Extension Communications Extension 4-H Youth Building Ames, Iowa 50011-3630 (515) 294-9915

10/11/04

Contacts:
Paula Flynn, Plant Pathology, (515) 294-0581, pflynn@iastate.edu
Del Marks, Continuing Education and Communication Services, (515) 294-9807, delmarks@iastate.edu

Garden column for the week of Oct. 15, 2004

Naturally Scary Pumpkins

By Paula Flynn
Extension Plant Pathology
Iowa State University

Sketching, cutting, scooping, scraping and carving pumpkins can be a time-consuming and messy job. However, this fall tradition is usually thought of as a fun chore, with results ranging from simple triangle eyes and square teeth to complicated silhouettes of eerie figures. It seems Mother Nature doesn't like to miss out on such Halloween fun, and has found her own way of creating scary pumpkins. There are numerous natural organisms lurking in the garden that can turn a pumpkin into a distorted, sometimes disgusting, tribute to Halloween.

Viruses can cause pumpkins to grow into strange and unusual shapes. These disease-causing particles are so small they cannot be seen by the naked eye or even with an average microscope. The viruses that infect plants do not cause diseases in humans. More than a dozen viruses can infect pumpkins, the most serious known as zucchini yellows mosaic virus. Small, soft insects called aphids feed on the sap of plants and spread this virus from plant to plant. Pumpkins that are infected by the zucchini yellows mosaic virus tend to be malformed, with bulging blisters and an interesting color mosaic of orange, yellow and green. These unique creations by Mother Nature are definitely an eye-catcher in a fall display.

Fungi are another group of very small organisms that can drastically alter the appearance of pumpkins. We have all witnessed the appearance of these often fluffy organisms, whether on old bread left on the countertop for too long or on a fruit or vegetable forgotten in the refrigerator. Certain weather conditions, usually wet weather, favor the invasion of fungi into plants. Some fungi invade pumpkin fruit and eventually cause them to rot. Species of the fungi Phytophthora and Fusarium are the most commonly found fungal foes. With time and moisture, they cause the appearance of fluffy white tufts of fungal growth on the fruit, sometimes erupting in scattered spots, large or small. In other cases, the fungus may cover the entire underside of the fruit. Many fungal organisms are found in the soil and find their way to the fruit by splashing rainwater. Unfortunately, once these fungi invade the fruit, it can rot and collapse quickly. Such a pumpkin set out on a front doorstep early in October can be transformed into a colorful, but flat display by Halloween night. Candy gatherers might shout "eeeewwww" instead of trick-or-treat!

Another fungus, Cladosporium, causes a disease called scab on pumpkins. This fungus decorates pumpkins in a slightly different way. It causes small, scattered sunken cavities on the fruit, creating the look of a jack-o-lantern with the measles or chicken pox. To make nature's scary look even more dramatic, secondary bacteria often invade these spots and cause the production of a sticky ooze with a foul odor.

Although the thought of enjoying the scary results that nature can bring to pumpkins is amusing, encouraging diseases in pumpkin patches is not a wise strategy for pumpkin growers. Producers hope to grow and harvest the fruit so it has a long life. Pumpkins that have been invaded by pathogens like the ones described above typically become soft and rotted, sometimes completely collapsing into a mushy mess. Unfortunately, timing the gruesome transformation to occur on Halloween night is virtually impossible.

Keeping pumpkins healthy is a season-long job that includes rigorous scouting. Some of the most serious pumpkin diseases start out as tiny spots on the leaves that can stealthily progress to the vines and fruit if they are not controlled early. In addition to scouting for microscopic pathogens, growers have to scout for insect infestations too. Since certain insects can spread diseases, the primary way to control pathogen problems may be to control insect infestations. Fortunately, new pumpkin varieties with disease resistance are constantly being developed. Avoiding diseases in the first place is the most economical way to keep pumpkins healthy. So, it's likely that we'll have to continue creating scary pumpkins with kitchen utensils and paint. That means growers may have some difficulty finding a big market for the scary pumpkins created by Mother Nature.

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ml: isugarden

Editors: Two color photos, suitable for publication, are available at right. Click on each thumbnail photo to go to the fullsized photo. The top picture's fullsize photo is 180K and the bottom picture's fullsize photo is 440K. Caption: Pumpkins that are infected by the zucchini yellows mosaic virus tend to be malformed, with bulging blisters and an interesting color mosaic of orange, yellow and green.

Caption: With time and moisture, the fungus Fusarium causes the appearance of fluffy white tufts of fungal growth on the fruit, sometimes erupting in scattered spots, large or small.

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