Orchids & Fungi

Fungi on Blossoms

• The blooms of orchids can contract a fungus on their flower petals known as botrytis petal blight. Symptoms start as brown or black spots on the individual petals. If untreated, the spots can merge to form larger blotchy spots and can further develop a mycellium -- a fuzzy mold covering the spots. Infected blossoms should be removed. Botrytis can be prevented by moving your orchid to a spot with less humidity. Avoid getting the flowers wet when watering your orchid.

Fungi on Leaves

• Several species of fungal pathogens can affect the leaves of orchids. Black rot, caused by either Pythium or Phytophthera, starts as brown or black spots on the leaves and can spread rapidly throughout the plant. Plants with black rot should be discarded, as there are few remedies that can effectively control it. Fusarium wilt blocks the flow of nutrients within the leaves of orchids. Affected leaves appear yellow and weak. Infected portions of the plant should be removed and discarded. A fungicide containing thiophanate methyl can be applied to the entire plant to help prevent further spread of the disease.

Fungi on Stems

• Sclerotium rot, or collar rot, causes a rapid decline of orchids. The base of orchid leaves and the lower stem are affected first. A white fungus appears on the stem and the surface of the potting soil. Small sclerotia that appear seedlike develop on the lower stem. Affected plants should be removed and destroyed. To prevent this rot disease, always make sure to use a sterile potting medium for your orchids.

Fungi on Roots

• Root rots, such as those caused by the fungus Rhizoctonia, can develop in orchid soil that is too wet and does not drain well. If the disease is discovered in time, the infected portions of the roots can be removed with sterile cutting tools. An overall decline of the plant is the most evident symptom of root rots. If growing multiple orchids, check the roots of all your plants if you suspect any may have root rot.