Insects and disease love conditions in greenhouses. The early warm air and ground temperature, plus the lack of beneficial insects, creates an artificial environment where some diseases and insects can damage plants before natural predators are established. To avoid serious infestations, insects and disease need to be spotted and treated before they cause problems.
The best control for insects and disease is prevention. Sanitation is a must. Weeds and dead or infected plants should be removed from the greenhouse on a regular basis. Sanitize recycled containers and materials. Selecting naturally disease-resistant plant varieties will also help control disease in the greenhouse, garden, and field.
Diseases that cause problems can be present in the growing medium. Most commercial mediums contain fewer disease organisms than unheated mixtures of soil and compost. If the composting process is managed skillfully, disease organisms and weed seed may be killed providing a disease and weed-free medium.
The tobacco mosaic virus is spread by smokers and by insects that have come into contact with infected plants. Greenhouse plants that are susceptible to the virus include tomato (most severely affected), muskmelon, cucumber, squash, spinach, impatiens, phlox, and zinnia. The best ways to prevent contamination are to eliminate weeds that serve as hosts for the virus and never allow customers or helpers to smoke in the greenhouse area or touch plants or supplies after smoking.
To prevent the further spread of insects and disease, respond quickly when they first appear.
Common pests in the natural greenhouse include aphids, whiteflies, thrips, sow bugs (also called pill bugs), spider mites, leaf miners, and scales. When you keep flats and pots off the ground, sow bugs are rarely a problem in a clean greenhouse.
I use biodegradable chemicals proven environmentally safe. Most are certified organic. There are three insect control products that I am never without: insecticidal soap, pyrethrum, and Bacillus Thuringiensis (Bt). Each is biodegradable and safe for humans, animals, and natural predators when used as recommended.
I use insecticidal soap to eliminate aphids, whitefly, mites, thrips, scales, and any other soft-bodied insects. Mites are spiders, but soaps and horticultural oils control their soft bodies. Scales in the adult stage are not easy to kill without harming the plant, but in the crawler stage their young are soft-bodied and can be controlled with insecticidal soap. Since the adult dies shortly after laying its eggs regular use of soaps will eventually eliminate scales from your plants. If only a few plants are infested, you may prefer to remove the infected plants.
Soak the tops and bottoms of the leaves. Spray uninfected plants near infestations to eliminate the spread of insects. Continue to watch for new infestations and re-spray infested areas in two or three days to make sure late-hatching pests are eliminated. Insects do little damage in small numbers but, when left untreated, they multiply rapidly in the greenhouse and move onto other plants, causing damage wherever they go.
Learn about the life cycles of these organisms. When natural controls are available to interrupt the life cycle, it may be unnecessary to spray.
Ants do not usually eat or infect healthy plants, but they transport aphids into the greenhouse and manage them for food. Small concentrations can be eliminated before they spread. Little white exoskeletons, shed by growing aphids, are more easily spotted than the aphids themselves, which are usually under leaves and out of sight. I use Terro, which ants carry back to their nests, eliminating the entire nest.
The active ingredients in Bt paralyze the digestive system of caterpillars, causing death soon after. This article does a good job of explaining how Bt works. If tomato hornworms have white eggs on their backs, you don’t need to use Bt. The eggs are the parasitic wasp, Trichogramma minutum, which will soon kill the hornworm and help control them in the future.
I use pyrethrum and occasionally a pyrethrum-rotenone combination for control of sow bugs, cucumber beetles, and squash bugs. On contact, pyrethrum irritates the insect’s nervous system, temporarily stopping feeding. Rotenone kills the insect by disrupting cellular respiration, killing it slowly. Using pyrethrum and rotenone together stops feeding quickly and eventually kills insects before they can recover.
By state law (in Ohio), if a product is registered for use in greenhouses the container will say “for use in the greenhouse.” Terro, pyrethrum-rotenone combinations, and many other natural controls are not registered for use in the greenhouse. This creates a dilemma for natural greenhouse growers. These products are far less toxic than many products labeled “for use in the greenhouse,” so it is likely that these and other products have never gone through the legal process required to be labeled “for use in the greenhouse.” Thus, the state labeling process doesn’t help me much. So, I have to think for myself and then do the safe and responsible thing. I carefully select natural insect control substances that are known to be safe for use around plants and humans. You will have to make your own informed decisions.
Disease, Fungus, and Mold Control
Avoid using plant varieties that are known to require anti-fungal sprays. Providing good sanitation and optimal growing conditions will significantly reduce problems related to disease, fungus, and mold.
When fungi appear, try changing the conditions that allow them to grow and spread. For example, damping off of young basil, leek, onion, pepper, vinca, and globe amaranth seedlings can be avoided by growing seedlings a little drier than other varieties. Water plants when the sun is shining to allow the soil surface to dry before sunset. If damping off occurs, a light dusting of agricultural lime on the growing surface dries and changes the pH on the top of the soil, stopping the spread of the fungi on my flats. Keep plants, susceptible to molds and fungi, in warm, well-ventilated areas. Check plants, regularly, until they outgrow the stage where fungi and molds are a problem.
You should also give your greenhouse at least one deep cleaning each year. This article does a great job of explaining how to do that.
Develop your own common-sense approach to disease and insect control in your greenhouse and garden. If you are having pest problems you can’t control, consult people who are knowledgeable about natural or certified organic methods of pest control.
Check in The Encyclopedia of Natural Insect and Disease Control by Rodale or take samples to your county Extension educator. ATTRA (Appropriate Technology Transfer for Rural Areas) is a national sustainable agriculture information service that can provide or help you locate almost anything you need to know. You can visit their website or call 800-346-9140.
Gini Coover is the author of The Natural Greenhouse, Growing Plants and Food for Profit. She has grown greenhouse plants and vegetables for over thirty years.