For organic gardeners, the two primary ways to fight disease are to take steps to kill spores in the soil or on plant surfaces—before the spores infect roots or leaves. Soil solarization kills many types of disease spores, along with some pests and weed seeds. Applying biocontrol agents to soil or plants can kill or outcompete pathogens. And as a last resort, sulfur and copper sprays will kill spores and in some cases even prevent a disease organism from spreading within a plant.
The more you learn about the common disease problems in your area, the less often you'll need to resort to using dusts and sprays. Keep records of the disease problems that occur in your garden, or ask fellow gardeners what diseases to expect in your area and when to expect them. This way, you can limit spraying or dusting to those seasons and weather conditions when your plants are most vulnerable to becoming infected.
It's also important to make sure you know what disease you're trying to control by applying a spray or dust. If you run into a problem that you can't identify, submit a fresh plant sample to a diagnostic laboratory or your local extension office for identification.
Always take appropriate safety precautions when applying sprays and dusts; see the Pests entry for guidelines. The following descriptions of disease-control methods and products are arranged from least to most toxic.
If an area of your vegetable garden has been troubled by disease, or if you plan to start a new planting of any kind and are concerned about soil-borne diseases, consider solarizing the soil before you plant. You'll need to plan ahead, because it's important to solarize soil during the hottest period of the year if you live in the North.
Solarizing is a simple procedure: You tightly cover the soil with clear plastic for 1 to 2 months. This can generate high enough temperatures in the top 6 to 12 inches of soil to kill many disease organisms, nematodes, pest insects, and weed seeds. The beneficial effects seem to last for several seasons. The illustration at right shows how to prepare a bed for solarizing. For even better results, support a second layer of plastic on wire hoops over the covered bed to provide added insulation.
Midsummer is the best time to solarize soil, especially in the North. Cultivate and remove crop residues from the soil, rake it smooth, and water if it is dry. Dig a trench several inches deep around the bed, and spread thin clear plastic film (1 to 4 mils) over the bed. Press the plastic into close contact with the soil, and seal the edges by filling the trench with soil. Leave in place for 1 to 2 months, then remove the plastic.
One of the most exciting areas of plant disease research focuses on using naturally occurring bacteria and fungi to fight against plant pathogens. The bacterium Bacillus subtilis kills or outcompetes the fungus that causes powdery mildew as well as some other plant pathogens. The fungus Trichoderma harzianum (sold as RootShield) kills the pathogen Rhizoctonia (one of the many fungi that cause damping-off). Trichoderma locates Rhizoctonia by a chemical the pathogen releases, then it attacks the damaging fungi and destroys it. In a study testing biological fungicides on vinca plants, researchers at Clemson University found that greenhouse plants treated with SoilGard (Gliocladium virens) had excellent shoot and root growth and were the equal of those treated with chemical fungicides.
Among the most beneficial root-inhabiting organisms, antibiotic-producing mycorrhizal fungi (sold as BioVam) cover plant roots to protect against pathogens, forming a "fungal mat," which also increases nutrient-uptake ability.
A thorough spray of vegetable or light horticultural oil coats plant surfaces, acting as a barrier to infection. Oils seem to help prevent fungal rusts and mildews.
Garlic appears to be a fungicide as well as an insecticide. Horsetail (Equisetum arvense) infusion sprayed on plants may help prevent fungal diseases.