Dendrobium orchids, a large and diverse genus of orchids prized for their fragrant, showy flowers, are hardy to U.S. Department of Agriculture plant hardiness zones 9 through 12. Occasionally dendrobium leaves become mottled with spots, lines, streaks or discolored patches, which indicates an infection that may be caused by one of several diseases.
Bacterial Soft Rot and Brown Rot
Bacterial soft rot and brown rot (Erwina spp.) cause yellowing of the leaves, followed by small, water-soaked spots that are surrounded by yellow halos. These spots later blacken and may produce an unpleasant odor. To control infection, remove infected tissue and spray the plant with bactericides containing alkyl dimethyl ethylbenzyl ammonium chloride as the active ingredient. Do not use copper-based compounds on a dendrobium. Prevent infection by avoiding overhead watering, because splashing water spreads the pathogens. When using bactericides, follow label instructions for application rates and timing.
Bacterial Brown Spot
Bacterial brown spot (Pseudomonas syringae pv. syringae) causes small, water-soaked blisters on the leaves of dendrobiums. Initially a dirty green color, the spots later enlarge, mottling the leaves with brown or black dried and sunken spots that ooze bacteria-filled liquid. To control the spread of infection, remove infected tissue and spray the plant with bactericides containing alkyl dimethyl ethylbenzyl ammonium chloride as the active ingredient. Keep your plant dry, and avoid overhead watering to prevent development of bacterial brown rot.
Anthracnose on dendrobium orchids is caused by Colletotrichum and Glomerella fungi. Leaves become mottled with dark brown or light gray patches that form in concentric rings or as dark bands across the leaf surfaces. As the infection progresses, fruiting bodies develop in the affected areas of the leaf. Treat anthracnose infections on dendrobium orchids with a systemic fungicide containing thiophanate methyl, and prevent infection in healthy plants with applications of protectant fungicides like mancozeb. Follow all label instructions for application rates and timing. Anthracnose can be prevented by keeping the soil around your orchid clear of debris and ensuring there is adequate air circulation around the foliage.
Fungal Leaf Spots
There are a few fungi that can cause mottling on dendrobium leaves. Cercospora fungi cause yellow spots on the undersides of dendrobium leaves, which later show on the surface and enlarge until they cover the entire leaf. The spots become sunken and turn a purple-brown to purple-black color, while the outer margins stay yellow. Guignardia and Phyllosticta fungi cause small dark purple lesions on the leaves as well, but these run parallel to the leaf veins and lengthen into diamond-shaped regions or purple streaks that eventually turn a tan color. Black fruiting bodies also develop in affected areas. Septoria fungi also cause mottling of dendrobium leaves, but first appear as tiny, sunken yellow lesions. These later enlarge and turn dark brown to black. When fungal leaf spots appear, remove the infected leaves and avoid overhead watering. Apply fungicides containing thiophanate methyl or mancozeb. Prevent leaf spots in healthy plants by reducing leaf wetness and keeping the area around your orchid free of debris.
There are about 30 different viruses that can cause chlorotic and necrotic spots, streaks or rings on dendrobium leaves. There is no treatment to cure these viruses, so dendrobiums showing symptoms of viral infection, such as leaf yellowing and mottling, should be destroyed to prevent transmission to healthy plants. Viruses can be prevented by good sanitation such as washing hands and sterilizing tools before and after working with your dendrobiums and other plants. Keep insect pest populations low by monitoring and treating as soon as you notice them.
- Brendan Smialowski/Getty Images News/Getty Images