All of the wet weather we’ve had this summer has led to ideal conditions for disease development. Because of this, many of our tomato plants are looking a little ragged. The most common diseases people have been encountering are early blight and septoria leaf spot. Both of these diseases are caused by fungus. One of the things needed for these diseases to develop is consistently wet leaves, of which we’ve had an abundance.
Early blight primarily infects the foliage of plants, but it can also infect fruit. It first appears on older leaves as irregularly shaped brown spots that have concentric rings (resembling a bulls-eye or target). The spots are often surrounded by yellow tissue. The spots can grow to be ¼ to ½ inch in diameter and will often grow together forming large brown areas. Eventually leaves will drop off of plants.
Septoria leaf spot is also primarily a leaf infection, but will also infect the stems. (It rarely infects fruit.) It forms small (1/16 to 1/8 of an inch) circular spots on leaves. These spots have a tan or light-colored center. Spots will grow to be around ¼ of an inch in diameter. Like early blight, the individual leaf spots will often coalesce, forming large areas of diseased tissue. Heavily infected leaves will turn yellow and fall off of the plant. Unlike early blight, it does not form concentric rings. But, if you look closely (may need to use a magnifying glass), you can often see small black pimple-like fruiting bodies in the center of the leaf spots.
There are a few things you can do for the remainder of this growing season to manage these diseases. First, remove and destroy any diseased foliage. Make sure leaves are dry when handling plants. If leaves are wet, you may end up spreading the disease. Fungicides can also be applied to plants. This will not get rid of the disease on infected leaves, but will protect healthy leaves from infection. When using pesticides, make sure to follow all label directions.
There are several different things you can do to manage these diseases in your garden for next year. Ideally you would not grow any solanaceous (tomato family) plants in the area for 3-4 years. (The tomato family includes peppers, eggplants and potatoes.) Early blight can survive in the soil, seed or infected plant debris for a year. While Septoria can survive for up to three years on infected plant debris as well as weedy hosts, it does not survive in the soil on its own like early blight. Because both of these diseases can survive on plant debris, it is important to remove any diseased plant tissues from your garden. Next year, if you are growing tomatoes from seed, look for disease-free seed. If you are purchasing transplants, inspect plants for any leaf spots before purchasing. Regardless of whether you are growing from seed or transplants, look for disease resistant varieties. Make sure to properly space your tomatoes. Adequate spacing will allow airflow between plants and it will allow them to dry out faster. It is also a good idea to stake or cage your tomatoes. This helps speed up drying of plants and keeps them off of the ground. Using mulch on the ground also creates a barrier between the soil, where disease spores may be, and the plants. Finally, avoid watering late in the day and try not to get the foliage wet. The longer the leaves remain wet, the greater the chance disease will develop.