Fungus on Dendrobium Orchids

Dendrobium orchids are prized by home gardeners for their colorful blooms and evergreen foliage. They make up the second largest genus in the family Orchidaceae, and most species are generally easy to care for. However, dendrobiums are susceptible to infection from several fungi, which typically cause spots, lesions or blemishes to form on your orchid leaves, flowers and stems.

Blossom Blights

Blossom blights are caused by several fungi at different times of year. Botrytis blossom blight (Botrytis cinerea) usually develops during the cool, moist winter months when temperatures drop below 70 degrees Fahrenheit, producing threads that form gray spore masses. Symptoms of infection include brown or pink spots on flowers that expand into almost circular translucent soft rots. If tan to brown spots appear during the summer months, your orchid may be infected by Alternaria alternata, Exserohilum rostratum, or one of several Bipolaris species. Alternaria fungi grow on dead plant tissue, while Bipolaris and Exserohilum fungi attack grasses. To control the spread of botrytis infection, remove and destroy infected blossoms and apply fungicides containing mancozeb, thiophanate methyl, or vinclozolin. Bipolaris and Exserohilum fungi typically die after a few weeks and your orchid will survive infection, but fungicides containing mancozeb or glycopene can be effective as preventative treatments for these and Alternaria alternata.

Colletotrichum Fungi

Salmon-colored spore masses on any part of your dendrobium orchid indicate infection by species of Colletotrichum fungi. Seedlings and new growth on mature plants are highly susceptible to infection, which can kill young seedlings. On mature plants, black rots may destroy new growth, including leaves and sheaths. Leaf and stem spots are typically dark brown and eventually buds may wither and fall off. Cool weather favors spread of Colletotrichum fungal infections. To control infections, apply fungicides containing mancozeb and remove all diseased plant parts and debris.

Phyllosticta Fungus

Phyllosticta capitalensis causes yellow spots on the leaves of dendrobiums that turn tan as the leaf ages. Some spots may blacken as the fungus forms small, dark fruiting bodies on the leaves and these produce spores that may be carried by splashing water or wind. Infection is favored by wet conditions, and according to the University of Hawaii, there is no chemical treatment available for Phyllosticta infections once yellow spots develop. Keep your soil clear of plant debris and remove all dead or dying leaves from the plant to prevent spread of infection to healthy plants. Ensure your orchid has good airflow around the foliage and water early in the day to allow the leaves to dry.

Fusarium Rot

Fusarium rot (Fusarium proliferatum) affects dendrobium flowers, leafs, sheaths and roots. Symptoms of infection include dark brown oval spots on the flowers and young leaves. Rows of three to four spots across the leaf blades close to the cane is a distinctive characteristic of fusarium rot, which spreads rapidly. To control spread of the infection, remove infected plant parts immediately and clean all tools and surfaces. Fungicides containing mancozeb, glycophene or thiophanate methyl may effectively prevent the spread of fusarium infections.

Phytophthora Rot

Several species of Phytophthora fungi may cause olive to greenish-yellow leaf spots on your dendrobium orchid that later darken to brown or black. Infected canes have dark, wet, light brown spots. If the fungus moves into the roots, plant decline and death may occur. Phytophthora spores are usually spread by splashing water and by contact. Infection is favored by warm temperatures and high humidity. Phytophthora infections can be prevented or reduced with applications of fungicides containing fosetyl-al or metalaxyl. In cases where infection is severe, the entire plant may have to be destroyed.