If your tomatoes aren't at their prime this season (or haven't been in the past), any number of culprits could be to blame. Below is a brief description of the most common tomato problems, along with some suggestions for avoiding them next year.
Anthracnose
This fungal disease is perhaps best known for killing thousands of dogwood trees around the country. It causes sunken spots on tomatoes that look water-soaked and that often form concentric circles that look like a bulls-eye. If you suspect an anthracnose attack, remove any infected plant parts from your tomato and throw them away; the plant may survive. To prevent further infections, pick fruits promptly, promote good air circulation, and keep the foliage dry. Never compost infected plants or fruit.
Blackheart
If you cut open a tomato and find a dark rotten part inside, the culprit is blackheart. This is a problem, but not a disease. It's caused by a lack of calcium. Your plant might have been "hungry" for calcium for any number of reasons, including drought, a variation in the amounts of soil moisture when the fruits were forming, root damage, or heavy soils.
Blight
This disease was the cause of the dreaded Irish potato famine. Late blight also affects tomatoes. The diseases shows up as spots on leaves and stems that appear water-soaked. These spots can grow into large, dark lesions. A white, fuzzy mold may appear on the bottom of the leaf as the disease progresses. Remove and destroy any affected parts promptly. Spots may also form on green fruits and expand, causing malformed fruit with dark lesions. To help avoid this disease, don't plant tomatoes and potatoes together.
Blossom drop
If all or some of the blossoms fell off your tomato plants, stress was the cause. Stress occurs when your garden suffers from sustained high temperatures (above 90 degrees F daytime; above 75 degrees F nighttime), large temperature swings, cold weather (below 55 degrees F), drought, soil that was too dry, or excessive moisture fluctuations.
Blossom-end rot
This common problem shows up as a dark, sunken area at the bottom of fruits. The rot enlarges and can cover a good portion of the fruit as it grows. As with blackheart, the cause is a lack of calcium.
Botrytis (gray mold)
This disease causes spots on tomato leaves that look water-soaked, while the fruits show pale halos that can be up to 3 inches across. The halos start out white and turn yellowish as the fruits mature. A gray, fuzzy mold may appear on the affected parts of the plant, too. Botrytis is spread by spores in the soil, so avoid splashing leaves when watering. A thick layer of mulch (3 inches or more) will help.
Bronze mite
A common pest in hot arid climates is the bronze mite (also called tomato russet mite). Thousands of these minute critters can turn a tomato's main stem bronze and its leaves brown. Spray plants with sulfur every few weeks to control.
Catfacing
Ugly, gnarled fruits with scars near the blossom end are caused by cold temperatures or uneven moisture while fruit is developing, or by viruses, fungi, or insect damage. Remove catfaced fruits to encourage later crops.
Cracking
Fruit that cracks at the stems or shoulders is caused by too much moisture while fruit is developing. Don't overwater at this time. Some tomato varieties are prone to cracking, and some are more resistant. Seed catalogs and packets should offer this information.
Frost damage
A frost-damaged plant has wilted leaves that look water-soaked, and damaged leaves and stems often turn black. (Because tomatoes are tropical plants, they can't tolerate cold temperatures. When they do get cold, the plant cells freeze and the cell contents leak out.)
Fusarium
The first symptom of this common soil-dwelling fungus is wilt. Bottom leaves will turn yellow and fall off first, and you might also notice that the insides of the affected stems are discolored. Eventually fusarium will kill the whole plant. The fungus loves warm soil--above 82 degrees F--so if you garden where it's hot, lay down a cooling mulch early (a layer about 3 inches or more is best). Once fusarium attacks your plant, destroy it. Don't compost the plant, though, because it may spread the disease to your compost, and then on to your other plants.) Resistant varieties are marked "F" or "FF."
Nematodes
Root knot nematodes are tiny, worm-like creatures that attack plant roots and kill affected plants. Nematode-affected plants may have yellow leaves, become stunted, and wilt. If you suspect nematodes, dig out the affected plants and inspect their roots. If nematodes are your problem, the roots will generally look stunted and knobby. Destroy any infected plants. To prevent this problem in the future, try planting resistant varieties (designated with a "N" on the packaging) and rotate your tomato-family crops each year.
Powdery mildew
Although you may be familiar with the type of powdery mildew that affects garden plants such as phlox and lilacs, a different strain of this disease can also attack your tomatoes. The main symptom of this fungus is white or gray spots that form a powder which eventually covers the leaves and fruits. Affected plant parts discolor and drop. Powdery mildew likes still air and lower light conditions. Keep lots of space between your plants to help your tomatoes stay safe from this disease.
Septoria leaf spot
This fungus attacks older plant leaves first, covering them with yellow spots that darken to brown or black and often become rings. The affected leaves eventually fall off. Remove and destroy infected leaves at first sign of infestation. If the problem continues, destroy the plants.
Sunscald
Just like people, plants can get sunburned. Sun-damaged fruits first show blisters, which dry out into pale, bleached areas. Leaves become shriveled and dry. Sunburn is generally only a problem when there is extra light reflected off of a surface onto the plant--such as if you plant your tomatoes next to a white building.
Tobacco mosaic virus (TMV)
Viruses are the worst diseases in the plant world. They cause abnormal, distorted growth, mottled leaves, blotchy fruits, and stunt entire plants. TMV, one of the worst viruses, is spread in many ways, including by human contact with tobacco products. Destroy all infected plants the moment you suspect this disease. To prevent infection, wash hands after handling tobacco. Resistant varieties are marked "T."
Verticillium
This shows up as a wilt, much like fusarium, except that verticillium thrives in cooler conditions. This disease causes leaves to become yellow or mottled, and like fusarium, it causes the bottom leaves of a plant to wilt first. Later, the whole plant dies. Remove and destroy affected plants as soon as wilt strikes. Resistant varieties are marked "V."
Prevention is the best care
Planting tomatoes that are resistant to the diseases most common in your area is the best way to prevent problems. Resistant varieties are labeled V, F (or FF), N, T, and A after the name of the plant to designate which diseases or problems they resist.