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Will late blight be a problem again in 2010?

May 5, 2010
By RICHARD GAST, Franklin County Cornell Cooperative Extension,

Last year was a sad year for tomato growers. For a while, it seemed like every other call I received at the Extension was from a gardener distraught about finding his or her once-healthy tomato plants dying in just a matter of days.

The cause of this devastation was late blight, a disease caused by the pathogen phytopthera infestans, which strikes potatoes as well as tomatoes, causing annual yield losses of around 20 percent in tomato and potato crops globally. It is, arguably, the most destructive of plant diseases.

Phytopthera is so quick and so agile that it can cause enormous damage within a very short space of time, especially during the damp weather conditions it favors. It is so versatile that not only has it so far survived every strategy used to control it, it has responded by evolving newly adapted strains.

Article Photos

Symptoms of late blight are shown on the leaves and stems of this tomato plant.
(Photo provided)

Technically, phytopthera infestans is an "oomycete," a classification that did not exist until recently. Oomycete means "egg fungus." Think of it as a fungus whose spores are dispersed by wind.

Fungi are unable to produce nutrients on their own. To survive, they must obtain their food from other organisms. Think of them as parasites that can easily and quickly consume a host. Under hospitable conditions, late blight spores will attack all parts of a host plant and damage translocation tissue, the tissue that allows water and sugar to move from one part of the plant to another, killing the plant in a relatively short period of time.

The ease with which late blight can spread from one home garden to another - from a home garden to a farmer's fields or from farm to farm, even over great distances - cannot be overstated. This was well documented in 2009. Because of this, late blight really needs to be thought of, and treated as, a community disease. And anyone growing susceptible plants needs to take responsibility for implementing a suitable management program. Growers must learn to recognize the symptoms and to understand and use all available disease management practices.

Late blight cannot overwinter through the extremely cold temperatures of northern New York. And for that reason, it is seldom a major problem for home gardeners in our part of the country. It does appear, but only sporadically.

Early infection from late blight is especially uncommon. The occurrence of late blight in 2009 was the earliest ever that the disease had been reported in the Northeast. And enormously favorable environmental conditions persisted through June and July and continued into August. The disease became explosive due to the quantity of spores produced and the rapid disease cycle.

The good news is, another year like 2009 is not expected. And with a little bit of luck, we won't have a relentlessly rainy early summer again this year.


What happened in 2009?

Last year, late-blight-infected tomato transplants were found being distributed at big-box-store home-and-garden centers in numerous locations across an area that extended from Maine to Maryland to Ohio. Such incredibly widespread distribution of late-blight-infected tomato plants was, until then, unprecedented.

Personally, I can't think of a better argument for buying locally grown nursery plants, which also supports the efforts of our region's small, local growers and family-owned garden nurseries, and helps local field-crops farmers and small fruit and vegetable growers protect their crops and their livelihoods.

I believe that the epidemic of late blight contamination of 2009 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.

Currently, the late blight pathogen is only known to be able to survive on living host-plant tissue. It is an obligate pathogen, meaning it cannot survive between crops on infested debris, unlike the early blight pathogen, which can. Late blight is also incapable of surviving in the soil.

This means that gardeners growing tomatoes do not need to rotate away from the planting area they used in 2009, specifically for late blight control. Nonetheless, if you have the space, crop rotation is not a bad idea.

Almost any experienced gardener will tell you that growing a crop in the same area for several years can, and often does, lead to increased disease problems. Crop rotation, which is the practice of not planting the same crop, or any other crop from the same family of plants, in the same location more than once every three years, allows you to naturally interrupt the life cycle of soil-borne disease organisms and soil insect pests in annual plantings. As a result, soil-borne insects and disease have difficulty becoming established. Crop rotation also helps maintain a balance of essential mineral elements in the soil.

It's important, too, to note that the pathogen that causes late blight cannot survive the winter in or on your tomato seeds. So, it's OK to start seedlings from tomato seeds that you may have saved.

Potatoes, however, are another matter. Stored potatoes simply should not be planted. And of extreme importance is the fact that potatoes, left unharvested in the soil or in compost piles, often do survive the winter and appear as volunteer plants the following season. Those surviving tubers are living tissue - just what phytopthera infestans needs to survive the winter. If infected tubors are allowed to sprout and grow, the over-wintered pathogen will produce spores on the new foliage.

Thus, it is critical for managing late blight successfully this year that any leftover potatoes and any volunteer potato plants be destroyed as soon as they emerge. Do not wait until symptoms appear and new spores develop.


Recognizing late blight

If tomato plants become infected, they develop irregular, greenish, water-soaked spots on the leaves. These lesions are typically found first on the younger, more succulent leaves in the top portion of the plant. Under cool, moist conditions, the spots rapidly enlarge to form purplish black lesions on the leaves and stems. White, cottony growth may be visible on the underside of affected leaves. As the disease progresses, lesions enlarge, causing leaves to brown, shrivel and die.

Late blight can also attack tomato fruit in all stages of development. Infected fruit are typically firm, with greasy spots that eventually become leathery and chocolate brown in color. These spots can enlarge to the point of encompassing the entire fruit.

Established lesions cannot be controlled, even with the most effective systemic fungicides available - those used only by farmers who are licensed pesticide applicators. Therefore, infected plants should be promptly destroyed to prevent spread to nearby gardens, fields or farms.

Growers can attempt to salvage apparently uninfected tomato fruit but should be aware that some fruit infections will not become visible for several days.

Keep in mind, too, that discovering spots on your tomato plants does not necessarily mean your plants are infected with late blight. Early blight and septoria leaf spot are two extremely common fungal disease affecting tomatoes.

For additional information, contact your local Cornell Cooperative Extension Association office or e-mail me:



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