Cannabis Diseases and Plant Pests
Plants are considered diseased when their health or development is impaired enough that the adverse effects become visible to the eye. The disease may be caused by infectious microbes, such as bacteria or viruses, by pests such as insects, or by nutritional deficiencies or imbalances. However, for diseases that might affect your plants, there should be no need for a plant doctor. You’ll be able to diagnose the symptoms after careful observation.
Leaves naturally drop from plants during the course of their lives. Not every leaf will develop perfectly or so. The small leaves that are formed during the first few weeks of growth normally die within three months. Leaves at the bottom of healthy plants often die because they are shielded by the upper instance, in a garden receiving only 80 watts of fluorescent light, the plants may stay green only up to three or four feet away from the lights. Lower leaves may turn pale and yellow and then dry to gold or rust colours.
Because Cannabis is not native to the Americas, most of the microbial diseases that attack the plant are not found in this country. Homegrown Cannabis is remarkably free of diseases caused by microbes, and there is little chance of your plants suffering from these diseases. Fungal stem and root rots seem to be the only ones of consequence. These occur only because of improper care. Watering too often, coupled with a stagnant, humid atmosphere, encourages stem rot to develop. Stem rot appears as a brown or black discoloration at the base of the stem and is soft or mushy to the touch. Allow the soil to dry between waterings, and be sure to water around the stem, not on it. Wipe as much of the fungus and soft tissue away as possible. If the rot doesn’t disappear in a few weeks, treat it with a fungicide.
Diseases due to nutrient deficiencies are common indoors, and their symptoms usually worsen with time, affecting more and more of the plant. Whole leaves may be pale, or turn yellow or white; the condition may first afflict the bottom, or top, or the entire plant at once. Deficiency symptoms often appear as spots, splotches, or areas or chlorotic (lacking green) tissue. Sometimes necrotic (dead) tissue appears that is copper, brown, or gray.
Common Plant Pests
The indoor garden is an artificial habitat where the plants live in isolation from the natural world. For this reason, few of you will have any problems with plant pests. Marijuana seeds you use are rarely a source of pests, but be sure to inspect them before germinating. However, indoor plants are particularly susceptible to pests once contaminated. In nature, the pest populations are kept in check by their natural enemies, as well as by the wind, rain, and changing temperatures. Without these natural checks, pests can run rampant through the indoor garden.
The most common and destructive pests are spider mites and whiteflies. Spider mites are barely visible to the naked eye; they are ovoid-shaped. Juvenile mites are transparent and change to green as they suck the plant’s tissue. Adults are tan, black, or semitransparent. False spider mites are bright red. Mites are usually well-established before you discover them because they are so difficult to see.
Whiteflies are white (obviously) but look like tiny moths rather than flies. The adults are about 1/16 inch long, and you may not see one unless it flutters by the corner of your eye. Then shake the plants. If the result looks like a small snowstorm, the plants are infested with whiteflies.
The symptoms of infection by mites and whiteflies are somewhat similar. Symptoms usually appear on the lower leaves and gradually spread to the top of the plant. The first indications are that the plant loses vigour; lower leaves droop and may look pale. Look closely at the upper surfaces of the leaves for a white speckling against the green background. The speckles are due to the pests sucking the plant’s chlorophyll-rich tissue. With time, the leaf loses all colour and dies.
Pests are easiest to find on the leaves that are beginning to show some damage. You can usually see mites and whitefly larvae as tiny dots looking up at the lights through the undersides of the leaves.
To find out which pest you have, remove some damaged leaves and inspect the undersides under bright daylight. With spider mites, if you discover them early, a leaf may show only one or two tiny dots (adults) and a sprinkling of white powder (eggs) along the veins. In advanced cases, the undersides look dusty with the spider mites’ webbing, or there may be webbing at the leaf nodes or where the leaflets meet the petioles. With whiteflies, you usually see the adults first. On the undersides of the leaves, the whitefly larvae look like mites, but there is no webbing, and there are tiny golden droplets of “honeydew” excreted by the adult whiteflies.
Take quick action once you discover your plants have pests. If the plants are less than a month old, you will probably be better off cleaning out the garden, in order to eliminate the source of the pests and start over. As long as the plants are healthy they can withstand most attacks. The more mature the plants are, the less they are affected by pests. Whiteflies and mites sometimes disappear from flowering plants, particularly female flowers. Mites are difficult to eliminate completely. Often a holding action will save a good crop.
If only a few plants in your garden are infected, remove them. Or else, remove any leaves that show damage. If the plants are three or more months old, you might consider forcing them to flower while they are still healthy. Plants that are good-sized and still vigorous will usually stand up well to mites once they are flowering.
If you don’t want to use insecticides, there are several alternative ways to keep the pests in check until flowering. Mix 1/8 to 1/4 pound of pure soap (such as Ivory flakes) thoroughly in one gallon of lukewarm water. Then cover each pot with foil or newspaper, invert it, and dip and swish the plant around several times in the soapy solution. Let it drip dry and rinse with clear water. Use the dunking procedure every week or two until the plants are larger. This is often enough to get the plants growing well and into flowering before the pest population can become a serious problem.
Two homemade sprays that can be effective are dormant oil sprays and hot pepper sprays.
To make homemade hot pepper spray, mix four hot peppers with one medium onion and one clove garlic (213). Grind or chop and mash them along with some water. Cover the mash with water and allow it to stand for a day or two. Add enough water to make two quarts. Strain through a coffee filter or paper towels in a funnel. Add one-half teaspoon of detergent and spray as you would an insecticide.
No one wants to use insecticides, yet they seem to be the only way to eliminate the mites. There are a number of insecticides on the market that are relatively safe. Insecticides such as pyrethrum, rotenone, and malathion are relatively non-toxic to warm-blooded animals when used as directed. These are effective against many different plant pests besides mites and whiteflies. Additionally, they break down into harmless compounds such as carbon dioxide and water in a matter of days; so they do not persist in the environment.
Safe insecticides are used for vegetables. Follow all the package precautions. Do not use more, or more often, than recommended. Overuse can kill the plant. The label will list the number of days to wait before you can safely ingest the plant, usually from two to 35 days after spraying.
Both mites and whiteflies generally complete their brief life cycles in about one to two weeks. Because sprays are not effective against the eggs, repeat the spraying about once a week for three successive weeks to completely eliminate the pests. Since their generations are short-lived, some pests may become resistant to the spray. This can be a problem with whiteflies. Try a different insecticide if the first one does not seem to be working.
Add a couple of drops of liquid detergent to each quart of insecticide solution. Detergent acts as a wetting agent and helps the insecticide to contact the pests and stick to the plant. Small plants can be dunked directly in the solution, the surest way to kill pests.
To spray the plants, start at the back of the garden so that you are working away from the plants already sprayed. Spray the entire plant and soil surfaces, paying special attention to the undersides of the leaves where pests tend to congregate. Stay out of the garden and keep the room closed that day.
Sulphur dust can also be effective against mites and many other pests and are safe to use. The easiest way to apply them is with a plastic “squeeze” bottle which has a tapered top. Make sure you dust the underside of the leaves.
Before using any insecticide, remove all damaged leaves. Do not use any insecticide during flowering. Rinse the plant with clear water spray about one week after applying any insecticide, and once more before you harvest. Otherwise, there may be residues left which will affect the taste of the grass.
There are several other pests that can be a problem, although they rarely seriously affect marijuana. Aphids are about 1/16 inch long and are black, green, red, or pink. They have roundish bodies with long legs and antennae. Some species have wings. They congregate on the undersides of leaves which may then lose colour and become curled or distorted. Aphids excrete honey-dew droplets on the undersides of the leaves which can attract ants. If ants are also present, set out ant traps, because the ants will spread the aphids to other plants. A few successive washing in soapy water or one or two sprayings of the insecticides mentioned above should eliminate aphids.
Mealybugs are white, about 3/16 of an inch long, and look like small, flat sowbugs. They don’t seem to like marijuana and avoid it if other plants are present. Mealybugs can be removed individually with cotton swabs and alcohol.
Gnats are attracted to moist soil that is rich in partially decayed organic matter such as manures. To discourage gnats when using manures, cover the top few inches in the pot with the soil mixture and no manure. Drench the soil with malathion solution for gnats or any other soil pest. Flypaper will also help against gnats as well as whiteflies.
Some people don’t mind having a few pests on their plants. Whether you want to eliminate the pests completely or simply keep them in check may come down to whether you mind hearing the snap, crackle, and poop as their little bug bodies heat and explode when the harvest is smoked. Commercial marijuana, or any marijuana grown outdoors, will contain innumerable bugs and other small lifeforms.
Inspect your marijuana seeds for any pests, let your cannabis seeds sit in a light bleach solution for half an hour before moving them to germination. This way you can prevent and kill any fungus that may be present on cannabis seeds surfaces. Whiteflies and spider mites are extremely contagious. Mites can be carried to the plant on hands, clothing, or an animal’s fur. Many houseplant pests can fly or float to the garden through open windows. Mites crawl through cracks in walls and foundations during autumn, seeking warmth.
Many houseplants are popular because they can withstand abuse and infections by common plant pests. Your houseplants may harbour mites for years without your knowledge. You can find out if your houseplants have mites by placing some marijuana seedlings among the houseplants. Mites seem to enjoy young marijuana plants so much that the plants show symptoms of mites in a matter of weeks of any are nearby.
Hopefully, you’ll never have to deal with pests. Prevention is the best policy. Use soil that has been pasteurized or sterilized to avoid bringing pest eggs and larvae into the garden. Keep the garden isolated from other plants. Use separate tools for the marijuana garden and for other plants. Screen windows in the garden with wire screen or mesh fabrics such as nylon.