Farmer's Weekly (South Africa)
Dealing with cucurbit viruses
Viruses are the greatest threat facing the cucurbit grower. It’s rare that a crop will be grown without some symptoms appearing.
With a disease such as powdery mildew, to which some cucurbit species are vulnerable, you can at least stop it from spreading, or reduce the damage through appropriate treatments.
In the case of a virus, however, you need to delay, or even prevent, its introduction, especially when the crop is still young. The alternative is to slow down its spread.
In South Africa, growers have to deal with four different cucurbit viruses, as well as combinations of these four: • Cucumber mosaic virus (CMV), perhaps the most common of the four;
• Zucchini yellow mosaic virus (ZYMV);
• Watermelon mosaic virus (WMV);
• Papaya ringspot virus (PRSV). Identifying the specific virus you’re dealing with requires a lab test known as an enzymelinked immunosorbent assay (ELISA). But there’s no need to go to this trouble, as there is no cure for any virus once it has infected the plant!
Instead, as already mentioned, you have to try and stop the virus from taking hold.
getting rid of broadleaved weeds
The first step is to prevent the virus from overwintering on infected weeds close to the lands. Do this by eliminating broadleaved weeds around the lands in the off-season with a suitable herbicide. Years ago, I made the mistake of not doing so and had a virus strike the first young plants.
Bear in mind that CMV, in particular, has a very wide range of host crops, including the ubiquitous chickweed.
Look under the leaves and buds of the plants at regular intervals
controlling aphids
Aphids are the main vector. You cannot prevent them from entering the land, but you can reduce the speed of infection within the land. For example, I often find a dark-coloured aphid attacking the young flower buds, and I sometimes fail to notice it until the population increases and the leaves start to distort. By this time, the damage has usually been done.
Look under the leaves and buds at regular intervals. Apply a suitable insecticide at the first aphid sighting, or even preventively.
Some virus damage is inevitable, but, as noted, you can take measures to reduce the spread.
When an infected aphid enters the land, it will take up to a couple of weeks for symptoms to start showing.
If only a few plants show symptoms and no further aphid activity occurs, the virus will not spread. (Many aphid species probe a few plants in the area before moving on to search for a more suitable host.)
It’s worthwhile looking for these isolated infected plants on a regular basis. When you discover one, carefully remove it and place in a clear plastic bag. Leave this in the sun with the opening folded over to generate heat and so destroy the contents.
Once you have discovered an infected plant, make a point of returning to the area and repeat if necessary.
• Bill Kerr is a vegetable specialist and a breeder of a range of vegetables. Email him at farmersweekly@ caxton.co.za. Subject line: Vegetable production.