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Beginner

Identification

• Symptoms vary between viruses, cultivars and conditions and include stunting, leaf and/or fruit mottling, etc.
• Interaction between viruses may occur, complicating visual diagnosis

Often Confused With
Herbicide injury

Period of Activity
In general, viruses may infect plants throughout the season.

Scouting Notes
Virus symptoms can be quite variable.  Although some are spread by aphids, the insects may not be detected in the field.  Aphids moving across the landscape earlier in the season could have spread the disease and continued on, looking for more preferred feeding sites.

Thresholds
None established.

Advanced

Identification
Cucumber mosaic virus (CMV) causes plants to be stunted and produce little or no marketable fruit.  Some leaves may be mottled or have large, necrotic ringspot or oak-leaf patterns.  Fruit may be deformed, with sunken ringspots or a rough surface. 

Tomato or tobacco mosaic virus (ToMV, TMV) may cause mosaic symptoms and puckering of leaves and mottling of fruit.  Fruit may be small and misshapen. 

Symptoms of tomato spotted wilt virus (TSWV) are extremely variable.  It may cause stunting of plants, necrotic spots or patterns on leaves, dark-coloured lesions or streaks on stems, and mottling and necrotic patches on fruit. 

Often Confused With
Herbicide injury

Biology
Depending on the crop, virus diseases may be spread by aphids and/or other leaf-feeding insects, such as thrips.  Insects can carry and transmit a virus for their entire life (persistent transmission) or they can acquire a virus during feeding (non-persistent transmission).

In persistent transmission, an insect can become infected after feeding on an infected plant for an extended period of time (at least several minutes to one hour).  After an incubation period (sometimes several days or weeks), the insect is able to spread the virus for the rest of its life. 

Non-persistent transmission occurs when an insect lands on and samples plant tissues from many different plants.  The insect picks up the virus from an infected plant and transfers it to the next plant.  As the insect feeds, it clears the virus from its mouthparts and is no longer able to transmit the disease to additional plants.  Soybean aphids are a common vector of many non-persistent viruses.

Period of Activity
Generally, viruses may infect plants throughout the season. 

Scouting Notes
Virus symptoms can be quite variable.  Although some are spread by aphids, the insects may not be detected in the field.  Aphids moving across the landscape earlier in the season could have spread the disease and continued on, looking for more preferred feeding sites.  While scouting for other disease, take note of any mottled, stunted or puckered plants. Record the scope and location of any infected areas.

Thresholds
None established.

Management Notes

• Control weed hosts such as pigweeds, nightshades and related weeds, milkweed, purslane and chickweed in and around the field.
• The green peach aphid, as well as potato and melon aphids spread the cucumber mosaic virus. It is thought that soybean aphids moving through the crop (searching for soybeans, by tasting) may also spread the disease. Limited spread can also occur in the field through handling plants.
• Spraying for aphids is not effective in controlling the common pepper viruses in Ontario.  Insecticides may cause the aphids to initially become agitated, probing more plants than usual.  Insecticides also kill off beneficial insects, which may result in a rebound of the aphid population before the population of beneficials is able to re-establish.
• TMV is spread by handling plants or on tools and machinery.
• Tomato spotted wilt virus is spread by thrips, primarily western flower thrips.  Transplants may become infested with thrips in the greenhouse, especially if bedding plants or ornamentals are grown in the same greenhouse.  Tomato spotted wilt virus is rarely of concern.