Denise's Garden Guide: Preventing and Treating Late Blight in Tomatoes

Late Blight is a tomato disease caused by a fungus that tends to appear late in the planting season when temperatures and humidity are high. Late blight is not only found in tomato plants, but also can be found in potatoes. Therefore, once it is identified it is important to check both types of plants for the condition.

Identified initially as a white mold growing on the lower surfaces of the plant, Late Blight can affect both mature plants and seedlings. The fungus spreads, causing blue-gray water soaked patches on the leaves and stems which will move to the fruit as well. These blue-gray patches will expand, causing leaves to shrivel and brown cork-like spots with a series of brown rings to appear on the fruit.

What can I do to control my Late Blight problem?

Because the fungus causes fruit to be inedible and causes the plants to die (you will be able to identify dying plants by their unpleasant odor), it is important to work to control Late Blight early. Several treatments are available to you, depending on your preference to use chemical sprays or to focus on more natural approaches.

The easiest "off the shelf" treatment is Ortho Garden Disease Control which can be easily ordered online or picked up at your favorite home improvement store's garden center. Using the product at the first sign of Late Blight and then repeating treatment every 7 to 10 days until dryer, less humid weather arrives will keep the disease from spreading to other plants.

One alternative option is to spray with Neem Oil, available at nurseries and some pet stores. Safe for animals and children, spray in much the same way that you would the Ortho Garden Disease Control until weather dries and the spread of the fungus stops.

Once treatment has been followed, remove and destroy any infected plants and mark the places where the plants were this season so that you can leave those places empty in your next planting.

Tip: Because the spores that cause Late Blight are quick to spread and hard to destroy, make sure that any infected plants are removed and destroyed and no part of the plant makes it into your compost bin. Once in compost, the spores will spread unhindered and will cause future plantings to become infected.

Want to learn more about Late Blight and other tomato plant diseases? Information on Late Blight for this article was found not only in Denise's garden, but also in the books The Vegetable Gardener's Bible by Edward C. Smith and Home Gardener's Problem Solver by Ortho.