Continuing Education and
Communication Services
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
10/8/01
Contacts:
Barbara Ambruzs, Plant Pathology, (515) 294-1741, ambruzsb@iastate.edu
Del Marks, Continuing Education and Communication Services, (515) 294-9807,
delmarks@iastate.edu
Yard and Garden Column for Week Beginning Oct. 12
Fall Holidays Without Pumpkins? Scary!!
By Barbara Ambruzs
Plant Pathology Specialist
Iowa State University Extension
Pumpkins may fall prey to seven diseases caused by bacteria, at least 39 diseases caused by fungi, 10 nematode-associated disorders and 18 virus diseases. How in the world is it that every fall there are pumpkins available for carving and pies?
Luckily not all 74-plus diseases occur in one location. In Iowa, only a few diseases have the potential to devastate a pumpkin crop. Some of the more common diseases are viruses, downy mildew, powdery mildew, bacterial leaf/fruit spot and black rot.
There's no treatment for virus diseases. The major symptom to look for is deformed leaves. Infected fruit may be discolored, spotty and/or misshapen. (Sounds good for scary Halloween pumpkins!) Many viruses are spread by insects, usually aphids. They also can be spread mechanically on tools or hands. Early-planted pumpkins are less likely to be damaged by virus diseases. Controlling weeds in the vicinity of a pumpkin patch can help since many weedy species are sources of viruses. Plants infected with a virus should be removed and destroyed.
Downy mildew is caused by a fungal-like organism. Usually only leaves are infected. Cool, wet weather favors disease development, so it doesn't normally show up until late in the season. The fungus doesn't survive winters in Iowa, so the spores must be blown up from the south. Symptoms to look for are angular, yellow spots on the tops of leaves. On the underside of the leaves a purplish-brown mold develops. Planting early season cultivars can reduce the threat of the disease since it develops late in the season.
Plants infected with the powdery mildew fungus may produce smaller and fewer fruit. The same applies for downy mildew infected plants. Powdery mildew develops as a whitish talcum-powder-like fungal growth on both the top and bottom of leaves. Other above-ground parts of the plant can be infected, too. Older leaves are usually affected first. The under-side of leaves may be more severely affected because they are shaded. Sunscald could be a problem on the pumpkins since infected leaves usually wither and die, leaving the fruit exposed. Unlike the other diseases mentioned here, resistance to powdery mildew is available in a few pumpkin varieties.
Bacterial leaf spot symptoms are small, dark, angular spots that can grow together when the disease is serious. Spots on the fruit are small, round, scabby and often clustered. To manage this disease, overhead watering should be avoided, since bacteria are spread by water and prefer moist environments. Copper treatments can give good results if used before a full-blown infection.
Black rot can be quite dramatic. All plant parts can be affected. However, this disease is best known for large, rapidly expanding, water-soaked spots on the fruit. Black, pimply structures, called pycnidia, form in the center of the spots. These are tiny vessels that are filled with fungal spores. If a planting site has a history of black spot, cultural practices, such as watering at ground level, rotation and sanitation, alone may not be sufficient for control. Fungicides are available that can help with management.
For disease management, it's important for plants to start out as healthy as possible. Getting seed from a reliable source will increase the rate of success. Some seeds come with harmful organisms or have poor germination. Moving the location of your pumpkin patch from year to year is recommended since many disease-causing organisms can survive the winter on residue. Use resistant varieties where available. Avoid wetting leaves when watering; water the soil rather than wetting the leaves. Avoid wounding plants and fruit. Plant in raised beds to aid with drainage issues. If using fungicides or other pesticides, also use good cultural practices to maximize effects. Remember to read labels and apply recommended rates.
Here's one last word on rots. Have you ever spent a wonderful fall evening carving, with such great care, the perfect Jack-o-lantern? In a few weeks what does that piece of art end up looking like? Usually a mushy, melted-down, smelly mess. Carving creates huge wounds. Fungal spores are floating in the air waiting for a food source to become available. There are plenty of sugars for a fungus to feed in your pumpkin's eyes, nose and mouth. The fungus produces digestive juices and absorbs nutrients until they're all gone. If you want your pumpkin to look its best for the holiday, hold off carving it until right before the trick-or-treaters come knocking.
Maybe this Halloween will be scary, but it won't be because of a lack of pumpkins.
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ml: isugarden
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