What Causes Orchid Stems to Turn Black?

Distinction

• Orchid types include epiphytes that grow naturally on trees in tropical jungles and terrestrial orchids that grow underneath the jungle canopy on the forest floor. Epiphytes typically grow as perennials in U.S. Department of Agriculture plant hardiness zones 10 through 12 and include cattleya and phalaenopsis, or moth orchid. Paphiopedilum, commonly called lady slipper or Venus' slipper, and phaius orchids are terrestrial types that generally grow in USDA zones 9 through 12 Regardless of genus, species or growth type, most orchids are susceptible to the same diseases.

Fungal Diseases

• Fungal diseases wreak havoc on orchids and attack all plant parts, including the stems. Black rot is particularly insidious, causing more damage than any other orchid disease. Pythium and phytophthora pathogens, commonly called water-mold fungi, are the primary culprits that cause black rot. This disease spreads fast and causes plant tissues to turn black before they die. The American Orchid Society notes that ground cinnamon is an effective organic fungicide, but only if you treat fungal diseases in their early stages. After you cut out the infected plant parts, you can sprinkle cinnamon directly on the pruning wound.

Bacterial Diseases

• Bacterial diseases may also cause orchid stems to turn black, but they are typically differentiated from fungal diseases because of the presence of soft or wet lesions that turn black as they rot. Bacterial rots are caused primarily by Erwinia and Pseudomonas species pathogens. Bactericides that are formulated and packaged for orchids may kill the disease organisms, but you must treat plants early. Typically, bactericides offer little control, so it’s best to discard any infected plants, particularly if you have other orchids, so that the disease does not spread.

Viral Diseases

• Cymbidium mosaic and odontoglossum ringspot are the most common viruses that infect orchids, according to Iowa State University’s Department of Plant Pathology. Symptoms vary, even on different plants with the same virus, and sometimes plants show no symptoms at all. Stems may have black areas that are spotted or linear. Of the 25 known orchid viruses, none is treatable. Because insects may transmit viruses, check your plants often to make sure that they are free from insect pests.