Jul 31, 2009
Leaf Spot Disease Threatens Maine Wild Blueberries

Valdensinia leaf spot, a fungus that thrives in wet years like this one, has moved from Canada into Maine and has been found in spots in eight wild blueberry fields there. The fungus spreads quickly, causing leaves to drop in crop fields and disrupting the bud set cycle in prune fields, and threatening the crop for the following year.

Maine has 60,000 acres of managed wild blueberries, so left unchecked the disease could be quite devastating. Seanna Annis, blueberry pathologist at the University of Maine, is telling growers to destroy infected patches when they find them by burning the areas showing symptoms and a 10-foot area on the margin. So far, the spots that have been found are about 50 feet square, and growers should burn from the edges in, using hand-held burners, and avoid moving infected leaves. There is no fungicide to eradicate the disease, so burning of the plants and leaf litter on the ground is the recommended procedure.

The blueberries are safe to eat, but harvesting infected areas of the field moves so many leaves around it is best to burn the spots before harvest, Annis said. This avoids spreading the fungus to healthy fields.

Since the fungus will survive over the winter in infected leaves, it can infect the plants at bloom time next spring, she said.

A sanitation protocol has been posted at www.wildblueberries.maine.edu. The key is not to move any diseased leaves around. Bleach solutions will not kill the fungus, so treating machinery or boots won’t help.

“Fungicides will only suppress the disease by protecting the plants,” the protocol says. “Once in a field, the fungus will produce new spores and infect plants after any three-day wet period throughout the season. Many fungicide applications will be necessary throughout the prune and crop years to protect plants if this disease is in your field. We want to avoid the trouble (hopefully it is not a disaster) that is hitting growers in Nova Scotia this year from Valdensinia leaf drop disease.”

Symptoms to look for are large (1/4-inch and larger), round, brown spots on leaves, having a bull’s eye appearance. The disease spreads onto stems. Infected leaves drop off. The fallen leaves are the major source of new spores. Walking or driving over infected leaves will spread it into new areas.

In Maine, growers manage wild blueberries by periodically burning or mowing fields to renovate them, so fields are not cropped every year. When infected with Valdensinia, crop fields lose leaves and produce smaller berries, while prune fields will not set flower buds for the next year.

If you find this disease in your field, this is the recommended procedure:

Do not enter the field when it is wet. Remove dead leaves from your footwear before leaving the diseased area.

Flag off the infected area so no one walks or moves equipment through it.

Check vehicles or other equipment that may have come in contact with the infected area for dead leaves. Steam clean all equipment and vehicles before moving them between fields.

Contact Seanna Annis or Dave Yarborough to confirm and report the disease. The Blueberry Hotline number is 800-897-0757.

Follow treatment recommendations (burning) once disease is confirmed.

– Dick Lehnert

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