Orchid Viruses: Don't Catch That Bug!
|Among all of the diseases that can affect orchid plants, viruses are probably the most feared. Bacteria and fungi will do their damage—many of them are lethal—but, if caught early enough and measures taken to eliminate their effects, can be brought under control. Once a plant has been infected by virus however, it’s as good as gone: there is no practical cure for viruses once they’ve invaded a plant’s tissues. What’s worse, infected plants that have been weakened by virus will often show signs of secondary bacterial and fungal infections that mask the viral presence, making the identification of a virus problem difficult if not impossible. There is currently no way to positively identify the presence of virus in orchids without special testing, but infected plants do show some tell-tale signs which should at least alert the grower that testing may be necessary.|
So what are these signs? How do you differentiate between a fungus or bacterial infection, and a viral one?
Fungus and bacterial infections are usually characterized by black or brown spots, often soft, watery, and mushy looking, that can appear on any part of the plant. These spread quickly but, if caught before the infection reaches the plant’s crown or rhizome, can be eliminated before the plant dies. Other fungal infections appear as spreading, chestnut-brown stains on the leaves, and still others, usually species of Cercospora, cause varying degrees of leaf spotting that appear as tiny dots peppering leaves in a haze. The usual remedy for these is to cut away the affected portions using a sterilized knife or razor blade, then to apply an appropriate disease control to the entire plant. Many growers use cinnamon powder as an effective, natural fungicide, and common Listerine works very well against disease-causing bacteria.
But viruses are a different story. The three most common viruses that affect orchids are ORSV (Odontoglosum Ringspot Virus) , TMV (Tobacco Mosaic Virus), and CYMMV (Cymbidium Mosaic Virus), and they debilitate the plant by disrupting normal growth and causing malformation of plant structures. There is no practical cure. Symptoms that appear include chlorosis (the breakdown of chlorophyll that causes yellow and light brown patches) in diamond-shaped, spiraled, or mosaic-like patterns, elongated yellow spots that grow longitudinally along a leaf’s veins, the distortion and curling of leaves and flower spikes, and, classically, color-break in the blooms. (Color-breaks are streaks and patches of darker or lighter color that “break” the flower’s normal color-flow.) To help with identification, photographs of virally infected orchid leaves can be found here: Orchid Virus Photos
Virus can be mechanically transmitted from an infected plant to a non-infected plant by any tool or procedure that exposes or comes into contact with the plant’s sap. These include knives, pruning shears, plant stakes, wire ties, even used pots--that have not been properly sanitized before re-use. Plants rubbing together in a breeze which induces the leaf edge of one to cut into the other can transmit virus. You can transmit virus yourself by running your fingers along the length of a plant’s leaves or exposing it to rough handling, and that risk is multiplied by orders of magnitude if you’re a smoker or have handled tobacco just before contact. Chewing and piercing insects can also transmit virus: scale, mealybugs, grasshoppers, and aphids are all suspected carriers, so growers whose collections are plagued by these pests run a far greater risk of viral contamination. And because viruses are so tiny, they can enter through wounds that may be completely invisible to the naked eye.
At this time, the best defense against viral attacks is good cultural habits. And while many of these practices may seem fussy, finicky, and outright paranoid to the new orchid grower, their value will be quickly appreciated once an expensive plant is lost due to virus and the loss could have been avoided.
Always sanitize / sterilize any implements you use on your orchids, before and between each orchid you work with. Knives, blades, and shears can be flamed using a soldering torch or, after wiping off any organic debris, soaked in a 10% bleach solution for five minutes. Wooden or bamboo stakes cannot be effectively sterilized due to their porosity, so should never be reused or transferred from plant to plant. Plastic pots, if reused, should be scrubbed clean and then soaked in bleach solution. Clay pots, again, because they are porous, cannot be effectively sterilized by soaking, but can be reused if heated in an oven during a “self-cleaning” cycle or baked for an hour at 500 F. Old bark medium should never be reused on another orchid plant. (Instead, mix it into the soil of your vegetable or flower garden. Your vegetables and flowers will love you for it.) Wash your hands with soap and hot water before and between each orchid you work with. If you choose to use latex gloves, be sure to change them for fresh ones before handling a new plant. The work surface you use for repotting should ideally be made of nonporous material you can easily wipe down with disinfectant before and after each repot. But if a non-porous surface isn’t available, put down sheets of newspaper to do your repotting on, and discard them after each plant that you do.
If an orchid of yours has tested positive for virus, dispose of it as soon as possible, preferably by burning. If burning is illegal in your area, wrap the plant in a plastic bag, tie it, and throw it out. There is no cure, and keeping the plant around for sentimental reasons will only dramatically raise the risk that your other orchids will become infected as well. Please don’t give the plant away to some unsuspecting soul or leave it out on the sidewalk for someone to pick up. Virused orchid plants are useless to everybody!
Viruses are a very real and persevering threat to orchid growers, and the risks are greater if you’re careless or sloppy in your growing technique. But if you keep your growing environment clean, buy plants only from reputable dealers, and remain fastidious in your orchids’ maintenance, your plants will stand a much better chance of staying virus-free and delighting you with beautiful blooms year after year.
Louis J. Aszod
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