Blight on Tomatoes

Steps to prevent the heartbreak of tomato blight

By: Sandy Feather ©2010
Penn State Extension

Q. All of my tomatoes died from the late blight that was so prevalent last year. I removed the plants and cleaned up as well as possible, but is there anything else I can do now to avoid a repeat of last year’s disaster? Should I treat my soil with something?

A. Last year’s late blight epidemic resulted from a perfect storm of environmental factors and a high level of infectious late blight spores in the air. Apparently, infected transplants were unknowingly sold by a mega-grower in the South to big box stores and independent garden centers throughout the Northeast. These transplants were sold and planted in home gardens throughout, and favorable weather conditions made the disease run rampant.

Even commercial growers who tend to make preventative fungicide applications when conditions favor late blight development may have been caught off guard since it arrived so early in the growing season.

late blight on a tomato
Ripened tomato affected by late blight

The fungus that causes late blight, Phytophthora infestans, requires live tissue to overwinter in our climate. The only way it survives winter in western Pennsylvania is on infected potato tubers that have been overlooked at harvest or in commercial potato cull piles. It does not survive on dead tomato plants or related weeds common in area, such as climbing nightshade (Solanum dulcamara) or American black nightshade (Solanum americanum). The fungus does survive the winter in the southern United States and gets blown to the north on summer winds. Spores can easily move 40 miles in a day when weather conditions are favorable.



Plant pathologists refer to device called the plant disease triangle to explain disease development. Three things are needed for disease to develop: a susceptible host (tomato or potato), the pathogen (Phytophthora infestans) and a favorable environment (moderately warm, humid, wet weather). Disease control requires interfering with at least one leg of the triangle. Acting on the susceptible plant would require a tomato resistant to late blight. Although some varieties tout resistance to late blight, none are considered reliably resistant, especially when conditions are as favorable as they were last summer. Although you cannot stop the rain or make the sun shine, you can provide optimum growing conditions by testing the soil and amending it according to the results, watering properly and controlling weeds to permit greater air circulation. These are ways to change the environment to favor the plants more than the disease to some degree. Acting on the pathogen includes good garden sanitation because it removes a lot of spores and reduces their number in the garden to cause disease in the future. Applying a fungicide is another way to act on the pathogen.


It is good that you cleaned out your garden thoroughly last fall. There are other common tomato diseases such as septoria leaf spot and early blight that DO overwinter on diseased plant material left in the garden. If you also grow potatoes, be sure to promptly pull any overlooked tubers that sprout this spring or through the summer. Destroy them or send them out with the trash, but do not compost them.


Avoid getting the foliage wet when you water as much as possible. Soaker hoses and drip irrigation are ideal because they put the water on the soil - where you want it - and keep it off the foliage – where you do not want it. If those are not an option, at least use a watering wand and keep it as is close the soil as possible without actually sticking the head into it. If you use a watering wand, try to water early in the morning so the foliage dries quickly when the sun comes up. That is less critical when using drip or soaker irrigation since it does not wet the foliage to start with. True, plants get wet when it rains or when there is heavy dew, but you do not have to add to the problem by soaking the foliage when you water. The longer tomato and potato foliage stays wet, the greater the chance of disease, including but not limited to late blight.
Mulching around the base of plants reduces the number of spores that can be splashed up onto plants by rainfall or watering. While this is not important for late blight control, it can help with early blight and septoria leaf spot.  Organic mulches such as clean oat straw or leaf mold have the added benefit of adding organic matter to the soil as they break down. Plastic mulches – especially red plastic mulch – have been shown to increase tomato yields, and they also function to keep infectious spores from being splashed onto susceptible foliage.

late blight on tomato leaves
Foliar symptoms on tomatoes

There is nothing you can apply to your soil that would keep your tomatoes from getting late blight later in the summer. Unless there are some overlooked potatoes there, the fungus is no longer present. It will be blown north on wind currents later in the season. Since the fungicides available to home gardeners are best applied preventatively – before symptoms develop – it is wise to spray when weather conditions favor late blight development. You can make preventative fungicide applications when cloudy, wet, and moderately warm weather conditions favor disease development. Follow label directions for mixing rates and application frequency. You may be able to turn things around if you catch late blight in the very earliest stage of development and begin spraying, but you cannot “cure” a plant that is even moderately infected. Even though commercial growers have more fungicides to choose from, many of those products cannot save a moderately infected crop.


Conventional gardeners can use Bonide Fung-onil, Ortho Garden Disease Control, and other chlorothalonil products labeled to control late blight on tomatoes and potatoes. Organic gardeners can use fixed copper fungicides, but they are not considered as effective against late blight as chlorothalonil. Always read and follow label instructions.


Penn State soil test kits are available for $12 each. If you purchase more than one kit at a time, additional kits are $9 each. Please make checks payable to Penn State Cooperative Extension (PSCE) and indicate the number of kits you would like. Send your request to Soil Test Kits, Penn State Cooperative Extension, 400 N. Lexington Street, Pittsburgh, PA 15208.


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