Viruses of cucurbits

Quick facts

  • Infection with virus commonly occurs on all cucurbit crops in Minnesota.
  • It is difficult to identify a virus by symptoms alone.
  • Virus-infected leaves often have a mottling or mosaic pattern in shades of green and yellow.
  • Aphids transfer most of the viruses that infect cucurbits.
  • Perennial weeds allow the virus to survive from season to season in a field.
  • There are no pesticides available to reverse or limit the symptoms of viral infection.

Infection with virus commonly occurs on all cucurbit crops in Minnesota. There are several different viruses including Cucumber Mosaic Virus (CMV), Squash Mosaic Virus (SqMV), Zucchini Yellow Mosaic Virus (ZYMV),

Watermelon Mosaic Virus 2 (WMV-2) and Watermelon Mosaic Virus 1 (WMV-1), also known as Papaya Ringspot Virus (PRSV).

Identifying virus symptoms

It is difficult to identify a virus by symptoms alone. Symptoms vary depending on the crop, variety, age of the plant at the time of infection, and in some cases weather. It is common to find plants infected with more than one virus at the same time, often resulting in combined severe symptoms. The best way to identify viruses is through analysis of a sample at the University of Minnesota Plant Diagnostic Clinic.

Virus-infected leaves often have a mottling or mosaic pattern in shades of green and yellow. This mosaic can be very distinct and obvious, or subtle. Leaves are often distorted or deformed. They may be puckered, cupped under, have deep lobes, or appear thin and string-like.

Young leaves often show the most severe symptoms and are abnormally small. Growth on infected vines is typically stunted. In CMV infections, vines may wither completely.

The virus's effect on fruit depends on when it infected the plant. Early infections often result in no or very low fruit production. Later infection can result in fruit that is small, deformed and discolored. Fruit may have a mottled or mosaic pattern, ring spots or exhibit color break on all or part of the fruit. Melons infected with SqMV often lack netting at maturity.

What causes viruses

  • All of the mosaic viruses can infect all of the cucurbit crops including melon, cucumber, pumpkin, summer and winter squash. In addition, many of the mosaic viruses can also infect common weeds.
    • Squash mosaic virus will infect weeds in the Chenopodiaceae family like common lambsquarters, maple leaf goosefoot, Russian thistle and kochia.
    • Watermelon mosaic virus infects legumes like clover.
    • Cucumber mosaic virus can infect plants from over forty families, including vegetable crops like tomato, lettuce and spinach, flower crops like gladiolus, petunias, impatiens and rudbekia, and a wide variety of weeds.
    • All of the mosaic viruses can also infect weeds in the cucurbit family.
  • Aphids transfer most of the viruses that infect cucurbits with the exception of SqMV, a virus transmitted by both striped and spotted cucumber beetles.
    • Aphids feed on virus-infected plants (weeds or crop) and then transfer the virus when they feed on a new plant.
  • Perennial weeds allow the virus to survive from season to season in a field.
  • Viruses can transfer from plant to plant on the hands and tools of workers through infected sap.
  • Squash mosaic virus can come on infected seed. The other mosaic viruses rarely transfer on seed
  • Once in the plant, viruses move systemically through the tissue infecting leaves, vines and fruit.

Preventing and managing viruses

  • Plant resistant or tolerant varieties whenever available.
    • Resistance is virus specific and it is necessary to determine first which mosaic virus is causing disease in order to select appropriate varieties.
  • Purchase clean seed from a reputable supplier. If saving seeds, do not collect seed from infected plants.
  • Control weeds within and around the field. Focus control efforts on perennials weeds that may allow the virus to carry over from one season to the next.
  • Keep aphid and cucumber beetle populations low.
  • If disease appears in a few plants, rogue and bury these plants to prevent further spread of the disease.
  • Clean tools and hands with soap and water after working with infected plants. Reduce maintenance tasks that require handling of infected plants as much as possible.
  • In greenhouse or hoop house production, use screening and other methods to exclude aphids. Eliminate weeds or other virus hosts within 350 feet of the greenhouse or hoop house.
  • Avoid planting cucurbit field crops next to hoop house or greenhouse production areas.
  • There are no pesticides available to reverse or limit the symptoms of viral infection.

Michelle Grabowski

Reviewed in 2018

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