Tomatoes are one of the most commonly grown vegetables in home gardens. While tomatoes are relatively easy to grow there are a few diseases you should keep your eye out for. Two of the most common diseases people encounter are early blight and Septoria leaf spot. Both of these diseases are caused by fungi. Consistently wet conditions are required for both of these diseases to develop, which we've had plenty of.
Early blight (Alternaria solani) primarily infects the foliage of plants, but it can also infect the stem and fruit. It appears as irregularly shaped brown spots that have concentric rings (resembles a bulls-eye or target), commonly on older leaves. The spots are often surrounded by yellow tissue. The spot can grow to be ¼ to ½ inch in diameter and will often grow together (coalesce) forming large brown areas. Eventually, leaves will drop off of plants.
Septoria leaf spot (Septoria lycopersici) is also primarily a leaf infection, but will also infect the stems (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 with dark purple or brown margins. 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 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 read and follow all label directions. Contact your local extension office to get a list of recommended chemicals.
There are also several different things you can do to manage these diseases in your garden for next year. Make sure you are practicing crop rotation. Ideally, you wouldn't grow any solanaceous (tomato family; includes peppers, eggplants, and potatoes) plants in the same area (this is often easier said than done in backyard gardens). Early blight can survive in the soil, on seeds 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. Because both of these diseases can survive on plant debris, it is important to remove any diseased plant tissues from your garden. Controlling susceptible weeds, like nightshade, and volunteer tomato plants that can act as a source of infection are also important parts of keeping your garden clean.
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 the 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.
Good Growing Tip of the Week: Most home gardener's stake or cage individual tomato plants. If you're growing numerous plants it may be worthwhile to trellis them. Two commonly used trellising techniques are the Florida (basket) weave and the t-post string trellis.
Source: Ken Johnson, Extension Educator, Horticulture, firstname.lastname@example.org