Dendrobiums are an epiphytic type of orchid, with long, narrow, fleshy leaves that grow in alternating layers on either side of the thick pseudobulb stem. Whether grown indoors as houseplants or outdoors in temperate climates, a number of conditions can manifest distress in the leaves of the plant. Insufficient or excess watering are common issues, as are over-fertilizing, too much direct sunlight, fungus or insect pests.
Under- or Over-Watering
Too little moisture around the plant roots can manifest in yellowing leaves, which shrivel, brown along the margins, desiccate and eventually drop off. Too much moisture at the roots, either from excess watering or inappropriately heavy potting mix such as soil, can suffocate the roots and cause the leaves to fade to yellow and drop from the pseudobulb.
Phyllosticta Capitalensis Fungus
Small circular spots in yellow or black will form on dendrobium leaves when fungus spores are present. A particular fungal form that dendrobium are susceptible to is Phyllosticta capitalensis. It will spread from leaf to leaf with the spots multiplying and when the plant is denuded of leaves, it will die.
One of the most frequently seen pests on dendrobium leaves are scales that colonize the bottom surface of the leaves. Scales appear as small round or ovoid spots that are white, tan or dark brown. Scales colonize and reproduce quickly and can only be controlled if caught early and arrested with chemical insecticides. Most dendrobium orchids with noticeable and established scale infestations cannot be saved and must be destroyed.
Too Much Sunlight
Excessive bright, direct sunlight can burn leaves and cause severe drought stress in the orchid plant, delivering a one-two punch that can quickly yellow and desiccate the orchid leaves. Only expose orchid to direct sunlight when it is weak in the morning or afternoon. Bright, indirect light is always the preferred exposure.
Too heavy or too frequent fertilizing can burn the orchids delicate epiphytic roots causing rapid leaf discoloration and drop. Fertilizer applied directly to the potting medium or roots without first being heavily diluted in water can cause enough root damage to kill an orchid.