Control Methods for Bacterial Diseases in Orchids (Part Two)

Bacteria are also single-celled organisms and can only be seen under a microscope. The harm bacteria cause to plants is mainly through the production of toxins, which cause plant rot or tissue death, or block and destroy vascular bundles, forming tumors. Since bacterial control is relatively difficult, the best control method is to prevent the occurrence of the pathogens.

1. Orchid brown rot

It is one of the common bacterial diseases worldwide. Its pathogenic bacteria are Erwinia cypripedii, Pseudomonas gladioli, etc., and it mostly occurs in humid, warm environments. It generally damages the buds or leaves of orchids. When affected, water-soaked yellow spots first appear on the leaf surface of the orchid plant, then gradually turn chestnut brown and may become sunken. The water-soaked area then becomes brown and rots, often spreading rapidly to successively emerging leaves, then destroying the leaves and causing them to fall off, and sometimes harming the entire plant.

Control method: Once an orchid plant is affected, the diseased leaves should be removed promptly until only the pseudobulbs remain, then spray with 200 mg/L agricultural streptomycin or 0.5% Bordeaux mixture, once a week, for 3 to 5 consecutive times.

2. Orchid soft rot

Also known as orchid leaf spot disease, this disease is easily triggered under high temperature and high humidity, and is mostly seen in tropical regions of Asia. It occurs relatively severely in southern China and can be transmitted through soil. It can also infect through wounds caused by transplanting and management operations and through feeding marks of pests, and can also spread with rainwater or watering. Its pathogenic bacteria are Pseudomonas cattleyae, Erwinia species, etc. It generally manifests as disease of the whole plant, mostly infecting from the rhizome. When leaves are affected, dark green water-soaked small spots appear, rapidly expanding into yellow-brown softened rot. Brown water droplets sometimes seep from the rotting areas, with a special foul odor, and in severe cases the leaves rapidly turn yellow. If the pseudobulbs are infected, water-soaked lesions also appear, brown to black, ultimately making the pseudobulbs soft, shrunken and dark, and rapidly rotting.

Control methods:

(1) Once an orchid plant becomes diseased, the diseased leaves should be cut off promptly, and the wounds should also be coated with Bordeaux mixture;

(2) The diseased orchid plant can also be pulled out and soaked in 1% potassium permanganate solution for 5 minutes, then after washing, exposed to sunlight for 15 minutes, dried, and planted;

(3) Spraying with 0.5% Bordeaux mixture or 200 mg/L agricultural streptomycin or methyl parathion, etc., can also be used.

3. Orchid leaf rot

It mainly infects Vanda, Dendrobium, etc. The pathogen is Erwinia chrysanthemi. In affected Vanda, translucent patches appear on the leaf surface in the early stage; in diseased Dendrobium, the leaves are yellow and water-soaked in the early stage, and finally the lesions all turn black and sunken. Ultimately, this can cause the entire leaf to rot and fall off.

Control methods:

(1) Once an orchid plant becomes diseased, the diseased leaves should be cut off promptly, and the wounds should also be coated with Bordeaux mixture;

(2) The diseased orchid plant can also be pulled out and soaked in 1% potassium permanganate solution for 5 minutes, then after washing, exposed to sunlight for 15 minutes, dried, and planted;

(3) Spraying with 0.5% Bordeaux mixture or 200 mg/L agricultural streptomycin or methyl parathion, etc., can also be used.

4. Orchid brown spot

It harms many kinds of orchids. The pathogens are Pseudomonas cattleyae and Pseudomonas cypripedii, and it is prone to occur under high temperature, high humidity and poor ventilation. It spreads through cultivation materials, flower pots, etc., from wounds or pest feeding marks. It mainly harms the young shoots and leaves of orchids. When it occurs on leaves, soft, water-soaked small spots first appear, then develop and expand into clearly outlined, slightly sunken brown or black water-soaked spots. Sometimes turbid droplets are produced at the rotting areas of the spots. Once this disease occurs, the lesions spread very rapidly, and can cause the death of the orchid plant within just a few days.

Control methods:

(1) During cultivation, attention should be paid to environmental ventilation, and avoid watering "overhead";

(2) If diseased plants are found, the infected organs or the entire plant can be cut off according to the situation, then sprayed with 0.5% Bordeaux mixture or 100 mg/L agricultural streptomycin, or soaked in 0.1% potassium permanganate solution for 5 minutes, washed, dried, and then planted.

5. Bacterial flower rot

It mainly harms Cymbidium, Cattleya, Paphiopedilum, etc. The pathogens are Pseudomonas species and Pseudomonas fluorescens. They can both parasitize dead plants and infect already damaged tissues. After orchid plants are infected, rotten spots mostly appear on the flowers, including some small, necrotic lesions, with water-soaked halos, and cause necrosis and rot of roots, stems, buds, and flowers.

Control methods:

(1) The most effective method to control this disease is to replace the growing medium and strictly disinfect it. Soil disinfection methods can use high-temperature sterilization, and spraying with 200 mg/L agricultural streptomycin and 0.5% Bordeaux mixture can also be used.

(2) Cut off the infected tissues, and soak the infected plants in 0.1% potassium permanganate solution.