An Early Invasion of Late Blight on Tomatoes

Early Warnings of Tomato Late Blight Problems in the Northeast

Cornell University was the first to sound the alarm on June 26th that a “very destructive and very infectious disease is killing tomato and potato plants in gardens and commercial farms in the eastern U.S. , “ says Dr. Meg McGrath, Associate Professor, who is a specialist in vegetable diseases from the Department of Plant Pathology and Plant-Microbe Biology. “Home Gardeners need to be on the lookout for late blight,” which she calls “worse than the Bubonic plague for plants.” “Late blight has never occurred this early and this widespread in the U.S.,” she also reports.

Late Blight on Tomatos showing stem lesions

Late Blight on Tomatos showing stem lesions

This early warning announcement was followed on July 3rd, 2009 by widespread news from Associated Press and Newsday, that a tomato disease known as late blight was found in 6 northeast United States big box retail stores (Home Depot, Lowe’s and Wal-Mart). The states that had the first outbreaks were New York, New Hampshire, Maine, Connecticut, Rhode Island and Massachusetts. While late blight is not an uncommon tomato problem, it usually appears in late summer or early fall and is not nearly as widespread.

One major big box tomato plant supplier is Alabama-based, Bonnie Plants where some suspicion has been directed. AP reported that Company representatives doubt that the disease originated in one of their 62 greenhouses (growing stations) since they ship to 38 states in total. Nonetheless, as a precaution the company pulled plants out of stores in the Northeast and this has reported to have resulting in $1 million dollars in lost sales, said Bonnie Plants General Manager Dennis Thomas. Thomas also has said that recent inspections of the company greenhouses in Pennsylvania, New Jersey and West Virginia found no evidence of the blight. A report in the online industry magazine published on July 7th, 2009 said that Bonnie Plants did not knowingly ship any infected tomato plants, but they speculated that the blight could have infected the plants after they arrived at the Northeast retailers. Most of the tomatoes that were sent to the region were shipped in April and May.

The late blight disease, caused by the Phytophthora infestans fungus, was traced to one possible source of shipments from a Bonnie Plants greenhouse in Georgia. As reported in GreenhouseGrower, Thomas said that the suspect greenhouse uses organic methods, which do not include pesticides that can keep the blight at bay. He also said that less than one percent of the company’s tomato shipments this spring came from that greenhouse. Thomas also commented that Bonnie Plants intends to have tomato plants back on store shelves in the Northeast by the end of July when they traditionally ship herbs and cold-weather plants.

Late Blight on Tomato leaf showing fuzzy fungal spores

Late Blight on Tomato leaf showing fuzzy fungal spores

Plants that are kept on large multi-shelf racks in big box stores are particularly susceptible to disease problems because they are placed close together, have restricted air movement and sun exposure. Late blight on tomatoes is much more prevalent when weather conditions are cool and damp – a recent trend in the Northeast. The University of Connecticut Hort Department suggests that cool nights with warm days and high humidity for 24 to 48 hours after a rain where the leaves do not dry support favorable disease outbreaks. For more information visit The weather could probably turn out to be more of a culprit in this case than the commercial grower or retailer.

Late Blight on Tomato stems

Late Blight on Tomato stems

Late blight is the same disease that caused the Irish potato famine in the 1840s. In tomatoes, the early symptoms are brown lesion areas on the main stems, which quickly enlarge. Under moist conditions, a soft rot is accompanied by a grayish-white powder on the underside of leaves and if unchecked it will cause stem collapse, olive-green or browning of large portions of the leaves and a very quick death of the plant within two weeks. Pictures, provided by the Cornell University Long Island Horticultural Research & Extension Center can be found here:

Late Blight on Tomato leaves

Late Blight on Tomato leaves

Late blight is a fast-spreading fungus disease that attacks members of the Solanaceae family – tomatoes, petunias, eggplants, peppers and potatoes. The infecting spores travel on the wind and can spread far and wide. Wild plants in the same family, such as the nightshades, can also get late blight and should be removed from vegetable growing areas even if they do not show signs of the disease.

Home Depot and Lowe’s are offering compensation for late blight infected plants. Contact your retail store to find out their policy. In the meantime, dig up, double bag and seal tightly any suspicious tomato or potato plants. Dispose of them in the garbage — do not compost them. Meg McGrath recommends leaving the bag-enclosed tomato plant to bake in the sun for several hours to kill the plant and fungus spores. Make sure all plant parts are removed from the garden. Do not leave any leaves, ripe fruit or stems behind. Near ripe or unripe tomato fruit that are large enough to be worth saving, and are disease free can be harvested from the plant and eaten. Late blight will not cause any human illness.

Consider planting vegetables other than tomatoes, potatoes or peppers in the same area next year or plant resistant varieties of tomatoes. For other preventative measures consider scouting your vegetable garden on a daily basis, making sure tomatoes are staked to promote air movement, do not over use nitrogen fertilizers, use compost teas – sprayed on all leaf surfaces, and watering only at the base of the plant – not on the leaves.

There is no control for late blight, but early symptomatic plants may be saved through use of fungicides containing Chlorothalonil (non-organic) or copper (organic). Meg McGrath stresses that gardeners who want to try and control late blight by spraying should use repeat applications (according directions on the label).

Late Blight on Tomato fruit showing fuzzy fungi growth and spores

Late Blight on Tomato fruit showing fuzzy fungi growth and spores

Late blight control isn’t a one application situation. Also, take note of the personal protective equipment required to apply the spray. Gardeners in the Northeast who started their own tomatoes from seed are not immune to this disease either. Healthy plants can still be infected by spores on hands, clothing, and tools or wind-blown from neighboring areas (some even think that the spores can travel more than state-wide distances). Meg McGrath also suggests that volunteer tomato weeds in the garden should be vigilantly removed and she suggests following a “no tomato left behind” policy where two buckets are brought to the vegetable garden. One for the harvested fruits and the other for the “rots” so that none are left to drop on the ground (no matter how little they are). Dispose of these “rots” by double bagging them and putting it in the garbage.

Lastly, consider the livelihood of your local commercial tomato grower or CSA farmer and remove any suspicious home garden grown plants immediately to reduce the risk of spreading late blight from your garden to their commercial plants. This outbreak is going to require help from many people (gardeners and commercial growers along most of the east coast) to keep it under control – if we want to eat USA grown tomatoes during the next couple of months.

All images accompanying this article are used with permission from Dr. Meg McGrath and Copyright by the Department of Horticulture Website at Cornell University.

2 comments to An Early Invasion of Late Blight on Tomatoes

  • heyrobertdavis

    Great post. I’ve linked to this from a site I am moderating to collect all kinds of tomato blight resources:

    Thanks for the great post.

  • Teresa

    What a great and informative article. I live in upstate New York, and although I have heard everyone complaining about the tomatoes and the blight, I really didn’t fully understand it until I read your post. The weather here has been unseasonably wet and cool which most definitly has contributed to the poor tomato crops. Mine are a lot less abundant than last year, and they don’t seem as lush and tall either. Now I know what to look for. Thankfully mine have not shown signs of this (yet anyway). THanks again for the great info.

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