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Late Blight Information

Home Gardeners -- Check your garden for Late Blight on tomatoes and potatoes!

By: Lutie C. Batt, Extension Educator

Home gardeners need to be on the lookout for Late Blight - a very destructive and very infectious disease that's killing tomato and potato plants in gardens and on commercial farms in the eastern U.S.

Late blight is the same disease that caused the Irish Potato Famine in the 1840s. It has never occurred this early and this widespread in the U.S. One of the most visible early symptoms of the disease is brown spots (lesions) on stems. They begin small and firm, and then quickly enlarge, with white fungal growth developing under moist conditions that leads to a soft rot collapsing the stem.

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). Sometimes the border of the spot is yellow or has a water-soaked appearance. Spots begin tiny, irregularly shaped and brown. Firm, brown spots develop on tomato fruit.

You need to act quickly to protect your garden-grown tomato and potato plants and to make sure that your plants don't become a source of spores that could infect commercial farms, as late blight spores are easily dispersed by wind.

Here are the steps you should take:

  1. Examine your tomato and potato plants thoroughly at least once a week for signs of late blight.
  2. Spray fungicides preventively and regularly and/or
  3. Be prepared to destroy your plants when late blight starts to become severe. Seal them in a plastic bag and put in the trash. Do not put them in the compost pile, do not burn them.

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. Even here, these products are only effective if used before the disease appears and should be reapplied every 5-7 days if wet weather persists. Chlorothalonil is a protectant fungicide, with no systemic movement in the plant, so thorough coverage is necessary. For organic growers and homeowners, the options are very limited, since only copper fungicides can be used, and they are not very effective. Commercial growers have a number of fungicides that if applied early and often, can reduce the spread of Late Blight. They would choose not to spray if they could, but this destructive disease does not give them any other option. Even with fungicide applied every week, there is no guarantee of success, especially if the rainy weather continues. And will you even get ripe tomatoes with a cool wet summer and short growing season?

One source of late blight in New York has been traced to tomato plants imported to garden centers from production facilities in the south. If you started your own tomatoes from seed or bought locally grown plants, they are unlikely to be infected, at least initially. If you purchased your plants at a garden center and they show signs of late blight, please contact your local office of Cornell Cooperative Extension at 786-2251.


Late Blight Food Safety Statement

From Meg McGrath, Assoc. Professor in Plant Pathology, 9/8/2009
http://blogs.cce.cornell.edu/community-horticulture/2009/09/08/late-blight-food-safety-statement/

Are the unaffected parts of blighted tomatoes and potatoes safe to eat?

Yes the unaffected parts probably are safe to eat. Parts with symptoms likely do not pose a health risk when consumed either, but they do not look appetizing and will have an off flavor. However, no published scientific study on this specific issue was found to confirm this conclusion, therefore consumers need to make their own decision on food safety. The conclusion that unaffected tissue is safe to consume is based on several points. This pathogen does not produce a toxin that can make people sick, as a few plant pathogens can do. Plant pathogens cannot infect people. No food safety issues have been found with other diseases that affect tomato fruit or potato tubers. Late blight appears to be like other more common diseases, e.g. anthracnose on tomato fruit and pink rot of potato (which incidentally is caused by/ Phytophthora erythroseptica/, a pathogen related to that causing late blight), in that these do not appear to affect plant tissue beyond the area of infection. Many home gardeners likely often cut off diseased tissue rather than throw out the entire fruit or tuber having found the healthy appearing part of these to taste fine. This has not been associated with any human health issues. Diseases like late blight and anthracnose are not considered a health concern for commercial tomato processing. Fruit are sorted to remove affected ones, but this is because of the impact on fruit quality. For home canning, only disease-free, preferably vine-ripened, firm tomatoes are recommended in the USDA Complete Guide to Home Canning because fungal pathogens may raise tissue pH and thereby allow growth of potentially harmful microorganisms.

CCE's Food Preservation Expert Judy Price echos the caution to only can food that is of the highest quality. The possibility of a dangerous product is great when inferior and questionable food is placed in an anaerobic environment. It is also recommended that only high quality food be frozen, although the same dangers do not exist. Rather, the quality issues of an off flavor and unappetizing product should be considered in freezing tomatoes affected by blight.

 

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