Organic farmers aim to maintain crop health through good management of the farm ecosystem, but pest outbreaks still happen, especially when the weather doesn’t cooperate. In my neck of the woods, this past growing season brought lots of rain, and as a result, plenty of plant disease. I thought it would be good to review some organically approved pesticides that can be part of your disease management toolbox. Used in combination with cultural practices like crop rotation, maintaining soil health and sanitation, such products can sometimes help prevent or reduce crop losses to disease.
One material that can be used for disease management on organic fruits and vegetables, as well as field crops, is Bacillus subtilis, a naturally occurring bacterium found in several commercially available products. To learn more about it, I turned to the “Resource Guide for Organic Insect and Disease Management” online. This publication provides the details on about a dozen different organic pest control materials—what they’re made from, how they work and how they’re applied—and also reviews the research results in regard to their effectiveness on different pests and crops.
The guide says that B. subtilis is widespread in nature—it’s been recovered from soil, water, air and decomposing plant material. Under most conditions, however, it is not biologically active and is present in the spore form. There are different strains of B. subtilis, and a few of these have been developed for use as biological control agents.
Where it comes from
For example, the B. subtilis strain QST713 was isolated in 1995 by AgraQuest, Inc. from soil in a California peach orchard. It is the active ingredient in the line of Serenade products, which are applied to crop foliage. The B. subtilis strain GB03 is the active ingredient in Kodiak, and was discovered in Australia in the 1930s. It is applied either as a seed treatment or directly to soil.
How it works
B. subtilis bacteria produce antibiotics, including some called iturins, which help the bacteria compete with other microorganisms either by killing them or reducing their growth rate. When applied directly to seeds, B. subtilis bacteria colonize the developing root system, competing with various disease organisms that attack root systems. According to the manufacturers, B. subtilis also inhibits plant pathogen spore germination and interferes with the attachment of the pathogen to the plant. When soil or seed-applied, it is claimed that B. subtilis feeds off plant root exudates, depriving disease pathogens of a food source.
B. subtilis is also reported to induce systemic acquired resistance (SAR) against bacterial pathogens. SAR is when a plant’s own defense mechanisms are induced by prior treatment with either a biological or chemical agent. The concept of SAR has been studied for many years and is an exciting prospect for disease management.
Using B. subtilis
The labels of these products generally require the use of personal protective equipment (long-sleeved shirt and long pants, gloves, shoes plus socks, dust/mist filtering respirator, etc.) to mitigate the risk of skin sensitivity and possible allergic reactions. The EPA Workers Protection Standard requires a minimum of four hours before reentering a treated field. PHI (days to harvest) is zero.
B. subtilis products are available in a number of strains and formulations, and the following are currently listed as approved for use on organic farms by the Organic Materials Review Institute (OMRI). Always check with your organic certification agency before using a new material.
Serenade MAX is a wettable powder that contains 14.6 percent B. subtilis, and Serenade ASO is an aqueous suspension containing 1.34 percent B. subtilis. Both products are made by AgraQuest and are labeled for many diseases in a wide range of fruits and vegetables. Kodiak Concentrate Biological Fungicide is a seed treatment that contains 2.75 percent B. subtilis. It is made by Bayer Crop Science and is labeled for suppression of rhizoctonia, fusarium and/or Pythium on almost all crops field crops. BioStart Defensor contains both B. subtilis and B. cereus. It is made by Bio-Cat Microbials and sold as a soil inoculant; it is not labeled for use as a pesticide.
The EPA has indicated that B. subtilis is nontoxic to birds and insects. No adverse effects on fish or wildlife are likely through labeled use of these products. In terms of human health, the B. subtilis bacterium has been found to be relatively benign—it is not a known human pathogen or disease-causing agent. Lab data suggests relative safety in terms of oral, dermal and pulmonary toxicity, as well as eye and skin irritation, although irritation may occur from eye or skin contact with commercial formulations.
Does it work?
In the many tests done with Serenade, it gave good results against onion diseases, downy mildew in grapes, and powdery mildew on greenhouse tomatoes. It has been shown to significantly reduce lettuce drop caused by Sclerotinia sclerotiorum. Other trials showed poor to fair efficacy.
Studies in Michigan found Serenade to provide moderate to good control of mummy berry disease in blueberries. In trials against fire blight in apples, Serenade showed some efficacy, though low—this disease is notoriously difficult to control. Researchers in Michigan tested the efficacy of Serenade for two years in field trials under heavy disease pressure on inoculated trees. In both years, Serenade provided 50 percent control, and there was better control in an on-farm trial relying on control of natural inoculum.
Several organic growers I know use Serenade to manage early blight on field tomatoes, and they claim it has been effective. The research results I’ve seen are equivocal; some show moderate effectiveness and some don’t. It appears that under high disease pressure it is less effective. The University of California IPM program lists Serenade as a tool for suppression of early blight, with applications starting when plants are 4 to 6 inches tall and repeated on a five to seven-day interval, or as needed.
Serenade is often used and trialed in rotation or combination with other fungicides, such as copper products. There is some indication that the use of Serenade may allow for reduced rates or frequency of conventional materials used as companion fungicides.
Kodiak seed treatment needs to be evaluated differently from foliar-applied products. Typically, yield is used as a measurement of efficacy, as is emergence or stand establishment. Kodiak showed significant yield increase in some trials, but not in others. Since the cost of treatment is small, even small increases can be worthwhile.
In four studies, Kodiak gave little or no visible control of root rot pests, but in one it gave a significant 22 percent control of fusarium root rot in beans, and in another its use resulted in an 81 percent stand increase in chickpeas. Kodiak appears to be widely used as an effective seed treatment to suppress root disease in cotton. Like Serenade, it appears to provide additional benefits when used in combination with conventional seed treatments.
As with any pesticide, the effectiveness of B. subtilis will vary depending on the intensity of pest pressure and environmental conditions, as well as application rate, method and frequency. Like most biological controls, this material is not a powerful, knock-out type of tool. However, it is low in toxicity, available at reasonable cost and has the potential to suppress a number of diseases on different crops if used in a preventative fashion. Therefore it’s a tool I would recommend that organic growers consider trying as part of an integrated program in crops with a history of disease problems on their farm.
The author is vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office.