Lou McLeod: Papaya Ringspot Virus in the Garden

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The spirit of Vincent Van Gogh visited my squash patch! Lest you think I have been sniffing the catnip, look at this squash! When we found this, we gazed at it in wonder. The intricate pattern is definitely not random. Did my artistic son sneak into the garden and prank me with this complex artwork? The vibrantly colored red Kuri squash looks as if someone took a teeny tiny woodburning tool and painstakingly etched a wonderful pattern just into the outer skin of the fruit, leaving the inner layer of the hull intact. When we found a second one, we knew this was not a prank. We posted a photo and a wise Common Sense Home reader told us that this was caused by a disease, the papaya ringspot virus, also known as papaya mosaic virus.

Say WHAT?! What is that and what is it doing in my Northeast Wisconsin garden? (We certainly don't have papayas nearby.) I had to do some research, which I'll share here with you.

What is Papaya Ringspot Virus (also known as Papaya Mosiac Virus)?

PRSV (papaya ringspot virus) is a plant disease more commonly found in warmer climates on papayas. This disease decimated papaya production in the 90s. There are two types, PRSV-P and PRSV-W. It seems that the first affects papaya and cucurbits, while the latter only infects cucurbits– cucumber, melon, pumpkins, and squash. The virus, spread by aphids, causes symptoms in the plants within three weeks.

In papayas, young plants die quickly, while older, productive plants show yellow leaves which drop. The fruits have blemishes, and the compromised trees produce fewer and smaller fruits until they eventually die. Apparently, all infected trees die. The papaya growing industry was nearly wiped after the disease first appeared in Hawaii in 1992.

Unable to find any way to prevent the spread of infection, even by aggressively removing diseased trees (a practice that goes by the name of roguing), research turned to genetic modification. No resistance to the virus was found in the cultivated papaya trees, but resistance was found in wild cultivars of the Carica species. Unfortunately, these were sexually incompatible with the papaya producing plants.

By splicing genes from the wild species into the Carica papaya, an immune response to the PRSV was developed. The two transgenic papaya strains are called Rainbow and SunUp. These transgenic varieties now comprise a majority of cultivated papaya in Hawaii. Consumption of the fruits from the transgenic papayas have demonstrated no adverse effects in humans, but studies have been limited.

Papaya Ringspot Virus also Called Watermelon Mosaic Virus

But what is PRSV doing in my squash patch? Despite my search, I have no idea how it got there. Apparently, the papaya ringspot virus type W was formerly called watermelon mosaic virus type 1 here in our growing areas. There are also a couple of other mosaic viruses that produce damage on our cucurbits.

Can I eat the damaged squash? Yes. The squash looks too beautiful to eat, BUT it is safe to consume.

Can other plants get this? PRSV Type W affects all cucurbits. This would be squash, pumpkins, watermelons, cucumbers, and gourds. Type P will infect papayas as well.

Should I collect all my cucurbit vines to destroy the virus? That is not necessary. The disease is spread by aphids in a non-persistent manner. It does not spread from plant to plant directly.

Can I still save the seeds? Yes, the virus lives in aphids, not in the plant material.

Papaya ringspot virus (PRSV-W) can infect a number of common garden crops. We'll discuss prevention and control, as well as using affected fruit.

Part of our 2017 squash harvest. Out of dozens of fruits, only a few were visibly affected.

Papaya Ringspot Virus Prevention

What can gardeners do to prevent future problems? Control the aphids! There are a number of ways to control aphids, including;:

Physically remove them with water pressure
Dilute soap solutions which work by dissolving the waxy coating on the little aphid bodies
Homemade bug control sprays or ready-made organic sprays
Creative use of plant materials– This includes cultivating trap crops away from your patch to attract the aphids TO the alternate plants or planting and interplanting repellent plants such as onions and garlic in your patch and around your garden. In my patch, the aphids had many lovely lambs quarters to dine on so this may have contributed to the viral presence in my squash. Weed hosts of the mosaic viruses include the lovely:
- wild cucumber
- lamb's quarters
- clovers
- pigweed
- mallows
Creative use of animals– Attract beneficial and predatory insects to your land. Ladybug larvae LOVE aphids, but there are other insects such as lacewings that will eat aphids, as well.

The first solutions will also affect beneficial insects. I prefer to concentrate on growing healthy plants with enough diversity of plant material to provide a balanced environment for our insect friends.

Overall, I did not have any significant plant loss or fruit damage, so we're not seriously concerned. Next year we'll keep a closer watch on the weeds, and focus on plant health and a balanced ecosystem with predatory insects.

You may also enjoy other posts listed on the Gardening page, including:

How to Start a Garden – 10 Steps to Gardening for Beginners

Small Garden, Big Yield – 10 Tips for a Great Harvest

The Ultimate Guide to Natural Pest Control in the Garden

References

Gene Technology for Papaya Ringspot Virus Disease Management – The Scientific world Journal

Papaya Ringspot Virus – Established on Maui – State of Hawaii Department of Agriculture

Recombinant DNA Technology and Transgenic Animals

This post is by Debra Ahrens. Debra lives with her family on a five acre hobby farm in northeastern Wisconsin which she often describes as ‘short on hobby, long on farm'. Besides the School of Hard Knocks (Life), she attended UW-River Falls, majoring in Dairy Science. Along with her husband Jerry and their three youngest daughters, they raise every kind of domestic poultry known to man, and maybe a few that shouldn't be known. Their furry animal family includes a flock of Suffolk sheep, dairy goats, a few rabbits, their dog and a lone beef heifer, Thelma. In her spare time, Debra is a poultry and sheep project leader for Kewaunee County 4-H.