Virus and vine crops

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Several viruses infect and damage squash, cucumbers and pumpkins in Michigan. Among these are cucumber mosaic virus (CMV), zucchini yellows mosaic virus (ZYMV) and watermelon mosaic virus 2 (WMV2). This year, CMV and WMV2 have been confirmed. Each of these viruses may infect squash, cucumber, zucchini and pumpkin alone or in combination. Infected plants may be stunted and/or distorted. ZYMV, CMV and WMV2 all produce mosaic on foliage and fruit of infected plants. Plants are especially damaged when infected young because the yields are reduced and fruit quality is often affected by bumps and discoloration. CMV was widespread in vegetable vine crops in 2005. In particular, pickle growers suffered widespread and serious crop losses. Young pickle plantings established midsummer were infected as early as the first true leaf stage, although viral symptoms were not always obvious. Symptoms caused by ZYMV are often more severe than those caused by CMV and WMV2. Along with causing severe distortion and discoloration of the fruit, ZYMV significantly reduces yields. ZYMV causes such devastating crop loss that plants with mosaic symptoms that test positive for this virus should be destroyed to limit spread of this disease within a region. Positive identification of the virus(es) infecting the crop is based primarily on serological tests such as ELISA (enzyme-linked immunosorbent assay). Identification based on field symptoms alone can be misleading.

CMV infects many vegetables and other plants unrelated to vine crops. WMV2 infects vine crops and legumes (beans and peas) and other plants outside the cucumber plant family, while ZYMV infects primarily vine crops.

Plants known to serve as hosts for WMV2:

  • African Horned Cucumber
  • Bean
  • Bishop’s Weed
  • Bitter Apple
  • Bluecurls
  • Bluewings
  • Calabash Gourd
  • Cape Marigold
  • Cheeseweed, Mallow
  • Chervil
  • Chinese Houses
  • Citron
  • Common Groundsel
  • Common Mignonette
  • Corn Salad
  • Cotton
  • Crimson Clover
  • Cucumber
  • Curled Mallow
  • Dyer’s Rocket
  • Fenugreek
  • Fiddleneck
  • Flax-Seed Plantain
  • Guar
  • Hedgehog Gourd
  • Henbit
  • Huban Clover
  • Lamb’s Lettuce
  • Lambsquarter
  • Lupine
  • Malabar Gourd
  • Mask Flower
  • Musk Flower
  • Muskmelon
  • Narbonne
  • Pea
  • Phasey Bean
  • Pheasant’s-Eye Flower
  • Pumpkin
  • Quinoa
  • Shepherd’s Purse
  • Snake Gourd
  • Snow-On-The-Mountain
  • Sour Clover
  • Spinach
  • Spring Vetch
  • Squash
  • Star Cucumber
  • Sweet Pea
  • Tree Mallow
  • Watermelon
  • West India Gherkin
  • White Cut-Leaved Mignonette
  • Wild Canterbury-Bell
  • Wild Cucumber
  • Winter Squash

Plants known to serve as hosts for ZYMV:

  • African Horned Cucumber
  • Bitter Apple
  • Bitter Melon
  • Bluewings
  • Buffalo Gourd
  • Chinese Okra
  • Coyote Gourd
  • Crowfoot
  • Cucumber
  • Fenugreek
  • Hedgehog Gourd
  • Henbit
  • Malabar Gourd
  • Muskmelon
  • Pumpkin
  • Snake Gourd
  • Sponge Gourd
  • Squash
  • Watermelon
  • West India Gherkin
  • Wild Cucumber
  • Winter Squash

It is unlikely that all hosts of ZYMV and WMV2 have been identified. CMV, WMV2 and ZYMV overwinter in biennial and perennial plants and are usually carried to new plantings by aphids. Within plantings, CMV is spread chiefly by aphids, but may also be spread by cucumber beetles. WMV2 and ZYMV are also spread by aphids. While these viruses can be transmitted in plant sap through mechanical wounding (i.e., harvesting knives), spread by this means is considered minor.

Applying insecticides to cucurbits to control these viruses has not been effective. However, growers should be aware of surrounding crops and weeds that may serve as aphid sources. Insecticide applications to surrounding crops that are serving as aphid sources may be helpful. Reflective mulches, plastic coverings, and oil sprays when used experimentally have delayed and reduced infections, but have not been used commercially because of cost and disposal problems of mulches and covering materials. Weed control in and around plantings may help reduce infections, but will likely be inadequate by itself. Known weed hosts of WMV2 include (but are not limited to) lambsquarter and shepherd’s purse. Planting a border crop that is attractive to aphids but not a host to the viruses around a susceptible vine crop has not been evaluated in Michigan. In theory, the aphids would feed first on the border crop greatly diluting any virus that the aphid might be carrying before feeding on the susceptible vine crop. Although this practice may be helpful, it would not eliminate occurrence of the virus. Currently, use of resistant varieties is the most effective approach in managing these virus diseases and should be coupled with weed control.

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