Viral Tomato Diseases
Just like humans, tomato plants can also catch viruses. Many of these diseases are very difficult to treat so prevention becomes paramount. Many of them are more prevalent when tomatoes are grown next to certain plants so one good form of prevention is to carefully select the locations of your tomato plants. As with other types of tomato diseases, viruses are easily passed from other nightshade vegetables to tomatoes and vice versa - potatoes are particularly notorious for passing infections to tomatoes so it is best to separate plantings of these vegetables.
Alfalfa mosaic virus typically affects the whole tomato plant but the most obvious symptoms are on the leaves, which turn yellow and eventually die. They also tend to curl though they curl downwards rather than upwards as is seen in other diseases. As the name suggests, this virus often affects tomatoes that are planted next to alfalfa. Red clover is also often found next to infected tomatoes.
Cucumber mosaic virus usually results in stunted plants with misshapen leaves. The symptoms vary depending on how early a plant is infected - the earlier the infection occurs, the more stunted the plant becomes as it grows.
Tomato plants infected with this virus are more likely to show symptoms if the plant was infected as a seedling. Symptoms include yellow, curled leaves.
A variety of virus strains cause the potato virus and each produces varied symptoms. The leaves of infected tomato plants may die or they may become mottled. As with alfalfa mosaic virus, leaves often curl downwards and potato virus usually results in stunted plants. Black streaks may also be seen on the leaves and stems of infected plants.
Tobacco or Tomato Mosaic
The symptoms of this disease are often similar to that of cucumber mosaic virus though sometimes tomato mosaic virus causes the leaves of tomato plants to take on a fernlike appearance. Black streaks may also be seen on the leaves and stems of infected plants.
Tomato Yellow Leaf Curl
As the name suggests, this virus causes curling and yellowing of the leaves of tomato plants. It is more common in warm, humid climates.
Tomato Spotted Wilt
When young tomato plants or seedlings are infected with this virus, 'dead' spots typically appear on plant leaves. In older plants, leaves may instead turn yellow, become stunted and wilt though the main structure of the plant may initially remain upright.