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Light blight found on tomato transplants

July 1, 2009
By Richard Gast, Franklin County Cornell Cooperative Extension

Recently, alerts have been circulating among Extension educators across all over the Northeast. Their concerns are focused on one disease in particular, a devastating plant disease simply called late blight, and their efforts are focused on raising awareness of late blight-infected tomato plants found at local home-and-garden centers in numerous locations across an area that extends from Maine to New Jersey to Ohio. Tremendous concern lies in just how quickly late blight can spread from one home garden to another or from a home garden to a farmer's fields. And in just how devastating the impacts of the spread of the disease can be.

Late blight specifically attacks tomatoes, potatoes and other nightshade crops, including eggplant and petunias, which are closely related to tomatoes and potatoes and can also become infected and show similar symptoms. The disease is caused by an extremely destructive fungal pathogen, Phytophthora infestans, which develops and spreads rapidly in host plants in all stages of growth and easily survives from one season to the next in infected tubers. Under the right conditions, spores from infected plants can readily be carried by the wind or on storms for miles, or moved from place to place by splashing rain or by animals wandering from an infected garden or field to a garden or field where healthy plants are growing. Under ideal, moist conditions, a single infected tomato or potato plant can transmit the disease to thousands more in just a few days.

That is how the Irish potato famine of the late 1840's began. Most likely, late blight fungal spores were brought to England in the holds of ships coming from North America. Winds then carried the fungus from southern England to the Dublin countryside, where unusually warm, wet weather allowed the infection to thrive and spread quickly to the west.

Millions of Irish peasants, who subsisted solely on potatoes, were left starving and destitute. Those that ate infected potatoes became sick with cholera and typhus. During six years of famine, more than a million men, women and children perished. Another 1.5 million migrated to the Americas.

Late blight arguably remains the single most important disease of tomatoes and potatoes in the world today, with new, more aggressive strains of the fungus causing widespread damage and creating epidemics in, among other places, practically every major tomato- or potato-growing region of the United States.

The ease with which late blight can spread from garden to garden, from garden to farm, or from farm to farm, even over great distances, cannot be overstated. This puts farmers who grow tomatoes or potatoes at serious risk of losing their entire season's income. To prevent that from happening, Extension educators are asking that we, as gardeners, neighbors and community minded individuals, unite in our efforts to protect both garden crops and the commercial field crops produced by the local farming community.

It is imperative that gardeners learn to recognize late blight symptoms and use all available disease management practices. An effective prevention program should include cultural and chemical management practices that will reduce the potential for occurrence, spread and losses from late blight. This is best accomplished by buying disease-free tomato plants and certified disease-free potato seed, planting late blight-resistant varieties, destroying volunteer tomato and potato plants (those growing from last year's planting, cull or compost piles) and regularly using protective fungicides.

The Cornell University Department of Horticulture is recommending that gardeners act quickly to protect their home garden tomato and potato plants and to make sure that their plants don't become a source of spores that could infect other plantings, including those of commercial growers and farmers. If you are growing tomatoes or potatoes in your home garden, you should take the following steps:

Examine tomato and potato plants thoroughly at least once a week for signs of late blight

Spray fungicides preventively and regularly and/or

Be prepared to destroy garden tomato and potato plants should late blight start to become severe.

A July 26th Department of Department of Horticulture statement goes on to say, "If you want to try to control late blight with fungicides, you need to begin spraying fungicide now - even before you see symptoms - and you need to continue spraying regularly. Use a product that contains chlorothalonil. Copper is not very effective on late blight."

According to Vegetable IPM Specialist Dr. Tom Zitter, whose work at the Cornell University Department of Plant Pathology has focused on the epidemiology and management of bacterial,

fungal and viral diseases of fresh market vegetable crops (esp. tomatoes, potatoes and cucurbits) grown in upstate New York, if symptoms are found on any plants in the home garden, it would be prudent to destroy the plants immediately.

Place the entire plant in a plastic bag and dispose of them into the garbage. If plants do not show signs of infection after thorough examination, then there still may be time to apply protectant sprays immediately, repeating if weather conditions warrant (warm temperature with rain or heavy dews). Dr. Zitter states that all tomato varieties (conventional and heirloom) are susceptible, but some varieties, not currently available (like Mountain Magic), do carry genetic resistance for late blight.

The incredibly widespread distribution of late blight-infected tomato plants is unprecedented and just plain mind boggling. I can't think of a better argument for buying locally grown nursery plants, just one more thing that we can do to support the efforts of our region's small, local growers and family-owned garden nurseries and to help local field crops farmers and small fruit and vegetable growers protect their crops and their livelihoods. This situation clearly brings into focus the fact that the choices we make as consumers and the actions we take as citizens can profoundly impact local agriculture today and in the future and, as a result, the very quality of our lives.

When we buy locally, we support our community, our neighbors and our friends. In return, we are rewarded with better quality, fresher food to eat. We can feel good about the food that we are eating and safe in knowing where the food we are eating comes from. And in a world threatened by the possibility of acts of bioterrorism, encouraging consumers to buy locally is being looked at as a quintessential component in increasing the integrity and the security of our nation's food supply.

Please contact Cornell Cooperative Extension of Franklin County to find out how to properly dispose of late blight-infected tomato and potato plants and to find out where you can purchase disease-free tomato plants, disease-free potato seed and fungicide, and for information about proper fungicide use, timing of fungicide applications and how to monitor for, recognize and report late blight infection.

One additional note: After I finished writing this article, I received an email from the NYS IPM Program confirming the discovery of late blight in four potato fields in the Monteregie area, south of Montreal.



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