Resisting Mosaic Viruses in Cucurbits

I grow most of my vegetables in a community garden plot: sunny, cheerful, with soil I’ve been improving with free compost for years now. It’s a good situation, but while community gardeners can share labor, tools, plants and seeds, they also end up sharing pests and diseases. Our garden has been suffering from a problem I never encountered at home or in the Derwood demo garden: mosaic virus on cucumbers and squash. Mostly I have dealt with this problem by the highly scientific method designated Not Even Trying, but this year I really want to succeed. So, some research!

Potyvirus infection of pumpkin
Potyvirus infection of pumpkin
credit Dr. Gerald Brust, UME

Like colds and flu in humans, viruses of plants are tricky. They can spread easily by multiple methods, are not straightforward to diagnose, and are difficult or impossible to treat. Prevention is also hard: you can’t take your squash plant to CVS for a flu shot, or rather a watermelon mosaic virus vaccination. And wait, watermelon? Why does a squash plant get a watermelon disease? First, watermelon and squash are both members of the cucurbit family, along with cucumbers, melons, gourds and other edible and nonedible plants. They are closely enough related that they’re subject to the same diseases and pests. These particular viruses were named for the plants they were first identified on, but that doesn’t mean those are the only hosts, or even the most common ones.

Here are some points to note about cucurbit mosaic viruses:

  • They can cause a variety of symptoms, including mosaic-like patterns on leaves; other leaf discolorations and malformations; stunting of plants, fruit and leaves; abnormal flowers; and fruit that is oddly patterned and deformed. You may get a crop from these plants, but it will not be normal or vigorous.
  • However, just about every symptom can have another cause, from herbicide damage to insect feeding, or even natural leaf patterning mistaken for disease. When they all occur together you can be pretty sure mosaic virus is present, but the only way to tell for sure (and to determine which type of mosaic virus you have) is to send a sample to a lab for analysis. (Ask HGIC!)
  • Mosaic viruses named for cucurbits can affect nonrelated plant species, including other vegetables like peppers.
  • The viruses can be harbored in many common weeds. This is yet one more reason to keep your gardens (and the areas surrounding them) weed-free!
  • Mosaic viruses are usually spread by aphids. Squash mosaic virus can be spread by spotted cucumber beetles. These are difficult pests to control with insecticides, especially since disease transmission may already have taken place before the insect is killed. Exclusion methods may work for a time, but aphids are often trapped under row covers.
  • Since virus spreads between species easily, farmers are often advised to keep susceptible crops widely separated. This doesn’t work for home gardeners, so our best bet is to seek out resistant cultivars.

I asked Chris Giannascoli, one of our community garden leaders, how she has dealt with the mosaic virus problem. She has had great luck with a hybrid zucchini called Dunja, which has performed reliably for the last 7-8 years in the garden, while many other varieties have failed. This past summer, she was able to grow Max Pack cucumber, a hybrid pickler, with success, after nearly giving up on having any cucumbers at all. I’m definitely going to try these seeds this year.

Chris didn’t know which mosaic virus we have, but given this short list of anecdotal resistance I can make a guess: both are listed as resistant to watermelon mosaic virus and zucchini yellow mosaic virus, whereas Dunja is not listed as resistant to cucumber mosaic virus. So I will look for WMV and ZYMV in the disease resistance listings of other cultivars. I’m going to try a few, and will report back.

By the way, I’m given to understand that “resistant” in this case means something more like “tolerant.” The virus may still infect these plants, but they will tough it out, keep symptoms at bay, and produce more or less normally. I also intend to keep weeds down and watch out for aphids, and will encourage my fellow gardeners to do the same.

Cucurbits, while they are known for overproducing when happy, have a long list of pest and disease issues to deal with. We don’t need to add more problems!

Further reading:

Virus Disease of Cucurbits fact sheet from Cornell

Article on some effects of mosaic viruses by University of Maryland’s Dr. Gerald Brust


By Erica Smith, Montgomery County Master Gardener

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