Symptoms and Control Methods of Orchid Flower Blight

1. Damage:Orchidflower blight has already been found in Guangzhou, Nanjing, Shenzhen and other places. Abroad, it is also very common in countries such as the United States, Singapore and Thailand. The disease affects aorchidwide range of hosts, and can harm not only the genus Cymbidium among the Orchidaceae, but also Phalaenopsis, Cattleya, Dendrobium, Vanda and Phaius, among which Dendrobium and Cattleya are more susceptible, while the flowers of Cymbidium and Phalaenopsis have relatively good tolerance to this pathogenic fungus. 2. Symptoms:Orchidspots are similar to those produced on gardenia, cyclamen, chrysanthemum, carnation and geranium. After any part of the petals and flower is infected, small semi-transparent water-soaked spots appear within 24 hours. As the lesions turn brown, there is sometimes a faint pink ring around the spot. The number of lesions on each flower varies, but once the flower begins to fade, the lesions all increase, and careful examination will reveal a fuzzy growth on the flower, which is the mycelium and spores of Botrytis. The asexual generation of the pathogen is most commonly observed. 3. Pathogen: The pathogen of orchid flower blight is generally considered to be Botrytis cinerea. The conidiophores are single to tree-like branched, with enlarged or tapering tips bearing small protrusions, on which the conidia are borne singly, spherical, elliptical or cylindrical, single-celled, colourless or pale. The fruiting bodies emerge from sclerotia or mycelium, the conidiophores are 280-350um X 12-24um, clustered, grey then turning brown. The conidia are subspherical or oval, 9-15um X 6.5-10um, but in some cases the situation is more complex. It has been reported that natural sporulation cannot be seen in naturally collected specimens of Dendrobium flower blight, and among 27 isolated strains, the spore morphology differs markedly, on which basis they are divided into 3 types. Among them, 2 strains belong to type 1, which is Botrytis cinerea (B. cinerea), with elliptical spores measuring 10.0um X 7.7um; 13 strains belong to type 2, with more than 94% of the elliptical spores measuring 17.3um X 12.2um, of which about 1%-6% are two-celled spores; type 3 has 12 strains, whose spores are top-shaped single cells similar to those of type 2, measuring 19.6um X 10.8um, but it also has 3%-14% top-shaped spores with a length of 15.4um and a width at the widest point of 14.6um; the top-shaped spores have a series of various shapes, some approaching obovate, wedge-shaped, top-shaped and lobed. Which species the strains of types 2 and 3 belong to has not yet been identified. 4. Disease characteristics: Orchid flower blight mostly produces conidia in rotting flowers, and many conidia are produced from rotting flowers. When the weather is cold (7-18 degrees), or when the orchid house is poorly ventilated, the flowers often suffer from orchid flower blight, leading to an epidemic of the disease. In other seasons when the temperature is higher, the disease is generally limited to older, fading flowers, and when the flowers begin to age or have begun to weaken, the flowers of variousorchidflowers can all be infected by this disease. The conidia of this pathogen are spread by water flow or air. After fertilisation, rapid leaf growth together with moist and cool conditions favour the occurrence of this disease. 5. Control methods: (1) Strengthen quarantine: Orchid flower blight is relatively rare in China, and the degree of incidence is not as severe as abroad, so when orchids are imported, quarantine measures must be strengthened to prevent the pathogen from being brought in. (2) Horticultural control: Pay attention to ventilation in the orchid house, and when watering, care must be taken not to water the flowers; if watering from above, it should be done in the morning so that the leaves dry as quickly as possible. (3) Manual control: Remove and destroy all diseased flowers and other infected parts; this measure can reduce the number of infection sources formed by the pathogen through saprophytic growth. (4) Chemical control: Newflowersare highly susceptible, so it is necessary to use fungicides with long-lasting efficacy. The chemicals that can be selected at this stage include: chlorothalonil, benomyl, zinc ion-maneb complex and hydantoin RP26109. When used at the old shoot stage, chlorothalonil and benomyl are more stable in terms of protection than the other two chemicals, but chlorothalonil can leave residues on the flowers; in addition, it also has a certain toxicity to plants, so attention should be paid when using it. The application concentrations are as follows: chlorothalonil: 180g/100L, benomyl: 30g/100L, hydantoin (RP26019): 20g/100L, zinc ion-maneb mixture (dithiocarbamate): 192g/100L. In addition, applying benlate 50% wettable powder has a good effect, used at a concentration of 1000 times dilution, once every 30 days. If orchid flower blight has already become a problem, then the flowers must be sprayed just before flowering, and 7 days later benlate must be applied at the above ratio, but for coloured flowers, benlate may leave faint visible stains. Lu Liyen of Taiwan, China reported that orchid plantlets just out of the flask should be sprayed with 58% metalaxyl-mancozeb (mancozeb Ridomil) wettable powder at 600 times dilution immediately after transplanting, and 5% metalaxyl (Ridomil) granules applied to the surface of the planting medium at 25g/sq m can control the harm of orchid flower blight. Applying metalaxyl-mancozeb alone can keep the incidence rate within 3%, but it is not as effective as using the two chemicals together; applying 5% metalaxyl granules multiple times at an interval of 8 weeks can also achieve a 100% control effect. For field application, metalaxyl granules can be applied once at planting time; for flask plantlets, metalaxyl and metalaxyl-mancozeb are applied as described above, and after 6-8 weeks metalaxyl granules are applied once more; in dry and cold seasons with fewer incidences, the chemicals can be applied once every 3-4 months, and applying 4-5 times a year is sufficient to achieve the purpose of disease prevention. (Liu Zhongjian)