Symptoms and Control Methods of Dendrobium Leaf Spot Disease

1. Damage: This disease occurs in the United States, Thailand, and Taiwan, China, but has not yet been reported in mainland China. The pathogen of this leaf spot disease was first discovered by Burnett (1964) in West Palm Beach, Florida. The host range of this pathogen onorchidsis extremely narrow, generally limited to damaging the Dendrobium genus only. In Taiwan, it only infects the Dendrobium genus, including spring Dendrobium and autumn Dendrobium; other orchid genera such as Phalaenopsis, Cattleya,Cymbidium, Oncidium, and Vanda are all uninfected. Among autumn Dendrobiums, the large-flowered type (Phalaenopsis type) is more severely affected, while the small-flowered type (antelope type) is more mildly affected. However, once the disease occurs, it spreads rapidly. 2. Symptoms: This pathogen only damages the leaves of Dendrobium. Initially, water-soaked spots appear, and the lesions slowly expand into yellow patches with indistinct edges. The lesions are circular to oval, 8-32 mm in diameter. Multiple irregular pale yellow spots usually appear on the leaves. After the lesions mature, they become slightly sunken and appear purplish-black, while the expanding edges remain yellow. Conidiophores emerge from the stomata on the underside of old lesions. On the leaf underside, especially near the leaf tip, small gray masses of conidia can also be seen. As the lesions mature, they gradually turn pale purplish-black or black and show symptoms resembling mite damage. Scattered lesions on the leaves merge with each other, causing leaf yellowing and defoliation. Severely affected leaves have a shortened lifespan and eventually drop from the plant. 3. Pathogen: The pathogen of Dendrobium leaf spot disease has been identified as Pseudocercospora dendrobii. The conidiophores are dark-colored, emerging from stomata, growing in clusters, erect or slightly curved, rarely branched, with bluntly rounded tips, and no obvious spore scars at the conidiogenous loci; size is 4-6 um * 25-75 um. The conidia are curved needle-shaped or needle-shaped obclavate, with 4-9 septa, size 2-3.5 um * 61-69 um. This fungus grows slowly on PDA plates, with mycelium raised and slightly wrinkled. The temperature range for pathogen growth and spore germination is 10-30°C, with 25°C being the optimum temperature. Above 35°C, the growth of the pathogen is significantly inhibited, with almost no growth. When cultured in different media, mycelial dry weight was highest in Mucor synthetic medium and potato dextrose medium, and lowest in sugarcane leaf decoction medium and hydroxymethyl cellulose medium. This pathogen is difficult to sporulate; in all culture experiments, no sporulation occurred. 4. Disease characteristics: In simple greenhouses, the disease begins to occur in late August each year. In orchid gardens, leaf drop becomes widespread from November onward. In severely affected areas, leaves have completely dropped by March to April of the following year. In greenhouses, there is a clear correlation between the occurrence of this disease and temperature. In locations with high temperature, good ventilation, and low humidity, the disease is not severe. High temperature can inhibit disease development, while excessive humidity promotes disease occurrence. 5. Control methods: (1) Strengthen quarantine, with special attention to the import of Dendrobium from abroad. (2) Strengthen management and pay attention to the ecological environment of the orchid room. Especially in simple greenhouses, reducing indoor humidity during the heat preservation period to suppress the epidemic is the key point of disease control. (3) Apply benomyl 50% wettable powder at 500 times dilution, with a small amount of wetting agent added, once every two weeks for 3 applications total; the effect is excellent. Thereafter, apply the same dosage once a month to prevent reinfection of the plants. (4) Using ferbam at 700 times dilution to control this disease is also relatively effective. In severely affected areas, the second application must be made 5-7 days later. (5) Because the application period for this disease coincides with the flowering period, pesticide application generally leaves chemical spots on the petals. However, spraying 75% chlorothalonil wettable powder at 500 times dilution results in less obvious chemical spots and significant control effect, so this chemical can be used during the flowering period. In addition, 56% benomyl-carbendazim sulfate wettable powder also has a good control effect on this pathogen. (Liu Zhongjian)