Pumpkin Disease Update – (Rick Latin) – Many growers have called expressing concern over an apparent lack of fruit set in their pumpkins. As you know, poor fruit set is not likely due to an infectious disease problem. High daytime and nighttime temperatures during most of July may be responsible for the production of fewer female blossoms. Also, high temperatures may keep bees from their normal activities and may interfere with the pollination process itself.
Finally, if there is any kind of fruit set and a stressful period begins, it is possible that young fruit will be aborted to save the earlier set. Be that as it may, it is possible for virus infection to contribute to poor fruit set. If the infection occurs early in the development of the fruit, or before fruit are formed, then it is likely that the plant will produce only a fraction of the fruit it normally would set, and those probably will be of very poor quality. The viruses are very common in the Midwest in late summer. We are all familiar with the mosaic or 2-4-D type of symptoms that appear on foliage. Severe damage is associated with early infection and appearance of these symptoms. If the symptoms are just occurring now, then it is not likely that the virus will affect your crop. The virus most likely survives in perennial plants and weeds in wooded areas, along fence rows, and in ditch banks. I suspect that transmission to other weeds occurs in the spring. Sometime during the summer, aphids may enter a field after feeding among the weeds, and transmit the virus to pumpkins. The viruses also can be mechanically transmitted. By that I mean that it can be carried and spread with plant sap if a tractor drives through the field or a crew of workers damage plants during cultivation.
It is my opinion that the aphids may be responsible for introducing the virus into the field, but the spread and increase is probably due to mechanical transmission. Powdery mildew apparently got off to a late start this year and is not likely to cause much damage across the region. If you can keep it in check for another week or so, then its affect on the crop will remain minimal. I might be concerned about downy mildew outbreaks between now and October. A few cases of downy mildew have been discovered.
A wet September might increase our concerns and downy mildew management efforts. Phytophthora blight remains a concern, despite the hot dry weather. I suggest that you continue protective sprays, especially if the field has a history of the disease or if you have heard of an outbreak nearby. It is probably too late to do anything about it, but the bacterial leaf spot pathogen appears to be fairly widespread. This pathogen also attacks young fruit.
Symptoms of fruit infection include raised white scabs or blisters on the face of the pumpkin. This disease was identified within the past few years. We are still in the process of learning about how the pathogen survives and spreads, and what kinds of conditions prompt serious losses.