Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
In the last week or so high tunnel tomato plants have been reported that look a bit squirrely. I thought it was possibly herbicide or virus or nutrient problems. After eliminating the first and third possibilities we had the plants tested for a battery of viruses. There were 3 viruses found. The most unusual one was the Pepino mosaic virus, which belongs to the Potexviruses. This virus is very easily transmitted mechanically and has a low seed transmission rate. Seed transmission occurs at rates of less than one in a thousand when seed is not properly cleaned. The virus is external, contaminating the seed coat and not the embryo or endosperm. Symptoms vary greatly with fruit marbling being the most typical and economically devastating symptom. You can also have fruit discoloration, open fruit, leaf blistering or bubbling, leaf chlorosis and yellow angular leaf spots. The severity of the Pepino mosaic virus symptoms is dependent on environmental conditions. As the infected plants mature the foliar symptoms usually disappear, but not the fruit problems. Prevention of infection is through stringent hygiene measures as the virus is spread primarily by mechanical methods. The Pepino mosaic virus is a newer one but is appearing more often in tomato production areas.
The other two viruses found were more common: Tobacco mosaic virus and tomato mosaic virus. Tobacco mosaic virus (TMV) is one of the most highly persistent tomato diseases because it can remain viable without a host for many years and it is able to withstand high temperatures. Both viruses are spread primarily by mechanical methods. Workers and their equipment can become contaminated when they touch infected plants. Symptoms are rather general and appear as yellow-green mottling on leaves with flowers and leaflets being curled, distorted, and smaller than normal in size. Generally, the fruit from TMV infected plants do not show mosaic symptoms but may be reduced in size and number and may develop an internal browning that most often appears in fruits of the first cluster. Severe strains of TMV and tomato mosaic virus can cause the lower leaves to turn downward at the petiole and become rough and crinkled. Some tomato varieties when infected with TMV or tomato mosaic virus can develop dead areas on leaves, stems and roots. As with the Pepino mosaic virus the best control for these two viruses is strict hygiene and not using contaminated seed.
Figure 1. Tomato plants infected with three different mosaic viruses