As pumpkin harvest nears, it’s a good opportunity to review your cultural practices to understand what steps you can take in future years to mitigate disease and have a better pumpkin crop.
At this year’s Virginia Association for Biological Farming (VAFB) conference, Steve Rideout, plant pathologist and Extension specialist at Virginia Tech, gave a talk on pumpkin research on farms throughout the commonwealth as well as general best practices for managing disease.
There are four main strategies for managing disease, Rideout said: pest exclusion, cultural practices, choice of cultivars and chemical controls. Before you can you choose which strategies to use, you need to understand what diseases may be present in your pumpkin patch.
In Virginia, pumpkin viruses are transmitted by aphids or cucumber beetles. The viruses cause stunted vines with reduced production. They can also lead to smaller fruit and fruit which is not fully mature.
To prevent viruses, Rideout advised using certified seeds or transplants. Some cultivars are virus resistant, but no cultivar has universal resistance. Also scout for insects two to three weeks after planting.
Plectosporium blight, Rideout said, “keeps getting worse every year.” This soil-borne fungal pathogen is thought to be more problematic in operations with poor crop rotation, but it has been found in fields where cucurbits have never previously been grown.
“We also see fields where pumpkins have been grown for 20 years and there is no Plectosporium,” Rideout said.
The pathogen survives both in the soil and on plant debris, and is favored by warm, wet weather. There are no known resistant varieties. The symptoms of Plectosporium include pinprick leaf spots, more visible on the underside of leaves; a chalky residue on the pumpkin handle; and pinprick tan lesions on the pumpkin fruit itself. The leaf lesions are tiny and circular – different than downy mildew lesions, which are more angular.
“I’ve seen Plectosporium infect both no-till and conventional till fields,” Rideout said. “On plastic and on landscape fabric. We’re also starting to see it on squash and zucchini.”
Plectosporium on pumpkin. Photo courtesy of Steve Rideout
He suggested a number of control approaches. First, look at sanitizing field equipment. One grower reused lay-flat hose and found Plectosporium only where the hose was.
Second, use a two- to three-year crop rotation. “It may not help,” Rideout said, “but if you have Plectosporium and don’t rotate it will get worse.” As for fungicides, research is ongoing.
Cucurbit downy mildew is caused by the pathogen P. cubensis. It can be identified by angular leaf lesions and sporulation on the underside of leaves. In the morning, that sporulation reveals itself as a fuzziness.
P. cubensis does not overwinter in Virginia but migrates to the region with warm weather (and also on transplants). The first to arrive is clade 2, which is more heat tolerant and has more fungicide resistance. Clade 2 generally shows up on the Eastern Shore during the first week of August. Clade 1 shows up the last week of September. There is a website which tracks the appearance of downy mildew in the eastern U.S.: cdm.ipmpipe.org.
To suppress downy mildew, plant pumpkins in well-drained, full-sun areas.
Cucurbit powdery mildew appears as a cottony white mycelial growth on vines, Rideout explained. “For me, management of powdery mildew starts with cultivar selection,” he said. Homozygous cultivars tend to be more powdery mildew resistant than heterozygous. Powdery mildew does not have different clades.
Phytophthora crown and fruit rot looks like powdery mildew when it affects pumpkin fruit. It can also affect the leaves of the vine. To manage Phytophthora, rotate crops and avoid low-lying areas. Avoid over-irrigation as well.
“Phytophthora can survive in standing water including irrigation ponds,” Rideout said. Varieties with thicker-rind fruit tends to stand up better to Phytophthora. Planting on raised beds can also help.
Rideout also spoke about a number of field trials. One variety he highlighted was Snowball. “If you want to grow a two- to three-pound white pumpkin, Snowball is fantastic. It produces really, really well and has good disease resistance.”
For more information on improving your pumpkin production next year, check with your local Extension agent.
Every year, VABF hosts an annual conference with trade show and dozens of seminars. In 2024 over 50 exhibitors and 600 attendees participated. For more information, go to vabf.org.
by Karl H. Kazaks
