Introduction to the Control Methods for Two Orchid Diseases

Based on my records over the past two years of theorchidgrowth and disease occurrence, control measures and results at the Xiamen Botanical Garden, I will discussorchidbrown rot and anthracnose diseases and their control measures. Due to the high temperatures of midsummer, excessive watering, overly wet and compacted soil, poor aeration, and mechanical damage to the pseudobulbs,orchidsare very susceptible to brown rot. The pathogens are quite diverse, mainly Phytophthora palmivora and a small amount of Phytophthora nicotianae. The disease mostly appears on the leaf bases and pseudobulbs of one- to two-year-old plants as water-soaked blotches, and the affected areas gradually soften, appearing light brown to dark brown; at the same time, the vascular tissue of the pseudobulbs turns black, the central leaves wilt, and yellowing proceeds from top to bottom. Orchids grow in dense clumps with interconnected pseudobulbs, and the pathogen spreads through the vascular tissue of the pseudobulbs and external contact, quickly infecting healthy plants around the diseased one. The disease progresses rapidly, and within a few days it can affect the entire pot. June to August, with high temperatures and frequent rain, is the peak period for brown rot, so observe orchids carefully every morning and evening and strengthen prevention. In addition to sterilizing the potting medium by roasting or sun-drying when repotting orchids, routinely spray every ten days with an equal amount of Bordeaux mixture or 50% Tuzet 800-fold solution. Once disease spots are found on orchid leaves in a pot, immediately isolate them from healthy orchids, change the potting soil, and use a sterilized single-edge razor blade to completely remove and burn the diseased plants. For the normal plants that are kept, rinse the roots with clean water and cut off any rotten roots. If only 1-2 seedlings can be saved and the root system is too poor, with orchid leaves dehydrated and shriveled, you may cut the leaves by half or more, then completely immerse the plant in 0.3% potassium permanganate and remove it, apply Tuzet or carbendazim powder to the wounds, place it with roots up and leaves down in a shady spot to air-dry for more than ten minutes, then mix sterilized potting soil or 65% zineb 600-fold solution with bamboo root mud and replant. The potting soil of diseasedorchidsmust not be reused. After repotting, orchids should be placed in a cool, shady area for care, with attention to ventilation and light, no fertilizing in the first year, keeping the potting soil slightly dry with good drainage, and watering without pouring into the leaf center. After spraying with clean water in the morning and evening, fine uniform droplets should appear but not run off. To the spray water, add 0.5% each of monopotassium phosphate and urea to supplement the nutritional consumption of the repottedorchidsand promote new root and shoot growth; after one month they can grow stably. With careful management, they will still send out leaf buds that year and grow to at least close to the height of the plant the following year. Orchid anthracnose occurs commonly and severely. Initially, the lesions appear as reddish-brown semicircular spots, and later expand into dark brown patches, sometimes with a yellow halo at the edges, while the center turns from grayish-brown to grayish-white and produces small black dots arranged in concentric rings. When leaf lesions spread downward, the leaf forms a dead section. When the disease develops severely at the leaf base, it can cause the entire plant to die. The pathogen is Colletotrichum orchidearum, which overwinters as mycelium on diseased leaves of leftover plants. Lesions on old leaves begin to appear in early April, develop rapidly in May and June, and after July when new leaves sprout, lesions also appear. Control methods: the orchid room should be well ventilated and lit, pots should not be placed too densely, place them in a spot sheltered from wind and rain, and water from the edge of the pot. Take care to protect from frost damage to reduce infection opportunities. In spring and autumn, spray 50% carbendazim wettable powder 500-fold or 50% benlate wettable powder 1000-fold solution or 50% thiophanate wettable powder 500-fold solution, once every 10 days for a total of 3-4 sprays, which has a certain effect. (Zhang Hong)