Watermelon Mosaic Virus: Treating Watermelon Plants With Mosaic Virus

By: Bonnie L. Grant, Certified Urban Agriculturist

Watermelon mosaic virus is actually quite pretty, but infected plants may produce less fruit and what they do develop is malformed and discolored. The damaging disease is introduced by a tiny insect so small they are difficult to see with the naked eye. These little troublemakers can cause serious adverse effects in watermelon crops. Here are some tricks on recognizing the disease and minimizing its damage.

Diagnosing Watermelon Plants with Mosaic Virus

Watermelon leaf mosaic disease stems from Potyviris, a common virus in cucurbits. The disease symptoms are different among the squash, melons, gourds, and even wild cucurbits that it infects. Peas and alfalfa are also affected. Mosaic virus of watermelon shows up on the leaves initially but persists to spread to the stems and fruit. Effective control can only be achieved by a gardener’s vigilance and good cultural practices.

The first signs of infection are yellowing of the leaves and marginal chlorosis. The yellowing is most often at the leaf veins and edges and is irregular, resulting in a characteristic mosaic form. Young leaves deform and distort. Leaves are smaller than usual and have blister-like regions.

If any fruit form, they are dwarfed, discolored, and may have mottling and a warty appearance. The flavor is not significantly affected but the marketability of the fruit is diminished. Because less fruit form, crop sizes are greatly reduced. Additionally, the disease spreads easily and can affect many other crops.

Controlling Mosaic Virus of Watermelon

Treating watermelons mosaic virus can be tricky, but the first step is recognizing the problem. It also helps to know how the disease is transmitted. It is only moved into plants through feeding activities of several species of aphid or from leaf miners.

The infection is only transmissible for a few hours but during high feeding time, insects can infect a host of plants. The virus can also overwinter in seed or host weeds. Plants installed in the later period of the season are more greatly affected because insect numbers are high.

The most important management strategy is cleanliness. Remove all old debris and keep manual and mechanical tools sanitized. Crop rotation is also a recognized method for minimizing the incidence of the disease. Keep the area free of weeds, especially wild cousins of the sweet potato, which can harbor the virus. Remove and destroy infected plants to prevent the spread of the disease. Insect control is essential.

Use insect barriers where applicable. Some gardeners swear by a mulch of reflective silver plastic mulch around the plants. Apparently, the insects do not like the shine, but it is only effective until vines and leaves cover it up. Insecticides are not useful as the insect has time to transmit the virus before it dies.

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Watermelons (Citrullus lanatus) are annuals that can be grown in most places with a warm growing season that is at least 70 to 130 days long, as that is how long it takes for watermelons to produce fruit after planting. While they are fast growers, many things can slow the rate of the growth of both the fruit and the vines. You may not be able to control some of the factors, but you can prevent or correct other factors to have better success next year when you plant watermelons again.

Difference between red and yellow watermelon

In addition to the color difference, red fleshed watermelon contains lycopene, an antioxidant that makes fruits and vegetables red and pink.

Yellow lacks lycopene. The lycopene content is the big difference.

Yellow, red and pink watermelons are high in vitamin A and C. Other nutritional benefits and health benefits are similar.

In addition, both red and yellow melons have the same type of crisp texture, grow with rinds, and can grow with seeds or seedless.

Maryland Grows

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!

Photo by Dr. Gerald Brust of watermelon mosaic virus on pumpkin. Wait, what?

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!

By Erica Smith, Montgomery County Master Gardener

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