Dendrobium leaf spot

1. Harm: the disease occurred in the United States and Thailand, as well as in Taiwan, but has not been reported in mainland China. The pathogen of this leaf spot disease was first discovered by Burnett (1964) on West plam Beach of Florida. The host range of the pathogen of the disease on orchids is very narrow, which is generally limited to endanger Dendrobium. In Taiwan, only infected Dendrobium includes Dendrobium and Dendrobium, and other orchids such as Phalaenopsis, Cadelia, Cymbidium, Oncidium and Orchid are not infected. The disease of large flower species (Phalaenopsis tyPe) is more serious than that of small flower species (antelope type). But once the disease occurs, it will spread rapidly.

2. Symptoms: this pathogen only harms the leaves of Dendrobium, which is immersed in water at the initial stage, the disease spot slowly expands into an inconspicuous macula with an inconspicuous edge, the disease spot is round to oval, with a diameter of 8~32mm, and there are usually several irregular yellowish spots on the leaves. When the disease spot is mature, it will sink slightly, showing purple-black, and the expanding edge will remain yellow. The conidiophores grow in the stomatal part of the leaf back of the old disease spot, and a small pile of gray conidia can also be seen in the tip part of the leaf back. As the plaque matures, it will gradually become lavender black or black, and show symptoms similar to mites. The scattered disease spots on the leaves fused with each other to cause leaf yellowing and defoliation. The life span of the seriously diseased leaves was shortened and finally fell off from the plant.

3. Pathogen: the pathogen of Dendrobium leaf spot was identified as Pseudocercospora dendrobil. The conidiophores are dark, growing from stomata, erect or slightly curved, rarely branched, the tip of the spores is obtusely round, and there is no obvious bubble mark at the spore seat; the size of the conidium is 4~6um*25~75um, the conidia is curved needle-shaped or needle-shaped inverted stick-shaped, 4-9 septum, the size is 2~3.5um*61~69um. The growth of this strain was slow on PDA plate, and the mycelium protuberance was slightly wrinkled. The temperature range of pathogen growth and spore germination was 10 ~ 30 ℃, and the optimum temperature was 25 ℃. When the growth was above 35 ℃, the growth of the pathogen was obviously inhibited and almost no growth. In different culture media, the dry weight of mycelium was the highest in Mucor synthesis medium and potato glucose culture medium, and the lowest in sugarcane leaf cut juice culture medium and hydroxymethylcellulose culture medium. The pathogen was difficult to produce spores, and no sporulation occurred in all the culture experiments.

4. characteristics of the disease: the disease began to occur in the simple greenhouse in late August every year, the defoliation in the orchid garden generally occurred in November, and the leaves in the serious disease areas had fallen off before March to April of the following year. In greenhouse, there is an obvious correlation between the occurrence of the disease and temperature. In places with high temperature, good ventilation and low humidity, the disease is not serious. High temperature can inhibit the development of the disease, and high humidity can promote the occurrence of the disease.

5. Prevention and control methods:

(1) strengthen quarantine and pay special attention to the import of Dendrobium from abroad.

(2) strengthening the management and paying attention to the ecological environment of the orchid room, especially in the simple greenhouse, reducing the indoor humidity during the heat preservation period to restrain the epidemic of the disease is the focus of disease prevention and control.

(3) apply 50% 5O0 wettable powder solution and add a little wetting agent, apply 1 every two weeks.

A total of 3 times, the effect is excellent. After that, the drug was used according to the above dosage once a month to prevent the plant from re-infection.

(4) it is also effective to use 7O0 double solution to prevent and cure the disease, and it must be used for the second time after 5 to 7 days in the severe disease area.

Medicine.

(5) because the application period of the disease is just in the flowering stage, the general application of pesticides will residual drug spots on the petals, but spraying with 75% tetrachloroisophenol wettable powder 5O0 solution is not obvious and the control effect is significant, so the drug can be used in the flowering stage. In addition, 56% Befen sulfuric acid wettable powder also has a good control effect on the pathogen of the disease. (Liu Zhongjian)