Occurrence and Control Methods of Anthracnose in Orchids (Part Two)

OrchidsAnthracnose is widely distributed and is one of theorchid's important diseases.

[Pathogen] Orchid anthracnose fungus (Colletotrichum orchidaerum) and orchid leaf anthracnose fungus (C. orchidaerum f. cymbidii Allesch), both belonging to the Deuteromycotina subdivision of fungi, genus Colletotrichum.

[Symptoms] In the early stage of the disease, small yellowish-brown slightly sunken spots appear on the leaves, gradually expanding into dark brown round or oval spots. Spots at the leaf tip can extend downward, causing the leaf to die back in sections; spots at the leaf base can merge into a single patch, causing the entire leaf to wither and die rapidly. In the later stage, small black dots arranged in concentric rings can be seen in the center of the spots, which are the pathogen's acervuli.

[Disease Development Pattern] The pathogen overwinters in diseased plant residues and on pseudobulbs; the following spring it produces conidia that invade through wounds or directly penetrate young leaves to cause damage. The optimal temperature for conidial germination is 22-28°C, and free water favors conidial germination. The disease can occur from March to November each year.

[Epidemic Characteristics] The disease is more severe under high humidity and muggy conditions or during the moldy rainy season; overly dense planting, poor ventilation, high environmental humidity, and water accumulation in the nursery also easily promote the occurrence and development of the disease. Different orchid varieties show significant differences in disease resistance.

[Control Measures] A. During the growing season, promptly prune diseased spots on leaves or remove diseased plant residues. B. Reduce environmental humidity and maintain ventilation and light penetration; watering should use drip irrigation or be poured along the edge of the pot, avoiding overhead watering. C. In the early stage of disease occurrence, spray 70% thiophanate-methyl wettable powder at 800 times dilution, or 50% carbendazim wettable powder at 500 times dilution, or 50% captan at 500 times dilution for control, spraying once every 10 days or so, with the number of sprays depending on the development of the disease.