What is orchid black rot? A detailed explanation of the fungal disease

OrchidBlack rot (Orchid blak rot), also known as crown rot (Crown rot), damping off (Damping off) or heart rot (Heart rot)] Distribution range: worldwide; common in Asia. Pathogens: Phytophthora palmivora and Phytophthora cactorum. Hosts: Cattleya, Coelogyne, Cymbidium, Dendrobium, Laelia, Oncidium, Paphiopedilum, Phaius, Phalaenopsis, Rhynchostylis, Vanda, Vanilla, etc. Symptoms: In the early stage of infection, small brown wet spots appear, with yellow edges. Irregular wet lesions are mostly seen on the lower surface of the leaves, then gradually expand. The centers of older and larger lesions become dark brown or black, and water seeps out when squeezed, while old lesions become dry and black. Infected leaves turn yellow, wither, and fall off, the area near the base of the leaf is noticeably discolored, and the pseudobulbs are also infected and turn black. The growth point of an infected plant easily falls off. Under extremely humid conditions, infection can also appear at the shoot tip. If the flowers are infected, pale brown (darker in the center) wet spots also appear. However, the symptoms also vary under different genera and differentcultivationconditions. Sometimes the young buds are infected first, causing the ungrown leaves to fall off; in other cases, the roots and rhizomes are infected first, then the infection spreads upward to the pseudobulbs and leaf bases. This disease ultimately causes the entire plant to die. Control: Remove the infected parts or organs, and burn them or bury them deeply. Copper-containing fungicides can be used, such as spraying with 0.1%-0.2% copper sulfate solution, or 50% captan wettable powder at 400-500 times dilution, 80% fosetyl-aluminum wettable powder at 400-600 times dilution, 25% ridomil (metalaxyl) wettable powder, or 1% Bordeaux mixture. Regularly spraying with 50% thiram at 100-150 times dilution is an effective measure to prevent this disease. In addition, maintaining ventilation and avoiding humidity can prevent the spread of this disease. Note: This disease is easily transmitted through contaminated potting soil, water, etc., and spreads especially quickly in high-temperature and high-humidity environments. Another type of black rot is caused by Pythium ultimum, with similar symptoms; the pseudobulbs first become hard, then dry up, and the disease can spread to the roots. The control methods are also roughly similar. (Chen Xinqi, Ji Zhanhe)