by Sally Colby
Growers in some western states and throughout the Midwest and East have had increasing problems with early season virus infection on cucurbits. “Normally, we see virus infections in late July and early August,” said Dr. Gerald Brust, University of Maryland extension. “But these virus infections are starting in late June and early July. When you see it then, it’s going to cut into yield quite a bit.”
Brust says some growers insist the virus is in the seed because they hadn’t seen any signs of aphids that commonly transmit the virus. “I wanted to look at why we were having early season virus,” he said. “One of the problems is that we don’t see it in every single field. We see it in some fields one year real bad, then it wouldn’t be there the next. It isn’t consistent.”
One grower who consulted Brust about the issue was rotating pumpkin fields from year to year. The grower reported that one field always had early virus infection and another field didn’t. Then he’d go check that field again and it would have early season virus infection.
To compound the problem, signs of infection are not evident until three to four weeks after the initial infection. “That makes it difficult to find out what’s causing the damage,” said Brust. “You’re seeing it now, but the infection took place two or three weeks prior. It’s impossible to go back and see what was out there feeding on your pumpkins.”
Brust says the virus tends to move toward parts of the plant that are growing rapidly, such as new growth. Once the virus is present, the fruit becomes deformed, even if the fruit has turned orange and appears to be developing normally. Brust noted that he has seen pumpkins that make it through the early infection and look like they’re growing normally but will start to rot earlier.
“The grower has a field of pumpkins that look normal and are growing as they should but are suddenly rotting,” said Brust. “You’ve done a good job spraying fungicide, but they shouldn’t be rotting in the field.” Some pumpkin fields with 35 percent to 45 percent infection rate will have yields reduced by 50 percent or more. Most infections were due to watermelon mosaic virus (WMV) or an undetermined potyvirus.
“Most viruses that hit pumpkins are potyviruses,” said Brust. “Watermelon mosaic is pretty much the same as it is throughout the Midwest and the Mid-Atlantic.” Brust added that there are several other causative viruses, including papaya ring spot virus and yellow mosaic virus, but after collecting and analyzing samples from pumpkins, the predominant virus was WMV. “It’s called watermelon mosaic-2 because watermelon mosaic-1 has been given a different name.”
Brust says early season aphids were thought to be responsible for the early infection. “In general, when we see potyviruses in pumpkin fields, aphid activity is very high,” he said. “That’s because only a small percentage of aphids carry the virus. Most do not. It can be as small as one percent, but it it’s in 10 to 15 percent of aphids carrying the virus, there will be a huge virus outbreak in the field.”
“Colonizers are the ones that stay behind and don’t infect pumpkins. It’s the ones that come through, infect the pumpkin then move to another field because they don’t like to feed on pumpkins. That’s why you don’t see colonizing aphids in the field, yet you have virus infection in the field.”
Infections usually start out localized, with patches of infection here and there, then if a lot of aphids are infected with the virus, more and more areas of a field become infected rapidly.
Brust described field studies to determine what was causing the spread of early virus. “We used yellow pan traps,” he said. “These traps simply tell us how many aphids landed in the field. We can’t count the number of aphids on a plant because most are transitory. They land on a pumpkin, leave the virus behind, then move on.”
Aphids prefer to land on a yellow surface, and with soapy water in the trap, the aphids can’t get out. “We count how many land to get an idea of how many aphids came through a field,” said Brust. “What we found is high aphid numbers coming through the fields in late June and early July. This eliminated aphids as the main cause of early season virus because the infection had to take place earlier, and there weren’t enough aphids to transmit enough virus to the field like we were seeing.”
What about striped cucumber beetles? “They can show up early in the season and feed very heavily,” said Brust. “They like pumpkins, especially when they’re small, and like the cotyledons when they first come up. If there are a lot of beetles, they will defoliate the plant and kill it. There are also beetle populations that are low enough that you don’t really notice them.”
Brust says using a neonicotinoid with the seed at planting will afford several weeks of protection, but feeding will be noticeable after that. Growers often believe they can stop aphids from transmitting viruses with frequent spraying, but even daily sprays are ineffective.
Research involved beetles raised in greenhouses with pumpkins. “Pumpkins that were fed upon by these beetles became infected, and the only thing that they became infected with is WMV,” he said. “Any pumpkin that had a lot of feeding and had more than five beetles in with it became infected. Supposedly beetles can’t carry and transmit WMV, but that’s what was happening.”
Brust explains that viruses enter the aphid’s stomach and leave via body fluids. “The aphid’s regurgitant contains enzymes that break down plant tissue so that when the aphid feeds on the plant, they can absorb it much more quickly,” he said. “When they do that, viruses can go into the plant.”
Brust reminds growers that striped cucumber beetles also transmit bacterial wilt. “When that bacteria is near an open wound, the bacteria move into it,” he said. “But it has to be an open wound. It’s the same thing with viruses – they need a fresh, open wound made within the last four to six hours – then viruses in the regurgitant get into the wound faster.”
Brust says before viruses can get into the plant, the beetle adds enzymes to help break down the food. The enzymes can also break down viruses that prevent them from moving into the plant.
Where are viruses coming from? The main culprit is volunteer cucurbits and weeds such as lambsquarter. “Lambsquarter is a problem in pumpkin fields due to herbicide resistance,” said Brust. “Lambsquarter was spotted in or near many of the fields
How does the virus get from lambsquarter to pumpkin? “For some reason, aphids picked up the virus more easily from a weed and were able to transmit to the pumpkins,” said Brust. “When aphids can go back and forth from weeds to pumpkins, you’ll have more early-season infection of pumpkins. If you can keep your field clean of lambsquarters, it will help.”
If cucumber beetles have a place to feed, they’ll stay there to feed, then overwinter, lay eggs in spring and continue to repeat the cycle. Fields that have problems will continue to have problems.
Brust suggests growers first determine whether striped cucumber beetles are a virus vector in the field. He says growers who use an intensive control program for striped cucumber beetle in problem fields have been successful in reducing the rate of early season virus.
Because beetles feed at the base of a cucurbit plant, Brust advises the use of a drop nozzle to make sure the spray reaches that area. He noted that seems to do a good job controlling cucumber beetles as opposed to using a fan spray over the whole plant.
Understanding early-season watermelon mosaic virus in pumpkins
by Sally Colby