Phalaenopsis is a botanical genus containing 60 species of tropical flowering epiphytic plants more commonly known as moth orchids. According to the University of Tennessee Extension, Phalaenopsis are among the easiest and most popular orchids to cultivate at home. Black spots on the flowers of Phalaenopsis orchids are the result of a fungal disease known as Botrytis. Botrytis control is difficult, but is not impossible.
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Remove Affected Plant Matter
Remove infected foliage and flowers as soon as possible. Cut affected foliage from the plant with sharpened and sterilised pruning shears. Destroying the removed plant matter prevents the Botrytis fungal spores from spreading to other plants. After pruning the diseased foliage, wash the pruning shears in a solution of warm, soapy water and dry them with a soft cloth. Moisten a second cloth with rubbing alcohol and rub the blades to kill any lingering Botrytis spores.
Sterilise Surrounding Surfaces
Infected moth orchids may spread Botrytis spores to surrounding surfaces. Disinfecting these surfaces prevents nearby plants from infection. The Saint Augustine Orchid Society recommends wiping down any hard surfaces around infected orchids with a broad-range disinfectant, fungicide, virucide and algicide. Create a homemade disinfectant solution by mixing 1/4 cup household bleach with 2-1/4 cups water. Discard any unused bleach solution.
Kill Fungal Spores on Plants
Botrytis spores linger on the moth orchid even after removal of the infected foliage and flowers. Wipe down the orchid's stems and foliage with a copper-based fungicide and bactericide. Always use fungicide products according to package instructions to avoid injury to yourself or your Phalaenopsis orchid. Copper-based fungicide products are not suitable for use on all parts of the moth orchid. Never apply fungicide to an orchid's flowers unless the product instructions explicitly direct.
Phalaenopsis Continued Care
Once the Botrytis is controlled, prevent future infections. Move the moth orchid to a spot offering better air circulation. Use a fan if necessary. Water moth orchids in the early morning and avoid wetting their foliage. Leaving an orchid's foliage wet overnight invites fungal disease. Provide daytime temperatures between 21.1 and 27.7 degrees C and nighttime temperatures above 15.6 degrees C. If the black spots reappear, swab the affected blossoms with a cotton ball dampened with hydrogen peroxide.
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- "Easy-Care Guide to Houseplants"; Jack Kramer; 1999
- University of Tennessee Extension Service; Growing Orchids in the Home; Linda M Seals, et al.
- Saint Augustine Orchid Society; Botrytis; Courtney Hackney; March 2007
- LSUAgCenter.com; Phalaenopsis or Moth Orchid; Sept. 2, 2010
- Colorado State University Extension; Phalaenopsis; Oct. 27, 2010
- Orchids.com: Frequently Asked Questions