Prevent Late Blight From Attacking Your Tomatoes

This scourge of organic growers everywhere can be addressed without chemicals.

July 28, 2017
late blight on tomatoes

Late blight, an increasingly common killer of tomato and potato crops, elicits dread in gardeners throughout the United States. This fungus-like pathogen has a lethal reputation, having contributed to the Irish potato famine of the mid-1800s. Today, late blight doesn’t cause such drastic public-health disasters, but farmers and consumers feel the economic pinch when it strikes.

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The fast-moving disease spreads through wind-dispersed spores, potentially sweeping across entire states in a single day if conditions—cool, damp, overcast, and windy—are right. Conventional growers rely on preventive sprays of chemical fungicides to keep the disease from setting in, but organic gardeners and farmers have limited options. Several copper-based products have been approved for use in organic agriculture; but copper, used often, can build up to unhealthy levels in soil, and it threatens beneficial insects. A better approach is to breed tomatoes that won’t succumb to the disease.

Related: 6 Tomato Growing Mistakes You're Making

While breeding potatoes resistant to late blight is a difficult task, one that has scientists using genetic-modification methods, it appears tomatoes won’t have to go down that controversial route to gain resistance. “I don’t personally see a need for genetic modification for late-blight resistance in tomatoes,” says Roger Chetelat, Ph.D., geneticist and director of the C.M. Rick Tomato Genetics Resource Center at the University of California, Davis. Traditional plant breeders create resilient varieties by seeking and incorporating traits from wild tomato relatives. In fact, virtually all of the disease resistance in modern tomato cultivars, including resistance to fusarium and verticillium wilts, tomato mosaic virus, root-knot nematodes, and bacterial speck, was initially acquired from wild species native to South America.

The quest for late-blight resistance focuses on wild growing tomatoes in river valleys high on the western slopes of the Andes Mountains in Peru and Ecuador. “Moderate rainfall there means the leaves are exposed to conditions that favor late-blight development,” Chetelat says. “Transferring a new trait typically takes many years of breeding.”


Related: Keep Your Plants Healthy By Understanding Diseases And Disorders

As hybridizers strive to reduce the impact of late blight, gardeners can pursue these disease-avoidance strategies:

Go For Resistance

Choose tomato varieties bred for late-blight resistance. ‘Defiant’, a slicer, and ‘Jasper’, a newer grape tomato, are available from Johnny’s Selected Seeds. Hybrids ‘Mountain Magic’, ‘Plum Regal’, and ‘Mountain Merit’ are also said to display resistance. During a widespread late-blight outbreak in 2009, cherry tomato types, including ‘Matt’s Wild Cherry’ and ‘Sun Gold’, were less impacted than other tomato varieties.

Related: The 8 Healthiest Nutrient-Packed Vegetables To Grow In Your Garden

Avoid Carryover

Start your own tomato seedlings or purchase from a grower that is not located near potato fields. Late blight may overwinter in potatoes left behind in fields or compost piles and spread once they sprout in spring. Remove volunteer potato plants that pop up in your garden, even if they don’t look diseased.

Recognize Symptoms Early

Late blight is among the most contagious and destructive of plant diseases. “The earlier a diseased plant is spotted and removed, the less likely the pathogen will spread to other gardens and farms,” says Meg McGrath, Ph.D., a plant pathologist at Cornell University in New York. Routinely scrutinize your garden for symptoms of late blight, which include dark lesions on the stems and brown spots on leaves, accompanied by fuzzy white fungal growth during humid weather. To be sure, take a sample to your local extension office for testing.

Related: 9 Tips From Professional Organic Farmers That Will Work Wonders In Your Garden


Respond Quickly

If late blight strikes your garden, immediately remove infected tomato and potato plants, double-bag them, and set the bags on the curb for garbage pickup. Tell neighboring gardeners and farmers and report the occurrence to, an online tracking resource.


If your garden’s tomato crop is wiped out, replant the space with crops that aren’t affected by the disease: green beans, fall brassicas, root vegetables, or salad greens. Or sow a soil-nourishing cover crop.