Alfaffa mosaic virus (AMV)
Aphids (Aphis gossypii, Myzus persicae and others)
Typically, yellow and purple areas develop in the young leaves, resulting in the foliage developing an overall yellow bronzing. Infected plants stop growing and leaves curl downward. The main stem has a dark brown discoloration of the phloem at the soil-line. which can be seen by lightly scraping the epidermis from the stem. This discoloration may extend into the upper shoot tips. Irregular. brown streaking in the stem pith is characteristic of this disease. Roots often show this same red-brown phloem discoloration. Depending on the age of the plant at the time of infection, developing fruit show various degrees of external and internal brown spotting and distortion. Generally, the disease incidence is greater the closer tomatoes are to alfalfa fields and gradually diminishes with increasing distance from the source.
Conditions for Disease Development
This virus has a wide host range on which it can survive. and is typically found in older alfalfa fields or permanent pastures. Aphids carry AMV in a non-persistent manner, and transmission occurs during feeding or probing of the plant. During hay mowing, winged aphids carry the virus from alfalfa to nearby tomato fields. Little or no secondary spread occurs within a field.
Locating fields away or upwind from alfalfa is a possible means of avoiding this disease. Spraying for aphids will generally not eliminate primary infection of tomatoes.