Mildew Problems With Orchids

Symptoms

• Symptoms of fungal diseases on orchids include brown and black spots or lesions on the leaves, stems and roots. One disease, botrytis petal blight, can affect the flowers of orchids. Bacterial brown spots and rots, fusarium wilt, anthracnose, and leaf spots all present on an orchid's leaves. Fungal root rots occur only on the roots. Other symptoms can appear as streaking or yellow spots in the leaves infected with a microfungus, a unique type of fungus that evinces symptoms like viral diseases.

Conditions of Diseases

• Most fungal diseases thrive in humid conditions. Lack of enough light and suitable host sites are the other two components of the disease triangle. Fungus can develop and thrive on all parts of the orchid, but favors dead or dying portions of the leaves and flowers. Diseases are often opportunistic and establish quickly on plants that are weakened due to insufficient nutrition. Feed your orchids regularly with a houseplant fertilizer especially during times of bloom when they use stored energy.

Control

• Control of fungal diseases in orchids starts with removal of the infected leaves or tissue. Use sterile cutting tools and disinfect between cuts to prevent further spread of the disease. Spray the affected orchid with a systemic fungicide containing the active ingredient thiophanate methyl or a protectant fungicide product containing the active ingredient mancozeb. These sprays can also be applied both as pre-emergent and post-emergent control of fungal problems.

Prevention

• Most diseases of orchids can be prevented with proper cultural care, starting with a proper planting media. Orchids will die if planted in typical potting soils. They need to be planted in a mix specifically blended for orchids or other epiphytes. These mixes are coarse and often comprised of tree bark or cork. Over watering of orchids also can set the stage for fungus and other diseases. Allow the potting media to get just slightly dry between waterings and do not water more than twice per week.