Consult Appendix 5 - Organic Certification Programs for a discussion of organic certification procedures and a list of organizations providing organic certification in the south. The following practices are allowed by most certifying groups, but growers interested in certification by a particular group need to use the materials list for that group. Practices discussed below include use of copper, sulfur, baking soda and mineral oil, disease suppressive organisms, solarization, and plant extracts.
Copper
Copper sulfate, alone or mixed with hydrated lime (Bordeaux mixture), is used to control leaf spot, anthracnose, mildew, blight and black rot. Copper compounds irritate skin and eyes and are highly toxic to fish. They are most likely to damage plants when temperatures are below 50 degrees F and the humidity is high. Copper in the soil can also build up to toxic levels.
Sulfur
Finely ground sulfur is effective against powdery mildew, rusts and some other fungal leaf diseases, but it can burn plant tissue when used with copper and oils. Also, sulfur sprays should not be applied within a month of an oil spray or at temperatures above 90 degrees F.
Baking Soda and Mineral Oil
Baking soda and mineral oil mixtures applied at a rate of four teaspoons of baking soda and four teaspoons SunSpray Ultrafine Oil in one gallon of water will prevent the spread of powdery mildew, Alternaria leaf blight, gummy stem blight and anthracnose on cucurbits. The mixture acts only as a contact fungicide with no residual activity, so complete spray coverage every 10 days is critical.Growers should be aware that baking soda has not been formally evaluated as a pesticide.
Disease-suppressive Microorganisms
Disease-suppressive soil microorganisms have been found in many places. In monoculture wheat the severity of "take all" disease often decreases within three to five years. This phenomenon is known as "take all decline," and is considered an effective natural control. Although the mechanisms are not completely understood, the decline is associated with changes in soil microorganisms that compete with and prey on the fungus. Melon plants grown in the Chateaurenard region of France do not show Fusarium wilt symptoms even though the fungus is present in the soil. Soils with suppressive characteristics tend to develop slowly and are usually found in fields where perennial crops or monocultures have been grown for many years.
Suppressiveness may be lost if the monoculture is interrupted even for one year, or if pesticides are applied. For example, researchers first recognized soils suppressive to cereal-cyst nematode when nematode numbers increased after application of a broad-spectrum biocide. Many species of fungi and bacteria in the genera Trichoderma, Streptomyces, Bacillus and Pseudomonas suppress diseases, but at this time only a few strains are commercially available. Additional commercial products may be available soon, however, as this is currently an active research area.
Gliocladium. Gliocladium is a disease-suppressive microorganism now available as a commercial product, 'Soil Guard' (V. R. Grace, Fogelsville, Pa.), which has been registered as a microbial pesticide for control of Pythium and Rhizoctonia.
Streptomyces. Streptomyces, a soil-dwelling fungus, includes both disease-causing and disease-suppressing strains, sometimes within the same species. For example, disease-suppressive strains of S. scabies control the disease scab in potato which is caused by pathogenic S. scabies strains. Streptomyces also controls soilborne diseases caused by Fusarium, Pythium and Phomopsis. Dried spores and mycelium of a Streptomyces griseoviridis strain are available commercially as 'Mycostop' to control seed rot, damping-off, and root rots of greenhouse vegetables and ornamentals. This product can be applied as a seed treatment, soil drench or through drip irrigation systems in the greenhouse. It is not currently labelled for field use.
Bacillus. Bacillus cereus when sprayed or wiped on cucumbers or applied as a dip suppresses cottony leak caused by Pythium aphanidermatum. B. cereus also reduced the severity of damping-off in alfalfa and southern blight in peanuts. Bacillus subtilis produced an antibiotic which significantly reduced severity of Rhizoctonia root rot of pea in Canada when soil-applied before planting. By itself, B. subtilis provided more disease protection than the fungicide Anchor, but Anchor combined with B. subtilis was even more effective. B. mycoides suppressed damping-off of cucumber seedlings caused by Pythium mamillatum. At this time, however, no disease-suppressive Bacillus species are available commercially.
Fluorescent Pseudomonas. Pseudomonas strains have been isolated that reduce incidence of southern blight in bean and Fusarium wilt in tomato and cotton, leafspot diseases such as early blight, Phoma and Stemphylium of tomato in the greenhouse, bacterial wilt caused by Pseudomonas solanacearum in tomato, and Pythium. At this time, none of the disease-suppressing Pseudomonas strains are com-mercially available, but substituting solarization for soil fumigation may encourage naturally occurring populations (see below). Naturally occurring populations are more effective and consistent than inoculants in suppressing diseases.
Solarization
In studies in California and Israel, soil solarization controlled certain diseases for at least two growing seasons. Although pathogen populations started to build up again when susceptible plants were reintroduced, preliminary evidence from these studies suggests that in some cases, pathogens reinfect solarized soils at slower rates than non-solarized soil. Effects of solarization on beneficial or suppressive microorganisms have not been widely tested. However, when measured, solarization appears to be less likely to reduce populations of beneficial organisms than fumigation. In some cases, as in disease-suppressive pseudomonads, solarization did not reduce populations at all. Researchers in California have also been experimenting with combining chicken composts, inorganic fertilizers, and residues of cabbage and other crucifers with solarization to improve control of nematodes and Pythium in lettuce. Solarization by itself was more effective than any of the amendments but the combination of solarization and chicken compost reduced both root galls and Pythium compared to solarization alone. See Weed Management for further information on soil solarization.
Plant Extracts
Although extracts from a number of plants are used as botanical insecticides, effective botanical fungicides are rare. The Swiss Research Institute found that an extract from dock (Rumex sp.) controlled downy mildew on cucumbers as well as applications of sulfur or Bayleton®, a commercial fungicide. The extract should be made from mature roots at least two years old. Freshly harvested roots are cut into small pieces, pureed, and mixed with a small quantity of water. After setting for an hour, the mixture is filtered and diluted with water (1 ounce puree to 6 gallons of water). Either the cut root pieces or the diluted solution can be frozen for later use. The dock extract will not stop infections that are already present but will reduce future infections when applied every 7 to 8 days.
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