The drought may stop the spread of some plant diseases in California this year, but what some growers and researchers are finding is the insects that typically feed in weedy areas and pastures moved to cropping areas earlier in the season as the weedy areas have dried up.
Looking for new places to feed, these insects migrate from the non-crop areas to crops, transmitting viruses and disease along the way.
“Everyone knows that California dries up this time of year anyway, but this has happened earlier with the drought,” says Steve Koike, plant pathology farm advisor, with University of California Cooperative Extension, Monterey County. “I think with the early drying of the whole state of California, all the insects that sometimes have the viruses in them are fleeing the dry areas early and going to the crops. As a result, we are seeing some early virus issues.”
Viruses Of Cucurbits
Viruses that typically impact cucurbit crops such as watermelon, squash, pumpkin, and cucumbers include cucumber mosaic virus, papaya ringspot virus, watermelon mosaic virus, and zucchini yellow mosaic virus. All of these are aphid vectored viruses. California also gets some squash mosaic virus which is spread by the spotted cucumber beetle, Koike adds.
The final malady seen periodically out West is the curly top virus, which is vectored by leafhoppers. This disease impacts cucurbits and a host of other crops.
Koike points out that it is very difficult to distinguish the symptoms of the different viruses. He adds, however, that in most cases, growers know that on all cucurbits, these viruses cause leaf deformities and discoloring of the foliage. Plus the fruit, although edible, is completely unmarketable.
“The fruit have bumps and are misshapen,” he says. “They are very much edible but not marketable because of how they look. Unfortunately, once the plant is infected, the grower can’t do anything to correct the problem.”
Infected But Productive
All is not lost, though. In some cases, the plant can be productive, and the fruit can be marketable, depending on when the insects fed on the plant.
“You may have a virus that causes the leaves to become yellow and deformed but you still get zucchini fruit, for example, that look pretty normal,” Koike says. “You have to observe what is happening with the crop because you may be able to still get fruit off the plants.”
It is a timing issue, he says, as growers can continue to harvest until they see that the fruit are no longer usable.
“These viruses are all systemic so they eventually move to all parts of the plants,” Koike explains. “Once that happens, and if it is a fruit deformity type virus, all the fruit will start showing deformities.”
Employ IPM Tactics
Koike advises growers to keep up with a sound IPM spray program keeping the viruses in mind, but warns that even though you have sprayed for the insect, whether it is aphid, beetle, or thrips, it doesn’t mean your plants are completely protected from the virus.
“It takes the insect anywhere from 30 seconds to 5 minutes to inject the virus,” he explains. “So the insect pest can feed for a brief amount of time, inject the virus, infecting the plant, but the insect then dies because you sprayed it. We know from research that vector control does not prevent diseases. However, the grower should still manage the vector.”
To further reduce the incidence of disease and virus pressure, Koike encourages growers to manage weeds and vegetation around cucurbit fields. He also says for growers who have the luxury of site selection, and are in an area that is at high-risk for viruses, to consider moving the cucurbits to another field. “Put your radishes in that field and put your squash someplace else where it is less of a risk,” he adds.
In the end, however, by paying close attention to what is occurring in your fields, you will be able to mitigate losses by keeping up with an IPM program and correctly identifying the insects and symptoms of viruses.