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What to do if you suspect a virus in your squash or melons during harvest

 Authors: Annie Klodd and Natalie Hoidal - Extension Educators-Fruit and Vegetable Production

Winter squash leaves with mottling and a yellow-green mosaic, characteristic of a virus. Photo: Annie Klodd

Last week, we responded to a disease inquiry in a pumpkin field, and found symptoms that looked like a mosaic virus. We immediately sent the samples to the UMN Plant Disease Clinic, and they promptly diagnosed the plants with Squash Mosaic Virus (SqMV). 

Additionally, Natalie found SqMV and watermelon mosaic virus (WMV) earlier in the summer, on several farms.

If you suspect a virus on your crop, do not hesitate to send samples to the UMN Plant Disease Clinic or another diagnostic clinic for diagnosis. Viruses cannot reliably be self-diagnosed in the field, as visual symptoms can look similar among the various mosaic viruses. 

Squash mosaic virus on zucchini. Photo: Anna Racer, Waxwing Farm.
Squash mosaic virus on zucchini, near Webster, MN. Photo: Natalie Hoidal.

Knowing which virus is present is very important for knowing how to manage it. For example: while SqMV is spread through infected crop seed and cucumber beetles, other viruses of cucurbits are spread mainly through aphids and not through infected crop seed. 

Mosaic viruses spread between weeds and the crop, or from an infected crop to a healthy crop. The insects referenced above facilitate the spread when they feed on infected plants and then carry the virus to the crop plants. 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, and cucumber mosaic virus infects weeds in over forty families of plants.

Therefore, knowing what virus is present aids in knowing how to manage it (i.e. which insect pests to target, how to focus weed control efforts, and how to talk with the seed company about virus-free seed). 

The following photos are of the samples submitted to the Plant Disease Clinic on Sept. 8, 2020, positively diagnosed with squash mosaic virus (SqMV). Again, viruses should not be self-diagnosed in the field. If you see any symptoms similar to those in the photos, submit them to the Plant Disease Clinic for identification. Then, contact Annie Klodd or Natalie Hoidal, or your county Ag Extension Educator, for management recommendations.

Photos: Annie Klodd

Significant deformities and mottling on "Jack Be Little" miniature white pumpkins. Older fruits on the same plant were not affected.

Warts, small size, and significant deformity on Delicata squash.

Mosaic pattern on leaves of "Jack Be Little" pumpkins diagnosed with SqMV. 

If a virus is suspected during harvest, take actions to make sure it is not spread further throughout the field. Clean tools and hands with soap and water after working with infected plants. Harvest from virus-infected areas last. Reduce maintenance tasks that require handling of infected plants as much as possible.

Several other actions can be taken now and into next year to reduce the incidence of viruses on your farm in the future. For more information on mosaic viruses and how to manage them, please read this web page by plant pathologist Michelle Grabowski: Viruses of Cucurbits - UMN Extension

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