavy overnight dews. When these lesions dry out late_blight_green-tomato_fr.jpgquickly, they may appear lime-green in color or even become beige. The edge of the water-soaked lesion, on either the top or the bottom leaf surface, will be covered with white fungal growth that contains the spore inoculum (visible with a hand lens). Spores are easily blown to surrounding areas, infecting plants and even some weed species in the family Solanaceae (the black nightshade family). Brown to almost black lesions develop on infected stems and the same lesions will develop on infected fruit, either directly on the plant or a few days after when they are sitting on a kitchen counter. It is not dangerous to humans.  Most of the fruit can be used if the affected area is removed.

late_blight_tomato-plantPlant examination and removal: Please inspect your tomato plants on a daily basis! If symptoms are already appearing on plants in your garden, these plants should be removed and put in a plastic bag for disposal. Do not put the removed plants in a compost pile, as spores can still spread from the debris. Your neighbors, not to mention commercial growers, will appreciate you taking this action immediately.

Plant treatments: Commercial growers have a number of fungicides that if applied early and often, can prevent infection and slow late blight development. They would choose not to spray if they could, but this destructive disease does not give them any other option. Homeowners do have a few products that are registered for use; the most effective ones have the common name of chlorothalonil, which will be on the label. These products are only effective if used before the disease appears and should be reapplied every 5-7 days if cool, wet weather persists. Chlorothalonil is a protectant fungicide, with no systemic movement in the plant, so thorough coverage is necessary. Copper fungicides are not as effective as chlorothalonil.

Diagnostic Services: For those homeowners interested in getting diagnostic testing for late blight on tomato or other diseases on vegetables in their garden, please contact your state university's plant diagnostic lab service. (These diagnostic services, in most states, require a fee, which some are not charging for suspected late blight. Reach them through your local County University Extension Service Office.) It is critical to get potential late blight samples to the diagnostic clinic as quickly as possible. Laboratories may be able to fax or e-mail results within 24 hours.

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