Insect and Disease Control for Organic Vegetable Gardeners

Pests you may find in your garden. (Clockwise from upper left) European corn borer in peppers, squash bug eggs, adult squash bug and green leaf beetle. (Photos by UNL Entomology Dept.)
Pests you may find in your garden. (Clockwise from upper left) European corn borer in peppers, squash bug eggs, adult squash bug and green leaf beetle. (Photos by UNL Entomology Dept.)

By Sarah Browning, Extension Educator in Lancaster County

Minimizing pesticide usage in the home garden is a great way to protect yourself from chemical exposure, while also protecting the environment and surface water resources. But pest control — insects, diseases and weeds — are challenging for the home organic vegetable gardener. Today, we’ll focus on techniques for insect and disease control. Gardeners using organic techniques may have to adjust their expectations at the outset and begin to accept a higher level of insect and disease damage in the garden. Start by deciding how much damage can be tolerated as a threshold for determining when control is needed. Then begin implementing any (or all!) of the practices below at the beginning of the gardening season. Don’t wait until pest problems appear to put control measures in place.

Start the gardening season by using these six basic principles to minimize disease problems.

1. Buy healthy, high-quality plants. Many disease problems are brought into the garden accidentally through diseased plant material. Buy only healthy plants from reputable growers.

2. Select resistant varieties. Using cultivars (cultivated varieties) with resistance to disease, reduces damage. Examples include: Tomatoes with resistance to fusarium, verticillium and nematodes; or, cucumbers resistant to bacterial wilt. For more information on resistant cultivars, check garden catalogs, seed packages or refer to Nebraska Extension NebGuide “Selected Vegetable Cultivars for Nebraska,” (G1896)

3. Rotate vegetable families. Rotation reduces insect and disease pressure on vegetable crops by preventing a high level of pests from building up in the soil or around the garden and is particularly useful in reducing disease problems. Crops are rotated by families, so you need to know which vegetables are related. Find more information at Washington State University’s resource “Using Crop Rotation in Home Vegetable Gardens,”

4. Use good garden sanitation. Removing diseased plants will limit the spread of disease to healthy plants. Many pathogens survive between growing seasons on diseased plant material, so removing diseased leaves, plants, fruits and vegetables from the garden as soon as you find them, slows disease spread. Keep gardens as weed-free as possible since weeds often serve as a reservoir of insect and disease problems.

5. Avoid overhead irrigation. Many diseases require leaf wetness for infection to occur, so plan to use drip irrigation this year to keep foliage dry and conserve water. If overhead irrigation must be used, water early in the morning so leaves are dry by nighttime. Avoid placing plants too closely together — this slows air movement through plant foliage and lengthens leaf drying time after heavy dew or rain.

6. Mulch. Summer mulch prevents rain-splash of soil containing fungal spores onto the undersides of leaves, which is the starting point for many fungal infections.

Create a physical barrier, preventing pests from getting to your plants by installing row covers. These are made using lightweight, fine-meshed fabric which can be loosely draped over crop rows and anchored to the soil at the edges. Row covers provide the greatest amount of protection if applied before insects appear. As an added benefit, they also moderate harsh summer temperatures, providing light shade. For crops that require insect pollination, row covers can be removed once flowering begins. Find more information at University of Wisconsin-Madison’s article “Floating Row Cover,”

Preserve beneficial insects. Natural populations of predators and parasites are valuable for reducing infestations of garden pests. However, some level of pest infestation must be tolerated to attract and maintain natural enemy populations. Should pest control be necessary, select a low-risk pesticide to minimize injury to beneficials. Several species of mass-reared beneficial insects can be purchased from commercial suppliers for use in home gardens. However, the artificial introduction of natural enemies usually does little good in the home garden because the insects often die or disperse into areas away from your garden. Creating good conditions for natural beneficial insect populations to increase is more effective than introduction of purchased beneficials.

Once insect problems have begun, what can be done to control them? Scout the garden for insects as often as possible — daily, if at all possible. Catching insect problems in the early stages makes management easier, rather than trying to play catch-up on a full-blown infestation. When insect numbers are high, injury may be reduced, but is seldom eliminated by non-chemical methods.

• Physical barriers, such as collars placed around young plants, will help protect against cutworms. The row covers mentioned above are effective against many insects and are especially useful on non-flowering vegetables.

• Handpicking is effective against low populations of large insects such as Japanese beetles, Colorado potato beetle, tomato hornworms and others. Crush them or drop them into a bucket of soapy water.

• Some trapping methods do work, such as the use of flat boards or shingles placed on the ground near plants to attract slugs and snails. They can be collected and destroyed.

• Syringing the undersides of foliage with a strong jet of water daily for 7–10 days can help reduce spider mite infestations.

Before using any pesticide, start by accurately identifying the insect or disease problem. Gardeners can submit pictures of plant problems to Nebraska Extension experts through our Digital Diagnostic Network,

Below are a few products useful in either disease or insect control that may be acceptable to organic gardeners. However, these products typically suppress a pest problem rather than eliminate it. And while they are less damaging to beneficial insects, that does not mean no damage. Always make sure any product purchased is labeled for use in the vegetable garden, and follow all directions on waiting periods after application before harvesting again.

• Bacillus thuringiensis — commonly referred to as Bt and marketed under the trade names Dipel, Thuricide and others. Consists of spores from a soil-inhabiting bacteria that kills the larvae of moths and butterflies, such as armyworm, cabbage loopers, cutworms, corn earworm and tomato hornworm. Will also kill desirable butterfly and moth larva, so apply carefully.

• Copper fungicide — one of the first elements used as a plant fungicide. Provides protection against infection by killing disease pathogens on a leaf or other surface before they infect the plant. Must be applied preventatively. Has no post-infection action. Many formulations of copper fungicides are available in garden stores.

• Diatomaceous earth – finely ground fossilized diatoms, a single-celled form of algae. Their sharp edges scratch and scrape the waxy or oily outer layer of soft-bodied insects causing them to dehydrate and die. Often used for slug control.

• Horticultural oil — Highly refined vegetable or mineral oil, which kills insects in several ways, but most importantly, by suffocating them. Oils act like a contact insecticide and provide no residual control, so the insects must be present and in a vulnerable stage of development for an oil application to be effective. Plant damage may occur if used when temperatures are too high.

• Insecticidal soaps — These products are liquid formulations of potassium salts of fatty acids and are effective at controlling some soft-bodied insects such as aphids, mites, leafhoppers and plant bugs. Requires thorough plant coverage and multiple applications. Use soaps with caution, as leaf injury is possible with certain plants.

• Kaolin clay — finely ground natural clay product. When sprayed, creates a fine film on plant surfaces and acts as an irritant, repellent or physical barrier. Heavy use has been found to be harmful to beneficial insects and can result in spider mite infestations.

• Neem — made from neem tree seed extracts and contains either neem oil or the purified active ingredient azadirachtin. Effective as a contact spray or through ingestion. Acts primarily by disrupting normal insect growth and, in some insects, has anti-feeding or egg-laying properties. Quickly broken down by sunlight.

• Spinosad — made by fermentation of a soil bacteria and disrupts the insect nervous system, resulting in paralysis and death. Effective as a contact spray or through ingestion. Kills affected insects in 1–2 days. Toxic to bees and parasitic wasps if sprayed or they come into contact with wet plant surfaces.

• Sulfur — finely ground sulfur can be used either as a dust or spray to prevent diseases and is sometimes used to control spider mites. The chemical may ‘burn’ tender foliage if applied when air temperature is 85°F or higher. Do not apply within 20–30 days of applying a horticultural oil — plant damage may result.