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 result 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.
Early Season Blight
Early-season blight or early blight is known as Alternaria leaf spot. It is a fungus that has the ability to infect a healthy tomato plant in just a few hours. Once infected, the plant's leaves will develop spots, lose leaves and the plant's health will deteriorate. Alternaria can damage seedlings, young transplants and established older plants.
Early blight starts with brown or black spots circled by a yellow ring around the perimeter. The spots can have a ring pattern throughout. The lower leaves of older plants are usually affected first with the infection moving upward through the plant. Leaves gradually change to yellow and start to drop. Stems will exhibit dark, recessed cankers also sporting the ring pattern. Infected fruits appear as leathery, dark spots and may display the ringed pattern as well.
Early blight is transferred by insects, on the wind or rain, and can survive in infected plant debris over the winter. The diseases enters the plant when leaves are wet for several hours and the first spots will begin to emerge in as few as two to three days. Early blight thrives in high humidity and when temperatures range between 60 and 75 degrees.
Plants should be spaced several feet apart to allow proper air flow which helps keep the plants dry. Water tomato plants early in the day to allow ample drying time. Good soil with an adequate amount of nitrogen will produce healthy tomato plants better able to fight off the diseases. Practice a three- or four-year crop rotation to ensure any plant debris left in the sol does not transfer to new plants. Cereal, legumes and corn are recommended rotation crops. The onset of infection can also be eliminated with the use of a fungicide.
Early blight can be prevented with regular applications of fungicide but it is no guarantee. Copper and sulfur should be sprayed when wet conditions occur along with temperatures between 55 and 85 degrees. Once infection is detected, a fungicide is helpful in preventing the spread of the disease. Discard any diseased leaves and stems. Remove those that have fallen leaves and collected around the base of the plants. Be sure to destroy all of diseased material to prevent the fungus from overwintering in it.