Tomato Blight: How to Prevent and Treat Tomato Blight

Tomato blight is a fungal disease that can swiftly destroy an entire tomato crop if left untreated. Dark spots are a telltale sign blight may be in your garden, but the severity of the situation depends on which type of tomato blight is afflicting your plants. It often affects older leaves first, but it can also attack younger, healthy ones, as well as the fruits themselves.

If you notice blight-like lesions on your tomatoes, you’ll want to deduce which type you’re dealing with in order to handle it appropriately. These are the three most common types of tomato blight:

1. 1. Early blight: Caused by Alternaria solani or Alternaria tomatophila, early blight is difficult to deal with, but there’s still a chance you can save your tomato crop. Keep an eye out for brown spots with concentric rings. They can be found all over your plant, including on the tomatoes themselves. Remove and discard any aspect of your plants affected by these spots.
2. 2. Late blight: If your plants look like they’re besieged by mold or mildew, it’s likely late blight, caused by the Phytophthora infestans fungus, has taken over your tomatoes. If you see any of its progenitors, take preventative measures right away to stop any sort of blight from overrunning your garden.
3. 3. Septoria leaf spot: Septoria leaf spot is the least damaging type of blight, caused by the fungus Septoria lycopersici. You can find its tiny brown and black spots on the lower leaves of your tomato plants. If you remove the affected portions, you’re likely to nip the problem in the bud.

As soon as you start noticing blight-infected leaves, it’s time to act quickly. Here are three ways you can mitigate a tomato blight problem after spotting one:

1. 1. Remove infected plant portions. The most essential aspect of treating blight is to remove and destroy any affected area of the tomato plant. If you notice any telltale signs, prune those sections from the plant and discard them far away from your garden. As long as any sections of the plant or soil are still suffering from the fungal disease, the spores are likely to spread again.
2. 2. Use fungicide. Utilizing a fungicide is one key way you can address your blight problem. After removing any infected leaves, you can spray the surrounding area with a copper fungicide or biofungicide to help contain the problem if it hasn’t spread too far.
3. 3. Add mulch to the soil. Using mulch can introduce nutrients into your soil and prevent the spores from spreading further through the air. This makes blight less likely to take root in the first place.

Given how insidious tomato blight can be, taking preventative measures to stop it before it starts affecting your plants is far more effective than treating symptoms after they appear. Here are six tips for keeping your plant material disease-free:

1. 1. Pick reliable tomato varieties. When you plant tomatoes in the first place, make sure they’re resilient and come from a reliable source. Many providers sell resistant varieties of tomato seed that have been bred to withstand blight and other diseases. If you’re using transplants, ensure none of the plant tissue displays signs of blight. The entire plant must be free of blight at the time of planting to prevent a future infestation.
2. 2. Practice crop rotation. Rotate crops often to prevent tomato blight from taking root in your garden. This process helps keep your soil fresh and ready to take on a new set of growers every year, whether you’re using a raised bed of soil, a traditional garden, or multiple pots submerged in the ground. Planting tomatoes in the same plot repeatedly invites blight and other diseases to do their handiwork during the next growing season.
3. 3. Mind the moisture. This fungal disease thrives in wet conditions, so it’s important to keep foliage dry. Try to prevent the tomato leaves from being wet for too long. Water at the base of the plant when possible. This will make it much harder for the blight fungus to survive and spread. One upside to wet weather setting in toward the end of the year is that tomato blight doesn’t overwinter—any hidden spores will die around the same time your plants go dormant. Having a drip irrigation system in place is also useful to regulate moisture.
4. 4. Set sprinklers properly. If you water your garden with sprinklers, program them to stay low to the ground and activate them early in the day. Letting soaker hoses pour more gallons of water onto your garden multiple times of day is a perfect recipe for inviting tomato blight onto your plants. Keeping the lower leaves from getting too wet makes it harder for the pathogen to spread, and giving the water plenty of time to evaporate before night sets in comes in handy, too.
5. 5. Maximize air circulation. Ensuring your tomato plants enjoy good airflow will do a lot to prevent tomato blight in your home garden. Make sure each tomato plant in your vegetable garden has plenty of space around it. This is a good gardening practice overall, as it can help prevent a host of plant diseases.
6. 6. Use the right compost pile. As a general rule, don’t use the plant debris of your tomato crop as compost for the next batch you plant. If they’re suffering from tomato diseases like blight, then you absolutely must avoid composting them as it could lead to fungal spores returning with a vengeance in the growing season next year.

Grow your own garden with Ron Finley, the self-described "Gangster Gardener." Get the MasterClass Annual Membership and learn how to cultivate fresh herbs and vegetables, keep your house plants alive, and use compost to make your community—and the world—a better place.