Regional Gardening Reports :: National Gardening Association

In the Garden:
Inland Northwest, High Desert
March, 2004
Regional Report

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Botrytis can prey on primroses in greenhouses if conditions are right. Prevent infestation with cleanliness.

Botrytis Can Spoil Your Whole Spring

It's still too cold to plant outside, but with our short growing season, we want to get a jump on summer. So we look for a leg up with greenhouses.

Trouble is, greenhouses make wonderful incubators for insect and disease infestations. The perfect conditions inside a greenhouse -- plenty of food and humidity, and often restricted air movement -- contribute to a pathogen's well-being. Because insects and disease thrive in greenhouses, we can see the progress of infestation as though we were watching on fast-forward.

Plant pathologist M.K. Hausbeck noted recently that during high humidity, botrytis reproduces spores that ride easily on air currents. If those spores land on a leaf, flower, or seedling, he said, and free moisture is present (such as water from overhead watering systems or even condensation) the spore germinates within three hours, penetrates the plant tissue within six to nine hours and, on some plants, produces symptoms within 24 hours.

The Bad Guys
The strain of botrytis called gray mold is the most common form of the disease. It infects violets, anemones, asters, begonias, calendulas, camelias, carnations, chrysanthemums, dahlias, dogwoods, geraniums, lilies, primulas, roses, sunflowers, and a lot more. New leaves are most often attacked, and flower buds can become infected, causing a soft rot. Leaves turn black and have a fuzzy look about them. They die in no time. Those fuzzies are the spores. Botrytis can set up housekeeping on dead leaves and reproduce on them, too.

Clean 'em Up, Move 'em Out
Picking off infected plant parts is usually recommended. But Hausbeck found that if infected plant debris is collected and put into the trash, you're still not in the clear. Your slightest movement causes a little breeze. That breeze is enough to help the spores float off the dead leaves you're carrying and land on every plant, all the way down the length of the greenhouse.

"Better to bring a covered container or plastic bag that can be sealed at the site of the infected plant to be discarded, rather than carry the infected plant through the greenhouse to a trash container," Hausbeck said.

You've got to clean up your act to beat botrytis and prevent its spread. Make Clorox or another bleach your friend. Cuttings should be rooted in sterile potting soil in clean containers. Clean everything with a 10 percent bleach solution. Lower the humidity when you can. When you get bare-root plants in the mail, dip them in a weak solution of 2 tablespoons bleach to a gallon of water to make sure you don't plant botrytis in the garden.

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