Questions about what disease might be affecting your potato tubers or tomato fruit? For plant disease diagnosis, please contact the UMass Plant Disease Diagnostic Lab online, or at

(413) 545-3209.

What does late blight look like during the growing season?

Symptoms on tomato early in the growing season

Late blight canker on tomato leaf and stem

The most common symptoms on tomatoes are sunken, dark green or brown lesions on leaves and brown lesions on stems, with white fungal growth developing under moist conditions. (See images above.)

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 lesion border is yellow or has a water-soaked appearance. Leaf lesions begin as tiny, irregularly shaped brown spots. Brown to blackish lesions also develop on upper stems. Firm, brown spots develop on tomato fruit.

If the lesion has a yellow border and is occurring on the bottom of the plant, it is likely due to infection of either early blight or Septoria leaf spot, two common diseases found in home gardens

Recognizing Tomato Blights

Online photo galleries of late blight symptoms

tomato late blight symptoms on leaves and fruit

potato late blight symptoms

Late blight identification and biology. Abby Seaman, Cornell Cooperative Extension

Photographs of late blight symptoms on tomato and potato, as well as other foliar diseases, courtesty of Cornell Cooperative Extension. Presented at the NOFA Conference forum on late blight, Augut 2009.

What about other leaf blights?

The "other tomato blights'" -- early blight, Septoria leaf spot, Fulvia leaf mold.

Printed in Vegetable Notes August 6, 2009.

early blight of tomato

What happened in 2009?

Late Blight caused by Phytophthora infestans – a very destructive and very infectious disease killed tomato and potato plants in gardens and on commercial farms throughout the eastern U.S. during 2009. As a result, many farmers across Massachusetts lost their tomato crops and incurred extra fungicide and labor expenses. While late blight occurs at some locations in the Northeast each year, the occurrence of late blight in 2009 was different compared to most seasons. In 2009 infected plants were distributed through large local retail stores throughout the region (Ohio to Maine) during June, and outbreaks were reported over this entire region by early July. Never before had such an extensive distribution of infected plants occurred, especially so early in the season. This, combined with the cool, wet growing season and the exceptionally contagious nature of the disease during cool, rainy, windy weather all contributed to a disastrous year for farmers.

Does late blight survive the winter?

The biggest threat for overwintered disease in New England is on potatoes. The fungus Phytophthora infestans needs live tissue to survive. Potato tubers that are infected with late blight and don’t freeze or decay during the winter can carry the pathogen over the winter to next spring. Tubers can survive in several ways:
    -Left in the ground at harvest, down several inches in the soil.
    -Disposed of in a compost pile that does not fully decompose and does not freeze.
    -Disposed of in a large pile of culled potatoes which does not freeze completely.
    -Kept in storage until late winter, and then put outside in spring.
    -Purchased for home use, and then disposed of (in compost or cull pile, as above)
Potatoes that freeze or fully decompose will not carry the pathogen overwinter.

Tomatoes wll not carry late blight over the winter, because freezing kills the whole plant. Tomato seed, even from fruit that was infected with late blight, will not ciarry the pathogen. Thus you can use your own seed or purchase seed to start next year’s crop without fear of late blight. Certain perennial weeds can become infected with late blight, but none of their aboveground tissues live through the winter. Greenhouses where tomatoes were grown could allow survival only if they never freeze and the crop lives all winter. Late blight will not survive on tomato stakes and cages.
In some parts of the world, late blight has two ‘mating types’ (the fungal equivalent of male and female) which can produce long-lasting ‘oospores’ that survive independently. So far, only one mating type has been found in the Northeast so we do not expect oospores to be present.

What to do this spring.

Tomatoes: Tomato plants started from seed locally (in the Northeast) will be free of the disease. Growing your own transplants from seed or purchasing from a reputable local grower will ensure a healthy start to the season for your customers and local farms. Disease-resistant or tolerant varieties of tomatoes exist, however seed is in limited supply this year. ‘Mountain Magic’, ‘Plum Regal’, and ‘Legend’ are three varieties with resistance or tolerance to late blight. Note that the variety ‘Legend’ is the only late-blight resistant variety for which seed is readily available this year. In addition to late blight, each year tomatoes become infected with early blight and Septoria leaf spot, which look very similar. If possible, also provide tomato plant varieties that are resistant or tolerant to early blight for your customers, such as the varieties ‘Mountain Fresh’, ‘Mountain Supreme’, and ‘Plum Dandy’ and others.

Potatoes: Purchase certified disease-free potato seed from a reputable source, and ask your supplier about their source of seed and if it was inspected in the field for late blight. During the spring (April – June), inspect last year’s potato plot and any compost or cull piles for volunteer potato plants that might come up. If you find potato plants, pull them out and put them in the trash or destroy them. If tubers were infected and survive, then the late blight could grow upward from the tuber, infecting the stem and producing spores when weather conditions are favorable. These spores could then disperse to other tomato and potato plants.

For both potatoes and tomatoes: provide good soil fertility, water drainage, air circulation, and use cultural practices to provide what the crop needs for healthy growth.

If you purchase plants to sell, inspect all transplants for stem, petiole cankers or leaf blight as long as plants are on the shelf. If you are a garden retailer or farmer, teach your staff what to look for, using the web-links below. If you suspect a late blight infection, use the Plant Diagnostic Lab to confirm if late blight is present. Again, purchasing from a reputable local grower is the best way to ensure that you are starting the season with disease-free transplants.

During the growing season, pay attention to weather conditions and pest alerts to learn about whether late blight has been observed in New England, and what actions you need to take to protect your crop.