Deadly disease compounds tomato growersí woes

August 31, 2015

Bill Marting looks over some of his cherry tomato plants in his backyard in Akron, Ohio, on August 21, 2015. His plants, that usually produce an abundant crop, have only been able to produce a handful of tomatoes this summer.

AKRON, Ohio ó It has been a tough couple of years for tomatoes.

Countless gardeners in Ohio and elsewhere have struggled with sickly tomato plants the last two summers, the unfortunate fallout of a perfect storm of weather conditions. And just as the fruit that managed to survive is reaching its luscious peak, another tomato killer is poised to strike.

Itís late blight of tomato, an especially destructive disease that spreads with abandon and can wipe out a whole field of tomato plants in a matter of days. It has been identified in one Ohio county and could spread fast and far, said Erik Draper, an assistant professor with the Ohio State University Extension and its director of commercial horticulture for Geauga.

"When it comes in this early, this is not a good thing. We will lose most of our tomatoes to this disease," he predicted.

Tomato late blight spreads in cool, wet conditions, exactly what we see this time of year when nighttime temperatures drop into the 60s and heavy dew forms, he said.

The disease used to be easier to control, he said, but in recent years it has run rampant. Thatís partly because conditions have been right for the disease to spread, but Draper believes itís also because the fungus-like organism that causes the disease, called an oomycete, has developed more aggressive strains.

Draper visited a commercial growerís tomato field in Geauga County recently and found a section in the middle with infected plants. When he returned a week later, "from one end of the field to the other, it was gone. All the tomatoes were gone," he said.

Typical signs of late blight are black or brown spots on stems. Large spots on leaves that look water-soaked at first and then turn brown, often with a border of light green, wilted tissue. Fuzzy growth on underside of leaf spots. Large, brown, firm spots on fruits.

Unfortunately most fungicides available for home use arenít effective at protecting tomato plants against late blight, because the culprit isnít a fungus, Draper said.

Thatís tough news for gardeners, many of whom have already been frustrated by tomato tribulations the past couple of summers.

This year and last, cool temperatures early in the season delayed planting and slowed early growth. So when the usual disease-causing organisms started moving in, the tomato plants were still young and vulnerable, Draper explained. Adding to the problem were heavy rains, which sometimes wounded plants and gave bacteria a way in.

West Akron gardener Bill Marting has managed to harvest only a handful of cherry tomatoes from his dozen or so plants, and there are few clusters of tiny tomatoes waiting to mature. Usually by early September, heís harvesting tomatoes by the bucketful, he said.

He blamed the heavy June rains, which turned his foliage yellow. Although the plants have bounced back and heís seeing some blossoms, heís resigned to a disappointing harvest.

"It was a great year for weeds and mosquitoes," he said wryly.

One disease that has given many gardeners grief this year is early blight of tomato, Draper said. Despite its name, early blight can strike throughout the season and is caused by a fungus, a different organism than the one that causes late blight.

Bacterial diseases called bacterial speck, bacterial spot and bacterial canker have also been common, he said. Septoria leaf spot, another fungal disease, has damaged some leaves and stems.

No matter whatís troubling their tomatoes, gardeners can improve their chances for next year by cleaning up plant debris thoroughly at the end of the season, Draper said. That way, thereís less chance disease-causing organisms will hang around in the soil, waiting to re-infect plants next year.

Throw away diseased plants rather than composting them, he said. Most home composting piles donít get hot enough to destroy the disease-causing organisms, especially in cold weather.

And watch for new plants that sprout from the seeds of tomatoes left on the ground. Remove and destroy them, Draper said.

Planting disease-resistant tomatoes can also help. Cornell University has a chart at, showing which varieties resist which diseases.

Still, Draper said planting the right tomatoes is no guarantee against disease. Just as a healthy person can get sick after sharing a plane with a sneezing seat mate, a resistant plant can become infected if the conditions are right.

Sometimes, the bad guys just win.


About late blight Ķ

How to prevent it: Spray healthy-looking plants with a fungicide containing chlorothalanil as the active ingredient. Fungicides containing copper are less effective but may help.

What to do if it hits: Remove diseased plants and debris and discard in a plastic bag. Do the same with new plants that may sprout. Donít compost them.

More information:



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