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September 26, 2018

Cooperative Extension Warns Gardeners: Watch Out For Late Blight on Tomatoes


<p>Late blight visible on a ripe tomato. Photo courtesy Dr. Steve Johnson, University of Maine Cooperative Extension.</p>

Late blight visible on a ripe tomato. Photo courtesy Dr. Steve Johnson, University of Maine Cooperative Extension.

Cornell Cooperative Extension of Oswego County alerts gardeners to be on the lookout for Late Blight. It is a very destructive and very infectious widespread disease that is killing tomato plants.

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

Protectant fungicide products are only effective if used before the disease appears and need to be reapplied every 5 to 7 days if wet weather occurs. If late blight symptoms have appeared on plants in your garden, it is crucial that you remove ALL plant debris as it can harbor spores for future crops. Place plant debris in a dark plastic bag, leave the bag in sunlight for several hours to kill plant and pathogen, and put them in the trash. Don’t put the plants in your compost as spores will spread from the debris. If you remove all tomato parts from your garden, you will be able to plant tomatoes next year. If you do not, consider planting another vegetable such as cucumber. Replacing soil is not necessary. For next year, look for Late Blight tolerant tomato varieties. A list is currently being compiled due to this epidemic.

Cornell Cooperative Extension of Oswego County has a fact sheet available on late blight with color photos of the disease. It you would like a copy, please call 315-963-7286.

Cornell Cooperative Extension is a key outreach system of Cornell University with a strong public mission and an extensive local presence that is responsive to needs in New York communities. The Cornell Cooperative Extension educational system enables people to improve their lives and communities through partnerships that put experience and research knowledge to work.

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One Response “Cooperative Extension Warns Gardeners: Watch Out For Late Blight on Tomatoes”

  1. August 25, 2009 at 3:08 pm

    We have a type of Blight on our tomato plants that is causing the plants to die off early. The fruit seems to be ok and not affected by the Blight.
    What we would like to know, is it ok to can the tomatoes from the affected plants? I have looked at different places on the Web but could not find anything to answer this question.

    Thank you
    E&S Robinson
    [email protected]

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