Odd Symptoms in Tomato Plants Turn Out to be Viruses

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

In Figure 1 you can see tomato plants that look a bit squirrely. I thought it was possibly herbicide or virus or nutrient problems. After eliminating the first and third we had the plants tested for a battery of viruses. There were 3 viruses found in the plants. 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 very 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 other two viruses found were more common: Tobacco (TMV) and tomato mosaic viruses. Tobacco mosaic virus 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.

The Pepino mosaic virus is a newer one but is appearing more often in tomato production areas. The overall odd thing was that the symptoms on the plants did not fit any one of the viruses well. I attribute this to the mixture of viruses in the plant making it very difficult to observe characteristic symptoms of any one of them. Add to this that the environment plays a part in the plants expression of the symptoms as does the properties of the viral isolate and it becomes clearer why symptoms were not typical. The infected tomato plants were not commonly grown hybrid plants. Growers need to be sure about where their tomato seeds are coming from before using them.

Figure 1. Tomato plants infected with three different mosaic viruses