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Orchid Disease – Bacteria and Fungi

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All orchids, whether in cultivation in a greenhouse, in a tropical garden or even in their wild state, can be subject to various fungal or bacterial infections. They are particularly troublesome in seedling houses, where once a rot starts to infect a seed tray it will quickly spread to the rest of the plants. If your pot-grown orchids begin to rot for no apparent reason, they can quickly decay until they are beyond saving. Some infections and fungi are air- or water-borne and can be introduced to your plants by these routes. Where a greenhouse is kept scrupulously clean and no rubbish is allowed to remain on the floor or benches, the spores from other decaying material are less likely to spread to the orchids. Even a compost heap dose to the greenhouse can be a source of infection. Hygiene is better than cure, and healthy, fit plants will fight off an infection naturally.

In the past, orchid growers often kept water tanks under the benches. During watering, the surplus water runs back through the slats in the benches into the tanks underneath. Continual recycling of the water through the plants can bring a real risk of water-borne diseases building up in the warm greenhouse. Each time the plant is watered, you are reintroducing the infection and it will soon reach a stage when it is impossible to combat the bacteria. A rainwater tank outside the greenhouse will have the same effect, particularly if trees shed their leaves into the tank. Although the use of soft water is recommended, rather than hard mains water, even soft rainwater that has been stored for long periods and recycled constantly will be more harmful than tap water.

The problem usually starts in the autumn or winter when the orchids’ growth rate has slowed down. Low temperatures, combined with too much moisture for plants that should be resting or growing very slowly, will quickly set up an infection at the base of the growth. Once begun, basal rot is difficult to stop and will spread right through the plant. Young seedlings are easily infected; the whole tray can damp off within a few days. Soft-leaved orchids such as phalaenopsis are very prone to rots which run rapidly through their foliage. Provided resting orchids are kept dry in the winter, given plenty of air movement and good light, the problem will not arise. Miltoniopsis and phalaenopsis that grow all the year round must be kept warm to prevent infections.

Flowers and buds can also be infected at this time of the year. Large white phalaenopsis very quickly show damp spots if left for two or three nights in a low temperature.

Cymbidium and phalaenopsis buds are very susceptible to low light levels. During the darkest of the winter months, several days or weeks with no sunshine at all result in the buds turning yellow and dropping off. Keeping the plants warmer to counteract the poor light will have the reverse effect, for while they can stand the heat in the summer when the light is good, they do not like poor light combined with heat. Some growers try to supplement the light levels with artificial light, but this is only successful with certain types of orchid, such as phalaenopsis and cattleyas.

Once a plant shows signs of rot at the base of the new growth or bulb, it is essential that the affected parts are quickly removed. Clean back to healthy material and treat with any of the fungicides or bactericides that are available. Flowers of sulphur can be used to help prevent the disease from spreading or occurring again. Be careful not to overdose with fungicide as this can act as a growth retardant. Cut back affected Phalaenopsis leaves with a sharp knife, then dry the edge and paint with fungicide. All cutting tools must be cleaned between each cut, either with a flame or with disinfectant.

Flowers that have become spotted are best removed to allow the plant to go back into growth and flower again later when it is healthier. Maintaining the right temperature, with the use of a small fan to assist in air movement on cold winter nights and long cloudy days, is the best method of keeping these problems at bay. Orchids growing on a windowsill are better off in the dryer conditions found indoors and, as a result, have less chance of contamination.