As a rule, most tomatoes are problem free but sometimes, usually in unusually wet or cool summers, they can have problems. Some of these diseases infect just the leaves and can be treated, other are soil borne and enter the stem of the plants which unfortunately kills the plant. Many of the diseases can be avoided if you remove all dead tomato and potato plants at the end of the season and grow your tomato in a different area each year. Watering the plants at the ground level rather than over the leaves can also reduce problems.
Early Tomato Blight: Although this blight shows up in the warm weather it starts its infection much earlier on. Typically if you grew tomatoes, or even potatoes, in the same area last year and left any debris in the vegetable garden, that debris harbors the blight. Symptoms include brown rings on older leaves followed by yellowing and withering of the whole leaf. If you have an infected plant, remove it and do not compost the remains.
Late Tomato Blight: This blight is more common when summer weather is cool and wet, but it can be transferred from one plant to another very quickly and easily. Typical symptoms are greasy spots on the leaves, sometimes accompanied by a mold. As with the early blight, good garden hygiene and clearing away old tomato and potato plants (including tubers rotting under the surface) is the key to keeping this blight under control. If the weather is cool and damp, keeping the plants covered so that the water does not get onto them can help. With container grown plants, bring the container into a garage or dry area. If you have this blight you need to remove all infected plants and dispose of them rather than compost them. Wash your hands carefully after touching the plants so that you do not transfer the blight to another plant.
Blossom End Rot: This is caused by irregular watering particularly at the early development phase. Symptoms include a flat base to the tomato that is dark or even black in color. Tomatoes are heavy feeders that need constant moisture, but not wet. Always water the plants at the soil level rather than an overhead sprinkler which can cause blight to infect the plants.
Cat Face: This is another deformity that creates lesions on the base of the tomato. A streak of discolored, hardened fruit is deformed slightly and is caused by inconsistent watering as well as cool weather when the young fruit is developing.
Verticillium Wilt: This is a soil born fungal problem that enters the stem of the plant where it can be seen as a brown ring around the soft green tissue. Outer symptoms include yellow leaves starting at the older, bottom leaves and working toward newer, upper growth. Small yellow circles are first, followed by general yellowing, wilting and finally leaf drop. To avoid verticillium wilt look for varieties that are less susceptible and virus free.
Fusarium Wilt: This fungus is similar to verticillium wilt but tends to affect first one side of the plant and then the other. The affected areas suddenly shrivel and turn black while wilting on the vine.
If your plant is infected with either Verticillium or Fusarium wilt you have to destroy the plant. Avoid growing tomatoes or potatoes in that same area for three years.
Fortunately these serious problems are uncommon and if you have good garden hygiene you can reduce the chance of your tomatoes developing them.