Symptom Identification and Control Methods for Cercospora Leaf Spot Disease

Several species in the genus Cercospora can causeorchidleaf spot disease. The symptoms and control methods of orchid leaf spot disease caused by the main species are described below. 1. Cercospora epipactidis (1) Damage: This pathogen has been found in Italy, Germany, the former Soviet Union and the United States. The hosts it damages include: E. palustris, Calanthe, Coelogyne, Scaphyglottis, Nervilia, Thunia, and Phaius, Spathoglottis and other varieties. (2) Symptoms: In Phaius, the initial symptoms appear on the underside of the leaves as depressed yellow spots of various sizes. Soon the lesions become visible on both sides of the leaf, expanding within just a few days, and finally becoming depressed purple-black spots with slightly raised edges of a somewhat deeper colour. Mature lesions average about 2-5 cm in diameter or slightly larger. Some lesions may merge into larger irregular patches. The centres of older lesions often break apart and fall out. Where lesions spread across thicker veins, after the remnants of the leaf tissue within the lesion have all fragmented and fallen away, the veins remain intact. When Bletia purpurea is infected by this pathogen, small oval or circular greyish-white areas usually appear first on the underside of the leaves. Eventually the infection becomes visible on both sides of the leaf. As the lesions expand, the centres become purple-black. When the pathogen has fully established itself, all leaves often become heavily affected. (3) Control: Benlate 50% wettable powder has a significant control effect on the aboveOrchidsThe symptoms and control methods of leaf spot disease are described separately below. 1. Cercospora epipaczlddis (1) Harm: This pathogen has been found in Italy, Germany, the former Soviet Union and the United States. The hosts it harms include: E. palustris, Calanthe, Coelogyne, Scaphyglottis, Nervilia, Bromheadia, as well as Phaius, Spathoglottis and other varieties. (2) Symptoms: In Phaius, the initial symptom is the appearance of sunken yellow spots of varying size on the underside of the leaves. Soon the lesions become visible on both sides of the leaf, and within just a few days they expand, finally becoming sunken purple-black with slightly darker raised edges. The average diameter of aged lesions is about 2-5 cm or somewhat larger. Some lesions may merge into larger irregular patches, and the center of older lesions often breaks apart and falls out. Where the lesion expands across thicker leaf veins, after the internal leaf tissue remnants have all broken away, the veins still remain intact. When Bletia purpurea is infected by this pathogen, initially smaller oval or circular grayish-white areas usually appear on the underside of the leaves, and finally the infection becomes visible on both sides of the leaf. As the lesions expand, the center of the lesion turns purple-black, and when the pathogen has fully established itself, all leaves often become heavily diseased. (3) Control: Benlate 50% wettable powder is effective againstorchidleaf spot diseases and all leaf spot diseases caused by Cercospora fungi. Use a 1000-fold dilution with a little wetting agent added, applying once every two weeks for a total of 3 applications. If necessary, thereafter spray once a month at the same concentration. 2. Cercospora dendrobii (1) Damage: This pathogen mainly affects Dendrobium orchid varieties. It generally does not cause orchid death, but it produces many spots on the leaves, causing the orchids to lose their ornamental value. (2) Symptoms: Symptoms first appear on the underside of the leaves, appearing as pale yellow areas. These lesions continuously expand into circular or irregular shapes, and may eventually cover the entire underside of the leaf. Irregular pale yellow spots usually develop on the leaf. After the lesions mature, they become slightly sunken and purple-black, while the expanding edges remain yellow. Soon corresponding yellow-green spots appear on the upper surface of the leaf. As the lesions mature, they gradually turn pale purple-black or black and show symptoms similar to mite damage. Severely affected leaves have a shortened lifespan and eventually fall from the plant. (3) Control methods: Apply Benlate 50% wettable powder at a 1000-fold dilution with a little wetting agent added, once every two weeks for a total of 3 applications. The results are excellent. Thereafter, apply once a month at the above dosage to prevent re-infection of the plant. Ferbam is also effective for disease control at a concentration of 800-1000 fold. In severely affected areas, a second spray must be applied after 5-7 days. 3. Cercospora angraeci (1) Damage: This disease has been found in the United States, Germany and the United Kingdom. Its hosts include: Angraecum fragrans, Odontoglossum crispum (O. alexandrae), Angraecum eburneum, A. comorense, A. orchidglade, A. veitchii and others. (2) Symptoms: They first appear on the underside of the leaves as pinhead-sized, brown or pale brown depressed lesions. Eventually the lesions turn dark purple-brown. As the disease progresses, the lesions may form nearly circular patterns, and in many cases the entire leaf becomes covered with lesions, most commonly on the oldest leaves closest to the base. The corresponding areas on the upper surface of the affected leaf appear yellow-green. Usually only the 6-8 leaves at the base develop severe lesions, but if the plants are planted too densely, older leaves near the top may also become infected. (3) Control methods: Spray once every two weeks with Benlate 50% wettable powder at a 1000-fold dilution with a little wetting agent added, for a total of 3 applications. If necessary, reapply once a month thereafter. Chlorothalonil or ferbam can be used instead of Benlate to protect healthy parts, at a 800-fold dilution. In addition, maintain spacing between plants to promote air circulation. If possible, remove and destroy all diseased leaves. 4. Cercospora cypripedii (1) Damage: This disease occurs in northern United States and southern Canada as well as the Philippines. Its hosts include: Cypripedium candidum, C. humile (C. acaule), C. parviflorum var. pubescens, C. pubescens, C. spectabile (C. hirsutum) and C. reginae, Paphiopedilum philippinense, Cypripedium philippinense. (2) Symptoms: Elongated pale red to dark brown or black leaf spots usually appear between the veins. The lesions are 1-3 cm wide and sometimes up to half the length of the leaf. Spores are produced on both sides of the leaf, but mostly on the upper surface. (3) Control methods: There are currently no feasible control measures for this disease on Cypripedium or Paphiopedilum orchids. However, Benlate has an inhibitory effect on Cercospora fungal infections on orchids. 5. Cercospora odontoglossi Perillieux and Delacrois (1) Damage: This disease has been found in the United States, Brazil, Venezuela and other countries. The species most severely affected by these two diseases include: Cattleya and Brassavola, Brassocattleya, Brassolaelia, Brassolaeliocattleya, Broughtonia, Diacrium, Epicattleya, Epidendrum, Epilaeliocattleya, Epitonia, Laelia, Laelonia, Potinara, Rodricidium, Schombocattleya, Schombodiacrium, Schomburgkia, Sophrolaelia, Sophrolaeliocattleya, Sophronitis and others. (2) Symptoms: After Cattleya-type orchid seedlings in seed trays are infected, pinhead-sized, slightly raised dark brown lesions first appear on the underside of the leaves, while the corresponding areas on the upper surface show chlorosis, and in severe cases eventually become necrotic. The symptoms of this pathogen infecting seedlings are often confused with mite damage. The difference is that mite damage spots are sunken, while the lesions of this pathogen are slightly raised. When mature leaves are affected, irregular sunken purple-brown lesions appear on the underside. The lesions are generally 0.32-2.54 cm in diameter, often surrounded by a brown or brownish margin. The spots may merge into very large patches. The corresponding areas on the upper surface appear as yellow sunken spots, turning purple-brown when mature. (3) Control methods: Benlate 50% wettable powder has an excellent control effect on this pathogen and can effectively control the spread of the disease. Apply once every two weeks at a 1000-fold dilution for 3 consecutive applications. Thereafter, reduce to once a month as a preventive measure against the disease. Some other fungicides also help control this disease, such as ferbam, captan and chlorothalonil, all at a 800-fold dilution. They must be applied twice a month. Adding a little spreader helps the chemical cover the leaf surface more thoroughly. Since C. odontoglossi and most other pathogens causing orchid leaf spot disease enter the plant through the stomata on the underside of the leaves, it is very important to achieve full coverage of the leaves with the chemical.