Black spots on tomato plant leaves are a sign of a fungus disease. Left untreated, the leaves will worsen and fall. The fungus will spread to other leaves and possibly to stems and blooms. The conclusion will be a sickly plant with few leaves left. Tomato production will dwindle and those that survive will be subject to sun scald without enough leaves to protect them.
Diseases That Cause Black Spots
There are two primary fungus diseases that can produce black spots on tomato plant leaves: Septoria leaf spot and Alternaria leaf spot, also known as early blight. Septoria and Alternaria differ slightly in appearance and prefer slightly different weather climates. Each has the ability to infect, defoliate the plant and potentially cause sun scald to the tomatoes. Alternaria can affect seedlings and transplants as well as older plants, while Septoria usually affects older plants.
Septoria starts with tiny water-soaked spots with a dark brown to black ring around them. They average 1/16 to 1/8 inch in diameter and appear to have a dark center where the fungus spores are produced. As the spots enlarge, they become tan or gray with a dark ring around them. The spots can appear on stems, blossoms and leaves, though not on the fruit. The plant will begin to defoliate to shed the affected leaves.
Alternaria forms brown or black spots with a yellow halo-like ring on the edge and a ring pattern throughout the spot. Spots generally appear first on the older leaves at the lower part of the tomato plant. Leaves turn yellow and begin to fall. Dark, sunken spots or cankers can appear on stems, also having the ring pattern. Spots on the fruits may have the ring pattern and appear as leathery, dark spots
Septoria begins in several ways. It can be carried in the seeds so make sure the variety you buy comes from a reputable source. It also can be carried on seedlings or grow on wild varieties of related plants such as ground cherry, jimsonweed and nightshade. While it does not live in the soil, it can overwinter in diseased plant debris. The fungus prefers mild temperatures and produces spores between 60 and 80 degrees. It also needs ample rain to grow and spread.
Alternaria is spread through wind, rain and insects and can survive in infected plant debris. If water remains on the plant for more than a few hours, the disease can penetrate and produce symptoms in two to three days. High humidity and temperatures between 60 and 75 degrees are its preferred conditions.
Both diseases are managed in the same way. Provide adequate spacing between plants, and irrigate early in the morning to allow the sun to dry moisture from leaves before nightfall. Provide adequate nitrogen levels in soil, dependent upon existing soil and type of tomatoes planted. Rotate crops on a 3- to 4-year rotation plan with cereal, corn or legumes if varieties planted are susceptible to either fungus. Use of a fungicide can also help prevent the infection.
Fungicides for black spot fungi are effective and ideally should be applied before any signs of disease appear. Sulfur and copper can be applied when conditions are wet and temperatures are between 55 and 85 degrees. If no fungicide was used previously, it should be applied now to prevent the spread. Once symptoms are present, remove diseased leaves and stems. Collect any fallen leaves and debris from the base of the plants and destroy.