Fall cucurbit production always proves to be more of a challenge for Southeastern growers due to increased disease pressure, especially from virus diseases.
These viruses cause serious losses every year during the fall, says David Langston, University of Georgia Extension vegetable pathologist.
There are four primary cucurbit viruses found in Georgia, says Langston. These include watermelon mosaic virus II (WMVII), zucchini yellow mosaic virus (ZYMV), cucumber mosaic virus (CMV), and papaya ringspot virus (PRSV).
“These viruses primarily are transmitted by aphids in a non-persistent manner,” says the pathologist. “This non-persistent transmission usually takes only a few seconds to acquire from and transmit to plants.”
The aphid populations, he adds, tend to build up during the summer, resulting in higher virus transmission later in the summer. “These aphids can travel long distances to initially transmit viruses to fields in Georgia. Once in a field, aphids can move from plant to plant and spread viruses within the field through secondary means.”
The primary inoculum for these viruses can come from naturally occurring weed hosts, abandoned cucurbit fields, nearby cucurbit fields or volunteer cucurbits, notes Langston.
“It's believed that most transmission comes from nearby sources because the retention time of these viruses is very short — only minutes or hours. Some retention times have been documented at 20 to 30 hours.
“Seed transmission is not thought to play a major role in the cucurbit mosaic disease complex. Seed transmission has been documented consistently only in CMV, and that was in weed hosts and cowpea,” he says.
Once a plant is inoculated with one of these viruses, says Langston, the virus particles can travel systemically throughout the plant. Variations in temperature, light, crop variety, viral strains, host vigor, viral concentration and mixed viral infections all can have an effect on the expression of symptoms.
“Symptoms usually show up anywhere from three days to two weeks following infection. Infected plants may appear asymptomatic because the incubation period for the virus may not have been completed, or the symptoms may be masked by certain environmental conditions.
“Sometimes, symptoms may show up on one part of a plant and not on the other, such as a symptomatic fruit on a plant with asymptomatic foliage. No one can rely on symptoms alone to accurately identify specific viruses. Lab testing is required to accurately identify a virus. The cucurbit compendium will have complete descriptions of virus symptoms.”
The best control for cucurbit viruses is planting resistant varieties, says Langston. These varieties have either traditional “host” resistance or “pathogen-derived transgenic” resistance.
“Another type of resistance that actually is more like tolerance is a ‘masking’ of viral symptoms through the use of a precocious yellow gene in squash. Most of the virus resistance is in squash. Of these types of resistance, the transgenic is the most expensive so far as seed are concerned.”
Cultural disease control practices such as destroying abandoned cucurbit fields, weed hosts and volunteer cucurbits all reduce available virus inoculum, he says. Avoiding late plantings of squash also can reduce virus losses, he adds.
“Insecticide sprays alone are not effective in controlling viruses. Stylet oil sprays can reduce the spread of aphid-transmitted viruses. These oil sprays must be applied when the foliage is first available to aphids, at or above 400 psi, and on a three to four-day schedule. If any of these are omitted in the oil program, virus control is greatly compromised. Reflection mulches that are bright and shiny have shown promise in other states.”