Organic Pest and Disease Control

A plant's main enemies are pests (such as rabbits, insects, slugs and snails, and nematodes) and diseases (like fungi, bacteria, and virus). Most can be controlled using ecologically friendly methods. Rather than setting a goal of a pest-free garden, learn how to work with nature to keep problems at an acceptable level. Vigilance is the key to growing good plants; paying close attention to how the plants are growing will pay off. Finding a pest or disease problem in the early stages, when it is relatively easy to deal with, is preferable to suddenly discovering that the crop or plant in question is seriously infected with some problem.

Pest control This can be approached in stages, depending on the problem. Animals (rabbits, gophers, etc.) can be kept away from food crops by surrounding the area with a wire fence with the base buried in the soil. In the flower garden, plant species they do not like to eat. Protect the stems of trees with a wire guard for the winter (make it high enough to allow for snow) and spray shrub stems with a hot pepper spray after the last rain in fall. Deer and elk are difficult to repel without enclosing the entire garden with a high fence, but there are plants that they are less likely to eat (see list on p. 68).

Large insects, such as caterpillars and beetles, can be picked off by hand and dropped into a bucket of soapy water. When larger plants, like trees, are attacked, the soil directly under their foliage can be covered with drop sheets and the plant shaken to dislodge the pests. The drop sheets can then be carefully lifted and the pests destroyed. Caterpillars that congregate in webbing "nests," like tent caterpillars and fall webworms, should be controlled during the day, when the young larvae have left the nest, by spraying them with Bacillus thuringiensis. Another alternative is to wait until evening when the caterpillars have returned to the nest. Prune off the nests and immerse them in a bucket of soapy water to kill the larvae.

Plants can be protected from damage by soil-dwelling caterpillars, like cutworms, by enclosing them in a barrier made from half a frozen juice can or a circle of heavy-grade tinfoil pushed slightly into the soil. Wireworms (orange and curl into a half circle) and millipedes (dark brown and curl into a spiral) both live in the soil and feed on plant roots or burrow into root crops. They are seldom numerous, except in reclaimed pastureland, but destroy them when digging.

Quick-moving small pests, such as flea beetles, carrot flies, and leafhoppers, can be kept away from young plants -- the most vulnerable -- by covering them with a floating row cover supported on wire hoops made from cut-down clothes hangers. This also gives protection against late frost and against sunscald on newly planted plants.

Slow-moving small pests, like aphids and mites, can be washed off many plants with a strong stream of water, however, this should not be used on plants with large, soft foliage, or on the fragile growing tips of plants. Mites can also be kept to a minimum by spraying frequently with water, using a hand sprayer, and soaking the undersides of the foliage. Planting flowers that attract native predators also helps to control aphids and mites.

Insects can be lured to bright yellow or red traps coated with a nondrying sticky substance, which holds them. There are also traps baited with scent lures called pheromones. These are scents released by insects to attract others of their species. When used in a trap, they may imitate a female scent and entice many of the males to enter. The unbalanced population results in a large reduction in the number of eggs laid, and young hatching.

Many natural predators can be used to help solve a pest problem. They can be released into the garden and will target a specific pest or range of pests (see p. 546). Naturally, it takes time for the predator population to build up sufficiently to bring the problem under control, so there is always a lag between introducing the predator and solving the problem. They rarely completely kill off all the problem pests, but they will bring the population down to acceptable levels. Remember, spraying for pest control will often wipe out the beneficial insects as well.

Nematodes are microscopic wormlike creatures, some of which attack plants, but others are beneficial and attack plant pests. They are especially useful for controlling some lawn pests and are simply mixed with water and applied with a watering can. In warmer parts of the country, one application will give several years control, but in the North, the cold kills them and they need to be reapplied if the problem occurs again.

Bacteria are also weapons in the fight against plant pests. Bacillus thuringiensis kurstaki, commonly known as BT or Dipel, was the original strain that attacks the caterpillars of certain species of moths and butterflies. There are now several other strains of this bacteria that can control Japanese beetle larvae, mosquito larvae, Colorado beetle larvae, and more. A large range of these predators and lures are available by mail or through your local nursery.

If none of the above methods control the problem to your satisfaction, you may have to resort to spraying with one of the organic controls listed on pp. 544–545.

Disease control. This is more difficult than pest control because the disease has usually got a hold on the plant before any symptoms appear. Many fungal diseases are spread by very small spores that float on the breeze and land on a plant leaf. They "germinate" and insert a small thread (called a hyphae) into the plant tissue. This feeds on the plant and grows, forming a network of hyphae between the cells inside the leaf. It is not until this point that the plant begins to show signs of stress -- different colored spots, wilting, or fungal tissue visible (as in mildew). Such leaves should be picked off as soon as noticed and put in the garbage -- not in the compost. If caught in time, this may be enough to stop the further spread of the disease.

Many plants, providing they are not under stress from poor growing conditions, can emit defensive secretions that can either kill fungal spores or limit the damage they can cause. It therefore makes sense to grow plants properly, giving them the soil conditions, acidity or alkalinity, and moisture they grow best in. In addition, allow good air circulation through plants, especially those, like phlox and bergamot, that are prone to mildew. Stagnant air in the middle of a large clump of stems is a mildew heaven.

Other fungi are great opportunists. Although they are not able to directly infect a plant, they can gain entry through stem and leaf wounds. Many cankers fall into this category. They can attack a plant only when it has been damaged by careless hoeing or gain entry through a wound caused by mower damage.

Fungi are important agents in plant decay, and most work in the gardener's favor. They help break down compost and are responsible for rotting wood in forests and returning the nutrients to the soil. Without them, the woods would be choked with dead trees. Some, such as coral spot, will also attack living material that is under stress from another cause. The appearance of small, bright coral-pink fungi on a branch indicates a problem.
Most of the fungicides listed in the chart on pp. 544–545 are preventative, rather than curative. They should be applied before the disease strikes to form a protective layer on the foliage that kills the fungal spores on contact.

Bacteria are minute organisms that can be rod-shaped, spherical, or spiral, and there are several million in a typical teaspoon of soil. They are important in breaking down dead plant material, but a few attack living plants, generally causing plant tissue to disintegrate. Soft rot of iris is a typical bacterial disease. They are difficult to control and long-lived in the soil, but generally specific to one species or group of plants. Avoid replanting the same species in soil where a bacterial disease has been diagnosed. Some bacteria attack certain insects and are used as insecticides.

Virus are submicroscopic primitive life-forms that live inside cells of plants and animals. They tend to be very specific, limiting their attack to a single genus or plant family. Some virus are used as insecticides but others attack plants. There is no cure and infected plants should be dug up and disposed of in the garbage. Infected plants usually have foliage with strange mottling or streaks, and are often puckering as well. The recently discovered virus attacking hostas is typical in this way. Plant infections can be spread by hand, shears, and other gardening tools. Newly infected plants may take several years to show symptoms, during which time the virus can be spread to other plants. Virus are also spread by sapsucking insects, such as leafhoppers, so controlling these insects is very important. Many modern varieties of vegetables, especially tomatoes, have built-in resistance to some of the virus and other diseases that attack them. This information is usually indicated by a series of code letters in seed catalogs.

Compost Tea

Homemade compost or special compost preparations available from garden suppliers are the basis of this tea. Simply put a shovelful of finished compost in a burlap sack and immerse it in a bucket of water for about a week. Strain the resulting tea through cheesecloth or some other material to remove all solids. Use the tea full-strength to water any and all plants in your garden. Compost tea not only provides a wide range of nutrients, but it also boosts plants' natural defenses against disease. Spraying plants with aerated compost tea can convey even greater benefits. To make aerated compost tea, follow the instructions that come with the compost preparations procured from a garden supplier.

Herbal Sprays

While herbal sprays do not appear to actually kill insects, they do seem to act as an effective repellent, and spraying plants with a tea made of garden herbs may help to keep them pest-free. Sage, thyme, rosemary, and white clover seem to help ward off attacks from leaf-eating caterpillars. To make, either soak 1 cup of fresh leaves overnight in 2 cups of water or pour 2 cups of boiling water over 2 cups of fresh leaves. To use, strain, dilute with an equal amount of water, and add a few drops of liquid soap (not detergent) to act as a spreader.

Stinging Nettle Spray

Stinging nettles grow as weeds in the eastern parts of the country but they can be used to make a spray that helps plants resist disease attacks.

When collecting nettles to make the spray, wear long pants, cover the arms, and wear good work gloves. Place about 1 pound of nettle leaves and young stalks in a bag and soak it in 1 gallon of chlorine-free water (tap water that has stood uncovered for 48 hours). Cover the bucket and leave it in a warm place for a week. The mixture will have a strong smell when uncovered and may need straining through a cheesecloth. Dilute with five times its volume of chlorine-free water and spray plants that are known to be susceptible to fungus diseases. Spray every 2 weeks for continued coverage. It also helps deter aphids and acts as a foliar feed. Store any unused spray concentrate in a glass jar, it will keep for a month.

Starch Spray

This forms a sticky coating on the leaf surface, which traps the pests and holds them until they die. It works best on small pests like aphids and thrips, rather than on large beetles and caterpillars. Mix 2-4 tablespoons of potato flour (available in health food stores) in one quart of water and add a few drops of liquid soap as a sticker. Shake well and spray onto the plants, covering the entire leaves. It will wash off in rain or can be hosed off after a few days.

Garlic Oil Spray

A mix of garlic, mineral oil, and soap gives very good results against many sucking and chewing insects. These include aphids, cabbageworms, leafhoppers, larval mosquitoes, squash bugs, and whiteflies.

Some plants are sensitive, so try it on a single shoot first. If there is no damage after 48 hours, spray the entire plant. Soak 3 ounces of finely chopped garlic in 2 teaspoons of mineral oil for 24 hours. Dissolve 1 teaspoon of insecticidal soap in 2 cups of water and add it to the garlic and oil. Stir well and strain. To use, add 1-2 tablespoons to 2 cups of water and spray on the pests. Store the remainder in a glass container for future use.

Hot Pepper Dust

Grow your own hot peppers to provide the source for a repellent dust that will help protect plants from cabbage maggots, carrot root flies, ants, and other pests. Dry the harvested pepper first, and then grind them with a mortar and pestle (always wear protective eye gear and gloves when working with hot peppers because the dust can be very irritating to your eyes). Sprinkle the dust along plant rows just after seeding or around the base of young plants. Apply more dust after rainfall or watering.

The above is an excerpt from the book The All-New Illustrated Guide to Gardening: Planning - Selection - Propagation - Organic Solutions by Edited by Fern Marshall Bradley and Trevor Cole. The above excerpt is a digitally scanned reproduction of text from print. Although this excerpt has been proofread, occasional errors may appear due to the scanning process. Please refer to the finished book for accuracy.

Copyright (c) 2009 Fern Marshall Bradley and Trevor Cole, editors of The All-New Illustrated Guide to Gardening: Planning - Selection - Propagation - Organic Solutions.

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