Orchid crown rot

1. Harm: Crown rot, also known as black rot, has not been systematically reported in China, but it often occurs in Singapore, Thailand, the United States and other places, and has become a destructive orchid disease. In China, it was recently found in the orchid in Shenzhen. In fact, all genera of orchids are susceptible to crown rot, while Cartland, Dendrobium, orchid, Phalaenopsis and Magnolia are more susceptible, while Arachnis, Renanthera and Arantheras are more resistant.

2. Symptoms: the initial symptom of orchid crown rot is usually a dark brown discoloration at the base of the young leaves near the new buds. when the disease is serious, the diseased leaves will easily droop and protrude outward, and show a deep purplish brown at the infected base, and the infection spreads rapidly, causing the leaves to blacken and rot, and these infected leaves can be pulled out easily. The leaf disease of young plants of Dendrobium is usually regular in shape, olive green to dark green brown, the edge of water stains will also appear blister, and the infected leaves will fall off finally. In a dry environment, the diseased leaves are sunken and often dark, ranging from brown to black, and similar symptoms can occur on the young leaves of older plants. Magnolia leaf disease is water-stained when it is wet, brown to brown and yellowing over time. Infection can be rapidly extended to the lower part of the stem, pseudobulm and root. Pseudobulbs and stems are easily decayed by leaf and polar infection, and occasionally rot directly. In many varieties, the rot of pseudocorms or stems is usually brown to black, and the progressive part of the disease is irregular. on the contrary, tender and succulent stems and pseudocorms have soft rot and black necrosis, such as diseases caused by bacteria. In the "enterprise black" varieties of Cymbidium in China, the buds are usually harmed. The symptoms first appear at the top of the leaves of the young buds, showing dark brown, and then gradually expand downwards. In serious cases, every leaf of the sprouting buds is infected.

3. Pathogen: the pathogen of orchid crown rot was identified as PhytoPhthora Palmivora. Synonyms are: P.faberi, P.theobromae, Pomnivorum var. Arecae, P. arecae, P. fici,P. Carica,P.carica and P.meadii. There are prominent papillary processes at the tip of the sporangium, inverted pear-shaped, small stalks, 40~60X25~35Pm, thick vesicles in diameter 32~42Um, ovary capacity male, oospore diameter 22~24um. Sporangia and chlamydospores are produced quickly (within two weeks) on the common medium, egg-hiding organs are rare and often not produced, and the optimum temperature for mycelial growth is 25,27.5 degrees.

4. Characteristics of disease: orchid crown rot can infect plants of all ages. The pathogen produced a large number of spores in the diseased tissue, and the spores were initially spread by splashing water or contaminated pot substrate. Chlamydospores were produced on the infected host tissue and overwintered, becoming the initial infection source of the disease in the coming year. The disease is most common during the rainy season or in moist weather for a long time.

5. Prevention and control methods:

(1) strengthen quarantine measures, because orchid crown rot has not been widespread in China, but it is very popular in some foreign places, so imported orchids, especially those susceptible to the disease, should strengthen quarantine and list the disease as a quarantine object.

(2) the pathogen of orchid crown rot has a wide range of parasitism, except orchids, it is common in papaya, cacao, ivy, palms and other ornamental plants, so it will bring serious problems about the cross-pathogenicity of these bacteria, that is, the pathogens parasitic on other strange hosts can also become the infection source of orchid crown rot. Therefore, other hosts (such as weeds, etc.) of the pathogen should be removed in order to avoid parasitism, and orchids should be planted separately from other ornamental plants that can be parasitized by the pathogen.

(3) artificial control: often remove diseased leaves and fallen leaves in order to reduce the initial infection source of the disease. If the infection occurs at the base of the young leaves and not in the lower depths of the new leaves, the diseased leaves should be removed and the incisions should be as low as possible to remove the diseased tissue, but if the neck buds are also infected, the infected leaves must be removed, leaving only healthy leaves on the plant. Try not to splash water when dripping water, it is appropriate to water at the edge of the basin rather than sprinkler irrigation, in the open field must also be sheltered to avoid direct rain.

(4) Biological control: some Difolatan and a few drops of water can be poured into the interleaf base to cover the wound, which can prevent further infection. In the case of severe infection, the tip uninfected part can be cut off at the distance from the infected leaf 5~8cm, then sprayed heavily with Difolatan, and replanted a little further away from the healthy plant. repeated use of Difolatan spraying every other week can improve its chance of survival. if the disease is already serious, the plant can be soaked in 1Difolatan 200O solution of 8-hydroxyquinoline sulfate or o-phenylphenol sodium. Soak the plants, pots and other utensils in the above solution for 60 minutes. If the rot continues to develop, it can be soaked repeatedly within 3 to 7 days. (Liu Zhongjian)