Septoria Leaf Spot is a fungal disease caused by warm wet weather. It is identifiable by the spots that appear on the plant's leaves. These spots are lighter in the center, grayish-white, and darker along the edges. At the very center, black spots may appear. The best way to prevent this disease is to water plants near the base of the plant in the morning so they can dry it. It is also advisable to place plants far enough apart that the leaves will not be touching the plants reach their full height. To prevent the spread of this disease, avoid working on plants when they're wet and remove plant debris at the end of the season. It is also suggested that growers rotate crops, tomatoes and those related to them, around growing areas. The only means of curing infected plants is to use a fungicide.
Similar to Septoria Leaf Spot is Early Blight, also known as Alternaria Leaf Spot or Target Spot. Treatment and prevention are similar to Septoria Leaf Spot, although Mountain Supreme, Pride, Gold, Fresh, and Belle varieties are all resistant to Early Blight. It is also prudent to avoid planting potatoes in the rotation since this fungus can affect potatoes, as well. Early Blight can be identified by its fuzzy appearance on the brown spots, which may seem to have concentric circles within them.
Anthracnose is another fungal infection, but this one affects the fruit. Like other fungal infections, prevention includes keeping plants as dry as possible. It appears as a circular, indented spot, often on overripe fruit. Sometimes these spots may appear cream or pinkish in color. Anthracnose is spread through raindrops and it frequently creates a pathway for infections by other diseases. It is especially important to remove plant debris and rotate crops with this disease, since it survives on vines and in the soil. Because it can typically be found on overripe fruit, it is important to pick fruit regularly as a preventative measure.
Fusarium Wilt is another fungal infection. Typically, it will attack one side of the plant by clogging the water conducting tissues which makes the leaves wilt. Eventually it will spread to the whole plant. If Fusarium Wilt affects indoor plants, treat the soil with steam. Otherwise, the same prevention measures for other fungi should be followed. Tomato varieties resistant to Fusarium Wilt may produce fruit while affected, but those that are susceptible will not.
Verticillium Wilt is very similar to Fusarium Wilt, but it attacks the whole plant at once. If plants survive, they will have a poor yield. Because a number of other plant species are affected by Verticillium Wilt, so it is best to rotate with corn and beans, as these are not affected the fungus.
Late Blight, another fungus, is most likely to afflict crops in cool, wet weather. On leaves, the fungus appears as water-soaked spots that become green-black blotches, which look frost-bitten and have a white growth underneath. On fruit, the fungus grows as brown blotches that turn fruit into rotten, smelly mess. Use the same treatment and preventative measures as with other fungal infections.
Bacterial Spot is caused by a bacterial infection. Spots on the leaves have a greasy feeling to them and exhibit a yellow halo. Over time, the spots become scabby with a pitted center. On the fruit, the fungus looks like black, pimply spots in water soaked areas. To avoid spreading the disease, be sure to avoid diseased transplants, and spray infected plants with a copper product designed to treat this kind of disease. It is important to use the same preventative measures as with fungal infections, and to exclude peppers from crop rotations, as they are also susceptible to the disease.
Bacterial Speck is another bacterial infection. While it may not kill plants, it will affect the quality of fruit. The spots are similar to those of Bacterial Spot, but they are smaller and can be scraped off the fruit easily. Bacterial Speck is found most often in cool, wet climates, and while it does not affect other plants, it can live on them. This is why it is important to destroy weeds growing in the area, as well. Spay any infected plants with a copper product.
Bacterial Canker is another bacterial infection, but unlike other bacteria, it is not easily treatable by copper products. It typically begins on one side of the plant before spreading to the whole plant. Spots appear as raised, white spots and then develop into spots with black centers and white halos.
Tomato Spotted Wilt Virus is a viral infection and cannot be cured. Because of this, it is imperative that infected plants be removed immediately. The virus causes yellow ring spots on fruit, and purple discoloration of leaf tips. The virus is spread by thrips and aphids, and any transplants should be bought from nurseries and greenhouses that can certify measures for prevention of these insects. Because this virus can also affect potatoes and peppers, do not plant these as part of crop rotation.
Other problems that can affect yield and plant health include various growing conditions. For instance, Sunscald occurs when the fruit is exposed to too much sun, such as from excessive pruning. A white blister will form on exposed area, and then it will collapse into a sunken area. Another problem is known as Leafroll, when the edges of the leaves curl under and inward. This is caused most often from irregular watering, and may eventually correct itself. It doesn't seem to affect the quantity or quality of the harvest. Plants that do not develop fruit are often suffering from extremely hot weather conditions, especially when there are high temperatures at night. Another issue is blotchy ripening, which is most frequently caused by low levels of potassium or poor light conditions. It may also be caused by high humidity, high soil moisture, low temperature, and excessive fertilization. To prevent this problem, use fertilizers with balanced formulas.
Source:
Gleason, Mark L. and Edmunds, Brooke A. "Tomato Diseases and Disorders." 2 November 2008. http://www.extension.iastate.edu/Publications/PM1266.pdf. aug. 2006.
