What to Do When an Orchid Gets Anthracnose? Symptoms and Control Methods

OrchidsThe anthracnose fungus mainly infectsorchidleaves and also infects fruits. On affected leaves, small yellowish-brown slightly sunken spots appear initially, gradually expanding into dark brown round or semicircular spots. If the pathogen invades from the leaf margin, the lesions are mostly semicircular or irregular in shape; if the disease starts from the leaf tip, the lesion extends downward and dries up, with the dead portion reaching 1/5-3/5 of the leaf; if it attacks from the leaf base, the diseased leaf dries up rapidly, and in severe cases the entire plant dies. In the later stage, the lesions turn blackish-brown with a grayish-white center, sometimes showing concentric rings, on which numerous small black dots appear, which are the acervuli of the pathogen. The size and shape of the lesions vary depending on theorchidvariety. OnCymbidium ensifolium, the lesions mostly occur at the leaf tip, are relatively large, and elongated; onCymbidium kanranand Welwitschia, the lesions mostly occur at the leaf margin, with a diameter of over 20 mm; on varieties such as 'Huanqiu Heding' and 'Yingchun Die', the lesions are scattered, small and dot-like, about 1 mm in diameter. On fruits, the lesions are irregularly elongated and blackish-brown. Orchid anthracnose is caused by infection from Colletotrichum orchidearum, which belongs to the Deuteromycotina, Coelomycetes, Melanconiales, Melanconiaceae, Colletotrichum genus. The conidia of the pathogen are cylindrical, produced within the acervuli, and infect and cause damage through these. The optimum temperature for the growth and development of the pathogen is 22-26 degrees Celsius; growth stops below 15 degrees Celsius or above 38 degrees Celsius. The orchid anthracnose fungus overwinters as mycelium and acervuli on fallen diseased leaves or diseased leaves of the plant, mainly invading through wounds, and can also directly invade young tender leaves. It spreads via wind and rain, with an incubation period of 14-20 days. The disease can occur from March to November, with a peak period from April to June. Factors such as high humidity and muggy heat, alternating sunny and rainy weather, poor ventilation, water accumulation in flower pots, dense plant clusters prone to friction damage, and severe damage by scale insects all aggravate the occurrence and spread of the disease. Almost all orchid varieties suffer from anthracnose damage to varying degrees. Generally speaking,Cymbidium goeringii、Cymbidium kanran, BaoChunlan, and 'Dafugui' are susceptible to the disease;Cymbidium faberi、Cymbidium sinenseare moderately susceptible; Tailan andamong Cymbidium ensifolium, 'Tiegengsu' is relatively resistant. Control measures: 1. Remove diseased residues; during the orchid growing season, promptly prune diseased leaves and lesions and burn them in a centralized manner. 2. Water at the pot edge or use drip irrigation to control leaf surface humidity, avoid overhead watering, so that water droplets do not splash and spread pathogen spores. 3. During the disease period, fungicides such as 75% chlorothalonil, 20% tricyclazole, 50% Rebice and 50% zineb can be used at 600-1000 times dilution, sprayed once every 10-15 days, for several consecutive applications; it is best to rotate several fungicides for better results.