Downy and powdery mildew, along with phytophthora and other blights, may be the nightmares of many a pumpkin grower. These fungal diseases certainly make those pumpkin vines look downright scary. They can also significantly reduce yields or cause problems if infected fruit is harvested and put into storage. Because these fungal pathogens can persist in the field for years, good rotation, removal of plant debris and careful attention to equipment disinfection are all warranted.
Fungal diseases are common in pumpkins in many growing regions, and most growers have protocols in place to mitigate damage. But there are other diseases – such as those transmitted by aphids or other insect vectors – which can cause damage and create some frightening fruit. Discoloration, bumps, warts and rough spots can all be attributed to pumpkin viruses.
Insect Virus Vectors
Aphids transmit several viral diseases of pumpkin: papaya ringspot, watermelon mosaic, zucchini yellow mosaic and cucumber mosaic viruses. Depending on when aphids feed on plants, the viruses they transmit can cause changes in the fruit.
Aphids infect plants by transmitting a virus during feeding. These aphid populations are typically transient, moving from place to place and not establishing themselves in one field. Aphids feed on infected plants, then feed on susceptible crops, transmitting the virus. Aphids take bites out of plants to see if they want to feed on them. Even if they don’t, that first bite is a viral one! This is why insecticides are of limited value – the damage is done before the aphid would be killed.
While only a few plants may become infected through this initial feeding by aphids, secondary spreading, as aphid populations increase throughout the season, can further disperse plant viruses. Later crop plantings may have more damage than early ones due to increased aphid populations. Numerous aphid species can be involved in virus transmission.
Insecticide use may contribute to aphid buildup, if beneficial insects are eliminated in an attempt to control the aphids. Hot and humid weather tends to favor aphid population buildup.
Cucumber beetles can also vector some of the pumpkin viruses – and are the vectors for bacterial wilt.
Viruses cause stunted growth of vines and fruit and may impede fruit set. Fruit produced by plants infected when young is significantly more deformed than that from plants which were mature when infected. Plants infected prior to pollination don’t typically set fruit, and any fruit that does form will be small, deformed and develop bumps. It may brown early. When plants are infected after pollination, pumpkins which are the most mature may only show some color changes, with patches of green or yellow color.
Later infestations can cause sooty mold, which arises due to the buildup of honeydew, which attracts mold onto the fruit. The mold colonizes the outside of the pumpkin, leaving black spots. These do not impact fruit quality.
Leaves of virus-infected vines can be mottled, with a mosaic color pattern, and deformed, stunted and twisty leaves are hallmarks. Leaves may appear shriveled. Viral symptoms can vary if there is more than one virus present in the plant. The viral load in the plant impacts symptoms, as does temperature. Different pumpkin cultivars can also show variety in symptom expression.
Older plants will take longer to show visible symptoms of viral infection, with a lag time of one to two weeks after they are initially infected. Younger plants will show symptom development within a few days of infection. Symptoms will emerge more quickly at higher temperatures. Shorter days and less intense light favor more severe symptom development.
Viral damage to pumpkins can’t be cured, but it can be mitigated. Late planted fields should not be nearby or downwind of susceptible fields. Planting grain sorghum or other summer grasses as trap crops around pumpkin fields may shield the crop from aphid damage.
Keeping weed populations down in and surrounding pumpkin fields is a control method. Papaya ringspot virus survives in weed populations along fencerows and in uncultivated fields. Watermelon mosaic virus 2 is readily transmitted from weed populations, as is zucchini yellow mosaic virus. Cucumber mosaic virus overwinters in perennial flowers. It can also be transmitted directly via seeds from infected plants. Leaves curl downward, and flowers may become deformed, with green petals.
Reflective mulches can deter aphids. Mineral oils and insecticidal soaps can inhibit aphid feeding, and disease resistant cultivars may be available.
Other issues can cause disfigured pumpkins too. Sunburn can cause discoloration, which then progresses to a shrunken spot several days later. This can occur when field curing pumpkins, or on fruit from damaged, diseased vines where pumpkins are left without adequate protection from the sun. Sunny days with low humidity when field curing pumpkins are risk factors.
End of the season frost damage can cause problems for late harvested pumpkins. Maintaining healthy green vines and leaves offers protection. Water-soaked spots which cause the rind to soften are indicative of frost damage, and this may progress to fruit rotting.
Bacterial leaf spot in pumpkins also causes changes to the fruit. Small, raised, colored spots proliferate on the pumpkin. These can eventually coalesce into flattened lesions as the fruit matures.
Pythium fruit rot will impart cringe-worthy damage to fruit, which is more than cosmetic. Pythium infection causes a cottony-fluffy looking white rot. Pumpkins with black rot – fungal disease – will have large, black oozing areas on the rind.
The fungal agent responsible for plectsporium blight can riddle pumpkin “handles” with lesions, weakening them, and cause white russeting on the fruit. Other fungal pathogens can cause severe fruit damage and rot too.
Although viruses can cause mild to severe cosmetic damage, these marred pumpkins may still be marketable as novelty decorations. While no one is trying to raise an unhealthy pumpkin, a bit of cosmetic damage may not mean that your pumpkins can’t be marketed – at least during Halloween season.