How to Prevent Late Blight on Tomatoes

Plant a diversity of tomato varieties to reduce the chance of disease.

Late blight, a disease that strikes tomatoes and potatoes, can quickly ruin an entire crop — and provide a source of infection for all the healthy plants in the area. It's important for gardeners to be aware of this disease so they can act quickly. Unfortunately, there's no silver bullet for control. The key is to be alert, be prepared.

Choose the right variety: Before deciding which tomato varieties to plant this season, consider choosing one that's shown some resistance to the fungus (Phytophthora infestans), which causes late blight. Though no varieties are immune to late blight, the ones on the list below stayed healthier than others. If you can, start your own plants from seed or buy transplants from a trusted local source. You might also want to plant some varieties that mature early, such as Early Girl, so if late blight does strike, you may still get a harvest.

Prevent overwintering: Fortunately, the fungus that causes late blight is unlikely to survive the winter, especially if the plants were properly destroyed. The disease needs living tissue to survive, so it can't overwinter on tomato cages or supports. However, infected potatoes (the other plant that gets late blight) can carry the disease through the winter. Be sure to destroy any volunteer potato plants that come up. If you plant potatoes again, be sure to buy seed potatoes that are certified as disease-free.

Give plants space: If possible, avoid planting tomatoes and potatoes where you had them last year. Be sure to give plants plenty of space, based on recommendations for the variety. Maximizing airflow and light around the plants will help them resist disease. Make use of trellises and supports that will keep the vines off the ground.

Avoid watering from above: Using soaker hoses or drip irrigation keep foliage dry, which makes it more difficult for late blight — and other diseases to spread. Avoid overhead watering techniques (sprinklers). Water early in the day so the foliage can dry before nightfall.

Late blight on a tomato leaf

Be alert: Learn to recognize the weather conditions that foster the spread of late blight. The disease spreads rapidly in cool wet weather, whereas dry weather tends to hold back the disease. Your local cooperative extension is probably monitoring disease conditions for home gardeners as well as farmers, so they may be a good source of information. Stay in touch with gardeners in your area so you'll know right away if late blight is near.

Spray preventively, if necessary: If the weather forecast calls for cool, wet weather, you might want to begin preventative spraying. The key word is preventively. Once plants are infected with late blight, it's too late to save them. Organic farmers and gardeners have had the most success with copper sprays (such as Garden Dust), and beneficial bacteria (Bacillus subtilis), as found in Serenade Garden Disease Control. Before using any of these sprays, read the label and use them accordingly. In most cases, effective protection requires that plants be sprayed as often as weekly throughout the growing season. Remember that these sprays do not prevent the disease, but they can slow its progress.

Know it when you see it: Make sure you know what late blight looks like. Two other types of blight, early blight and septoria leaf spot, are similar but unlikely to kill your plant.

If your plants succumb to late blight, take action immediately. Pull up the plants and either seal them tightly in a trash bag, or place them under black plastic, where the sun's heat can kill the spores. Do not compost blight-infected plants. If left unattended, the disease will spread quickly from your plants to those of your neighbors and local farmers. Please, garden responsibly!