EAST LANSING, Mich. — While there has been an increase in consumer demand for so-called “ugly” pumpkins, sometimes traditional jack-o-lantern, pie and mini pumpkin varieties are found with unwelcome warts, indentations and coloration. This may seem like it could be some kind of seed mix-up, but the culprit may actually be a plant virus.
There are many viruses that impact all kinds of vine crops, all producing similar symptoms that make identifying the exact virus impossible without doing lab tests. Exact virus identification isn’t as critical for virus management since there isn’t any way to cure or alleviate symptoms in an afflicted plant. One of the most common viruses is cucumber mosaic virus, which can infect a variety of weeds in addition to cucumbers, melons, squash, pumpkins and other vegetable crops. This multitude of hosts allows the virus to be present across the landscape, and it moves into cultivated crops through aphids, who introduce the virus into new plants after feeding on an infected plant. Other viruses include watermelon mosaic virus 2 and zucchini yellows mosaic virus, which are also transported via aphids.
Inspect the plant itself, not just the fruit, when confirming a virus incidence. Look for contorted leaves that are shriveled and puckered. Another symptom is a change of coloration in the affected leaves, where seemingly random portions of the leaf are a few shades lighter or brighter. These kinds of leaf distortion are also symptomatic of herbicide injury. However, if you also see discolored, deformed fruit, you have good evidence that a virus has infected the plant.
Undersized fruit is another symptom. It is possible that only the more recently developed portion of the vines and newly formed pumpkins will show symptoms, as the virus can only move outward from the point of infection.
While these immigrant aphids can infect a portion of the crop, they can further spread the viruses from one plant to another if aphids are allowed to build up in the field. This is complicated by the fact that many perennial weeds can host viruses and aphids, making weed control another aspect of the virus management equation. Checking regularly for aphids and intervening before their numbers get high will likely reduce the overall number of plants with virus symptoms. Aphids are notorious for quickly building up large populations in hot and humid weather. Often, the reason behind this is that natural enemies—insects such as lady beetles that eat aphids—are wiped out by insecticide applications, and aphids can reproduce without control. For this reason, using insecticide judiciously or choosing products that are less harmful to beneficial insects, such as stylet oils, Fulfill or Beleaf, can save natural enemies while suppressing aphids.
If you direct market pumpkins, simple marketing strategies may allow striped or bumpy pumpkins to be sold with other novelty pumpkins. While these pumpkins may be smaller, greener and lumpier than what you envisioned, the virus infection only impacts appearance, not the shelf-life of the pumpkin.
For more information on viral infections of pumpkins and other vine crops, see the Michigan State University Extension articles “Developing a virus reduction strategy for cucurbits” and “Virus and vine crops.” Always follow label rates and instructions for pesticides.