Black Rot on Orchids

Orchids are amazing houseplants.

Beautiful orchids enhance your home's decor with a spectacular display of fragrant flowers. Black rot (Phytophthora sp) will kill your orchid plants if left untreated. The soil-borne fungi, Phytophthora palmivora and Phytophthora cactorum, infect several different orchid genera. Save your orchids by understanding the disease and how to treat it.

Symptoms

Roots and pseudobulbs are first infected with the black rot disease from the soil-born pathogen, Phytophthora. Symptoms are small black lesions. The lesions cover the orchard leaves by spreading through the rhizome, which are horizontal underground stems with roots attached. The disease continues to spread until it kills the plant.

Life Cycle

Wet conditions favor the growth of black rot from resting spores that survive in moist soil until a host becomes available. Then, the resting spores germinate and motile spores penetrate roots. The roots do not need wounding to become infected.

Cultural Control

Ensure your orchids have good water management by growing them in a good orchid potting mix. Soggy soil encourages the growth of black rot, and water stress or salinity make orchids more susceptible to the infection. Sanitation is important for avoiding infections with black rot. Avoid growing your orchids near any area where black rot has been observed. Keep tools and watering cans sanitized. Purchase only certified orchid plants from a reputable source. Carefully inspect the plants when you buy them for any signs of black rot.

Chemical Control

Use copper-based compounds on orchid plants infected with black rot; compounds include copper hydroxide, copper oxide, basic copper sulfate, copper oxychloride, and copper ammonium carbonate. Apply them sparingly because too many copper ions can injure your orchid plant. Fungicides maneb, mancozeb, and zineb are effective in controlling black rot. Organic tin compounds are also effective but more toxic to the plant. Use the systemic chemicals phenylamide, phosphonate, cinnamie acid and quinine, since they move up from the roots to kill black rot in the leaves. Apply them when the plant is in active growth. All of these chemicals can be used as soil drenches or foliar sprays. Use foliar sprays when leaves are infected because soil drenches will become too dilute after moving to the leaves for effective control of black rot.

References

• University of Florida Extension: Black Rot of Orchids Caused by Phytophthora palmivora and Phytophthora cactorum
• University of California Pest Management Guidelines: Phytophthora Root and Crown Rot in the Garden
• Oregon State University Disease Control: Diagnosis and Control of Phytophthora Diseases