What to Do When Orchids Get Anthracnose? Detailed Guide to Prevention and Control Methods
Anthracnose occurs in all orchid-growing regions of our country, with damage being particularly severe in Tianjin, Shanghai, Nanjing, Guangzhou, Chengdu, Lianyungang, and Xi'an. When the disease is severe, it not only affects the normal growth of orchids but can also cause entire leaves to wither and die. Besides harming terrestrial orchids such as Cymbidium goeringii, Cymbidium sinense, Cymbidium faberi, Cymbidium ensifolium, and Cymbidium kanran (that is, Chinese orchids, referred to simply as orchids), anthracnose also harms epiphytic orchids such as Cymbidium hookerianum and Cymbidium lancifolium, as well as various ornamental plants including Dieffenbachia picta, Matthiola incana, Calendula officinalis, Hibiscus rosa-sinensis, and Osmanthus fragrans.
I. Symptom Characteristics.
Anthracnose mainly harms orchid leaves, and sometimes also infects stems and fruits. Leaf spots initially appear round or elliptical, with a light brown or grayish-white center, dark brown or blackish-brown edges, and a green fading halo ring around them; later, black small dots form on the spots, scattered or arranged in a slightly concentric pattern, and under humid conditions, orange-yellow sticky substances appear. As the disease progresses, leaf spots can expand into irregular large spots up to several centimeters long, or spots can merge into patches, ultimately causing leaves to turn yellow and wither. Leaf spots range in size from 3-20 mm, varying greatly. Stems and fruits develop irregular or elongated blackish-brown spots when affected.
II. Disease Occurrence Patterns.
The disease is caused by infection from several species of anthracnose fungi (Colletotrichum orchidearum, Colletotrichum gloeosporioides, and Colletotrichum gloeosporioides). The pathogen overwinters in diseased leaves, diseased leaf debris, and withered leaf base bracts, and in the following year, under suitable climatic conditions, spreads through wind, rain, and insects; it mainly invades plants through various wounds. The disease can occur almost year-round, but is most severe in summer and autumn. For example, during the plum rain season in the south, or when typhoons accompanied by heavy rain occur in autumn, causing orchid leaves to be wounded, or under conditions of high humidity, stuffiness, overcrowded placement, poor ventilation, and water accumulation in pots, the disease is most severe at these times. Watering from above the plant (overhead watering) easily spreads the disease. Orchids divided and propagated in the current year often suffer severe disease because their roots are easily damaged. Heavy, compacted potting soil also exacerbates the disease.
III. Prevention and Control Methods.
1. Strengthen cultivation management: Greenhouses should be well-ventilated and well-lit. In winter and early spring of the following year, special attention should be paid to keeping warm, preventing damage from freezing and frost, so as to enhance the plants' ability to resist disease. After being moved outdoors, they should be placed in shade houses to prevent wind and rain damage, while the shade house should have sufficient sunlight. Watering is best done by slowly pouring from the edge of the pot or by subirrigation, which can reduce the spread of pathogens from splashing water droplets caused by overhead watering. Pots should not be placed too densely. Fertilization of orchids should be done scientifically. It is well known that orchid fertilizer should be decomposed rather than raw. Applying raw fertilizer easily induces anthracnose. For potted plants, new soil should be replaced.
2. Eliminate sources of infection: Timely remove diseased leaves and their debris; especially before bringing plants indoors in winter, thoroughly remove diseased leaves from plants and diseased debris from pots, and burn or deeply bury them to eliminate sources of infection and prevent the disease from spreading. In winter and spring, spray the ground, pot surfaces, and plants comprehensively with 0.5-1% Bordeaux mixture 1-2 times.
3. Chemical control: Before disease onset, spray with 0.5-1% Bordeaux mixture, or 65% zineb wettable powder at 600-800 times dilution, once every 7-10 days, for good protective effect. During disease occurrence, spray with 50% carbendazim at 800 times dilution or 75% thiophanate-methyl at 1000 times dilution, both of which can control the spread of the disease.
4. Non-pesticide control methods: (1) Vinegar. Spraying vinegar on orchid leaves can prevent and control anthracnose. Mix 1 tablespoon of vinegar into 1 kg of clean water and spray on orchid plants. Vinegar can also be added when using pesticides. (2) Chive juice. Mix chive juice and clean water at a 1:60 ratio, spray on orchid plants, spray twice daily, repeating for several days, to treat anthracnose. (3) Pig bile. Mix pig bile and clean water at a 1:100 ratio, add a small amount of laundry detergent, and spray on orchid plants to prevent and control anthracnose and soft rot. (4) Quicklime water solution. First use an appropriate amount of water to slake quicklime into powder form, take the slaked quicklime powder and clean water at a 1:60 ratio, place in a container to soak, and take the clarified liquid to spray around orchid plants to prevent and control anthracnose. Quicklime is alkaline and can also prevent and control orchid white silk disease.