The tomato is one of the easiest fruits to grow, but care must be taken so diseases do not affect the plant and the fruit. There are several diseases that strike tomato plants. Prevent diseases by properly watering and fertilizing plants, and by protecting them from excessive weather conditions.
Bacterial canker is caused by Clavibacter michiganensis subsp. Michiganensis. It affects tomato plant crops in the north central United States. Symptoms include wilting of young plants and leaflets turning brown on older plants. Young and old plants eventually die back after the plants become infected with bacterial canker. If the plant has already produced fruit, the bacterium causes white, slightly raised spots on the fruit. As the disease progresses, the white spots grow dark centers of up to 1/16 inch in diameter. As the spots grow older, the white "halo" turns brown. Copper sprays are used to control bacterial canker, but according to the University of Iowa, the sprays have minimal impact on the disease.
Septoria Leaf Spot
Septoria leaf spot is caused by Septoria lycopersici. The first symptoms are small, water-soaked spots. The spots grow to about 1/8 inch in diameter, then develop grayish-white centers. The edges then turn dark. The spores of this fungus are spread through rain and from watering from above. Eventually, the leaves turn yellow and fall off. While the infection can invade the plant at any time, it is most commonly seen after the plant sets fruit, according to the University of Iowa. Control septoria leaf spot by planting healthy plants with the proper amount of space between them, watering at the base of the plants, watering in the morning, removing dead and decaying branches, and keeping any fallen plant debris cleaned up. Control existing infection with fungicides.
Bacterial speck is caused by Pseudomonas syringae pv. tomato. It starts out with 1/16-inch dark spots on the leaves, and the spots eventually develop yellow halos. Slightly raised specks develop on green fruit. The bacterium prefers cool, wet conditions of less than 70° Fahrenheit, according to the University of Iowa. It is most commonly seen after severe rainstorms. The disease overwinters in seed and plant debris, so if you notice the disease, treat it with fungicides. If treating does not eradicate the disease, remove the plants, including the root ball. Do not plant tomatoes in the same spot the following year.
Early blight is caused by Alternaria solani, a fungus that is also known to cause Alternaria leaf spot. The disease is common to tomato plants, and causes lower leaf loss and brown or black spots of up to ½ inch in diameter. The spots merge and form brown or black patches on the leaves. If the disease progresses far enough, dark rings form in the spots. The leaves eventually turn yellow and dry up. The disease also affects the fruit. It starts at the stem end, causing sunken areas and giving the fruit a black, velvety appearance. The blight spreads fast in warm, wet weather. Control the blight with fungicides.