Black Spots on the Leaves of My Oncidium Orchid

Sunburn

• Dark, crispy spots on orchid leaves are usually a symptom of sunburn. Oncidium orchids are medium-light orchids that cannot tolerate the sun beating down on them for long. The New York Botanical Garden recommends 1,500 to 2,000 foot-candles of light for Oncidiums -- meaning even setting them in a south-facing window sometimes gives them too much light. Though they thrive in bright locations, too much direct sunlight zaps them of moisture and causes dark, dry spots.

Fungal Disease

• Compared to other orchids, Oncidium orchids prefer less humidity, and without proper ventilation, excess moisture can cause fungal disease. Small round spots may spread to become dark webs that engulf and kill leaves. Applying a fungicide and removing affected leaves can help manage the problem, but eventually the orchid may die from the fungus. You can prevent future fungal diseases by watering orchids in the morning, giving them plenty of time -- and physical space -- to dry before nightfall.

Oncidium Care

• Proper care can prevent dark spots and the problems they indicate. An east-facing window is ideal for most orchids, though Oncidiums prefer brighter locations without the blazing heat of direct sunshine. University of Tennessee Extension recommends humidity levels between 30 and 60 percent for Oncidium orchids -- a healthy level for a home and easily achieved with a humidifier.

Expert Tips

• Though Oncidium orchid problems are sometimes difficult to cure, you can keep those dark spots at bay with a few simple tricks. For instance, if your Oncidium orchid starts taking a beating from the sun, shade it with a sheer curtain to prevent scorching, as Michigan State University orchid specialist Don Garling recommends. And rather than grouping orchids close together to increase humidity, University of Tennessee Extension recommends giving them plenty of space on trays filled with wet gravel; this will help prevent fungal diseases.