Insect Vectors of Vegetable Virus Diseases

Entomology News and Information:
Insect Vectors of Vegetable Virus Diseases

In recent years, a number of vegetable viruses have been diagnosed in Utah, including curtoviruses (such as Beet Curly Top virus in tomato and pepper, A); Watermelon Mosaic virus, B and C (in cucurbits); Pepper Mottle virus, D (in pepper and ground cherry); Alfalfa Mosaic virus (in legumes, pepper, and tomato); Tomato Spotted Wilt virus (in tomato and pepper); and Iris Yellow Spot virus (in onion). Viruses are named for the first plant on which they are identified; however, most have multiple hosts.

 

Green peach aphid (left) can vector several virus diseases, the beet leafhopper (right) vectors Beet Curly Top virus in tomato and pepper, and the tiny thrips (bottom) vectors Iris Yellow Spot virus or Tomato Spotted Wilt virus, depending on species.

Aphids, thrips, and leafhoppers are the primary vectors of vegetable viruses in Utah. Plant viruses affect commercial and home garden producers, and can devastate a crop if severe. In Utah, the vegetables most commonly infected with viruses are cucurbits, such as squash, pumpkin, melon, and cucumber, and solanaceous crops, such as tomato, pepper, and sometimes potato. Symptoms may be undetected early in the season, but become more noticeable as foliage expands and fruits develop. Often the populations of insect vectors have subsided by the time disease symptoms are noticed.

Most leafhopper- and aphid-vectored viruses in Utah are non-persistent. This means that the virus is picked up on the insect’s mouthparts within a few seconds of feeding on an infected plant, and moved to a new plant by winged adults during subsequent feeding bouts. The virus does not replicate in the insect’s body and is not passed to its offspring. The tospoviruses, which include Tomato Spotted Wilt virus and Iris Yellow Spot virus, are vectored by thrips and are persistent. This means that plant hosts of the virus must also be reproductive hosts for the vector in order for the immature stage, which does not move among plants, to acquire the virus. After the virus is acquired by young thrips larvae, it replicates within its gut, and is then transmitted by older larvae and adults. To manage virus diseases in vegetables, it is essential to understand the biology of their insect vectors.

The beet leafhopper vectors Beet Curly Top virus which is primarily a problem in tomato and pepper in Utah; however, the insect has a broad host range that includes weeds, ornamentals, and many vegetables. This disease is common in the southern regions of the state, but can also be severe in the north in some years. Severity of the disease is dependent on plant age when infected. Infected seedlings often die before setting flowers. Plants infected after the seedling stage survive, but are yellow and stunted. Infected leaves of tomato and pepper become thickened, roll upward, and can have purplish veins (but not always). Leaves of beets become twisted and curly. Immature fruits are typically dull in color and wrinkled. Yields are reduced and fruits ripen prematurely. Curly top tends to be particularly severe in home gardens where a diversity of host plants occurs, including those that attract the beet leafhopper.

Beet leafhopper does not prefer to feed on tomato and pepper, but it only takes a brief feeding bout to transmit the virus. Beet leafhoppers like to feed on Russian thistle and weedy mustards. The leafhopper migrates north each spring, spreading the virus as it moves into gardens and fields to feed. Some tomato cultivars, such as ‘CVF 111’ and ‘Saladmaster’ have shown some tolerance to the disease, but ‘Roma’ types are highly susceptible.


Tomato Spotted Wilt virus causes stunting, leaf necrosis, plant dieback, and fruit distortion (external and internal) including ringspots.

Cover young plants with floating row cover fabric to reduce early season infections. “Wall-of-water” cold protection products have also been shown to reduce leafhopper feeding. Good weed control around the perimeters of gardens and fields will reduce attraction of leafhoppers. Planting alternate rows of different vegetables (for examples outside row of carrots or melons followed by a row of tomatoes or peppers) can reduce feeding on susceptible plants. Reflective mulches are ineffective for this disease.

There are three primary species of aphids that occur on vegetables in Utah: green peach aphid, potato aphid, and melon aphid. These aphids can vector Watermelon Mosaic virus, Pepper Mottle virus, Alfalfa Mosaic virus, and others. Reflective mulches have been shown to reduce early-season aphid populations, and insecticides with good aphid-killing activity, including horticultural mineral oil and insecticidal soap, can reduce the incidence and spread of aphid-vectored viruses. There are resistant or tolerant cultivars for some crops and viruses. Good weed control and separating fields of susceptible crops have shown some success for cucurbits. There are many natural predators and parasitoids of aphids; however, because the virus can be spread quickly and early in the season, reliance on biological control is often inadequate for virus prevention.

The tospoviruses, Tomato Spotted Wilt virus (TSWV) and Iris Yellow Spot virus (IYSV), are primarily vectored by western flower thrips and onion thrips (IYSV) in Utah. TSWV has an extremely broad host range including many weeds, ornamentals, fruit crops, and vegetables. About a dozen hosts have been identified for IYSV so far. Symptoms of tomato spotted wilt on tomato are stunting, fruit color distortion (external and internal) including ringspots, leaf necrosis, and plant dieback. Symptoms of iris yellow spot on onion leaves are spindle-shaped lesions that can coalesce to cause leaf necrosis and early death. For both diseases, it is critical to start with clean transplants and to reduce thrips densities, especially during the early season. Insecticides are the most common thrips control tool, but cultural practices such as reducing nitrogen applications, weed control, and avoidance of planting tomato or onion adjacent to attractive thrips host plants can be helpful. Remove infected plants when detected to prevent spread of the virus.

Better management of insect vectors can decrease virus incidence and spread, and reduce crop loss to common virus diseases in vegetables.


-Diane Alston, Entomologist; Claudia Nischwitz, Plant Pathologist; and Erin Petrizzo, Small Farm-IPM Project Scout