Biopesticides for Plant Disease Management in Organic Farming

Introduction

Organic growers have available a large array of biopesticides that may be applied for the management of plant diseases. This article will define what biopesticides are and provide general information on their potential benefits and limitations. While information discussed is applicable to both organic and conventional crop production, this article will address only those materials approved for use on organic farms at the time of writing.

Before applying ANY pest control product, be sure to: (1) read the label to be sure that the product is labeled for the crop you intend to apply it to and the disease you intend to control, (2) read and understand the safety precautions and application restrictions, and (3) make sure that the brand name product is listed in your Organic System Plan, approved by your certifier, AND registered for use in your state. For more information on how to determine whether a disease management product can be used on your farm, see the eOrganic article Can I Use this Product for Disease Management on my Organic Farm?

What are Biopesticides?

Many biologically-based products are currently available, and these products are often referred to as “biorationals” or “biopesticides”. Biorational is an undefined term used in broad reference to any biologically-based product used in agriculture that includes fertilizers, pesticides, herbicides, plant growth regulators, and various other products. A biopesticide is defined by the U.S. Environmental Protection Agency (EPA) as a pesticide derived from natural materials. The EPA divides biopesticides into three categories: biochemical pesticides, microbial pesticides, and plants containing added genetic material.

Biochemical pesticides contain naturally occurring substances that control pests. Substances that control diseases in this category include potassium bicarbonate, hydrogen dioxide, phosphorous acids, plant extracts, and botanical oils. Microbial biopesticides contain microorganisms as the main active ingredient that function as biological control agents, affecting the pathogen directly or indirectly through the compounds they produce or by stimulating specific plant responses. Plant-incorporated protectants (PIPs) are the least common type of biopesticide. These are pesticidal substances produced by plants that contain genetic material added to the plant often through genetic engineering. Biopesticides also exist for the management of weeds, insects, and nematodes.

While generally a logical fit for managing diseases in organic crops, there are some biopesticides that are NOT approved for organic production. Some biopesticides, like salts of phosphorous acid and all genetically-engineered PIPs, are not allowed for organic production. Note that some fungicides approved for use in organic production systems are not biopesticides, including mineral oils, copper, and sulfur. It is critical to verify that a biopesticide, like any other product, is approved for organic production and registered in your state prior to any application. For more information on how to determine whether a disease management product can be used on your farm, see the eOrganic article Can I Use this Product for Disease Management on my Organic Farm?

Benefits and Limitations of Biopesticides

As Benjamin Franklin once said in regards to fire prevention, "an ounce of prevention is worth a pound of cure." The same holds true for disease control, especially when using biopesticides. Products should be used in a preventative and not a curative manner, as they typically lack the breadth of activity, efficacy, or residual activity of conventional fungicides. And, it is more important to apply biopesticides to the specific targets for which they have been shown to be most effective during the interval when those pathogens are most likely to be active. Failure to match the biopesticide to the appropriate disease or timing thereof will reduce the value of such applications.

Many biopesticides function by interfering with a pathogen’s ability to infect a susceptible plant either directly through parasitism or indirectly through the production of secondary compounds. Therefore, timing and frequency of application are critical. Effective control requires that the application of materials begins prior to conditions favorable for disease development or immediately following the first symptoms of disease. In foliar applications, these materials provide a protective barrier on plant surfaces that interfere with the pathogen. Repeated applications are required, often on a weekly interval, due to loss of product from degradation or rain/irrigation washing materials off foliage and fruit, or with the growth of new foliage. Some biopesticides, notably Regalia and Companion, are thought to turn on plant defense responses. Such products should be applied before infection to be effective.

Similar strategies are at play when biopesticides are applied to seeds, roots, or soil. These materials can provide temporary protection to roots, but results can vary depending on the crop, pathogen, and environment. Subsequent root growth or rain/irrigation may wash biochemical materials away from the root zone. Many biological agents can grow within the root zone and can persist longer than chemicals. However, because they are often unable to compete with native soil microorganisms, they tend to persist at levels ineffective for control. Many biopesticides are effective in the control of root rots associated with seed germination and seedling/transplant establishment. Again, the efficacy of biopesticides and most fungicides is limited against established infections, so the sooner applications are started, either prior to or during a disease outbreak, the more likely control will be realized.

The frequency of application of any product is related to its residual time, or the time required for a product to degrade in the environment. Biopesticides in general have a short residual time, typically much shorter than compounds such as copper. On the other hand, this short residual time also limits most concerns about build-up in the soil that are associated with copper. In addition, the activity of biopesticides is generally targeted to specific pests and closely related organisms, and they are usually inherently less toxic to non-target organisms. Therefore, they do not have the same potential to affect birds, beneficial insects, and mammals. Because of the short residual time, low toxicity, and reduced risk to nontarget organisms or the environment, the EPA generally requires less data to register a new biopesticide compared to a conventional pesticide. However, sufficient data regarding the composition, toxicity, degradation, and other characteristics of the pesticide is still required to ensure that the product will not have any adverse effects on human health or the environment.

The EPA, recognizing that biopesticides tend to pose fewer risks than conventional pesticides, has taken steps to promote development and adoption of new products. The registration process is quicker for biopesticides than conventional products, often taking less than a year, compared with an average of more than 3 years for conventional pesticides. To facilitate their registration, the Biopesticides and Pollution Prevention Division was established in the Office of Pesticide Programs in 1994. Some biopesticides are defined as minimum-risk pesticides through the FIFRA Section 25(b) rule because their active and inert ingredients are generally recognized as safe (GRAS). These consequently are exempted from the regulation requirements of FIFRA and thus can be used on any labeled crops for any target since they do not need to be registered as a pesticide. "Exempt from EPA registration" is stated on the label of these products.

When selecting a biopesticide it is important to obtain information about the product's mode of action, residual time, and target disease(s). Unfortunately, data on the efficacy of biopesticides is limited. Many products are broadly labeled with separate lists of registered crops and labeled diseases. Data proving efficacy is not required for the registration of pesticides in the USA. Most biopesticides are produced by small companies that lack funding to support replicated field trials needed to obtain sufficient efficacy data by experienced university and other independent researchers. To help fill this gap, the IR-4 Biopesticide and Organic Support Program funds grants to obtain efficacy information for biopesticides in development as well as those already registered. These funded projects help the program meet its objective, which is to further the development and registration of biopesticides for use in pest management systems for specialty crops (which include all vegetables) or for minor uses on major crops. There are databases of labels and projects at the IR-4 web site.

Several factors can affect performance of biopesticides, or any product used for disease control. These need to be considered when reviewing results from an efficacy experiment or on farm use. It is especially challenging to assess efficacy of a product used on-farm because there are no plants left untreated or treated with products known to be effective for comparison. A product may appear to be effective when actually conditions became unfavorable for the disease to develop or the pathogen was not present. Conversely, conditions may be so favorable for disease development that it is not possible to suppress the pathogen even with a conventional product demonstrated previously to be effective. The earlier in disease development that applications of a product are started, the more effective the product will be. Disease spots (lesions) cannot be ‘cured’ and once a pathogen has infected a plant it cannot be killed, even with most conventional fungicides, in contrast with insect pests which remain on plant surfaces and are accessible to treatment. Product performance can also be affected by spray coverage and frequency of application. Treatment timing is a common potential explanation for poor control with a product that has been effective in other situations. Sometimes in efficacy experiments the pathogen is introduced artificially rather than relying on natural inoculum. This may result in disease pressure that is greater or less than what would occur naturally Some products have continued to be developed and improved following registration, thus results obtained with an early formulation might not reflect the degree of control obtainable with the current formulation. Performance of some products can be improved by using an adjuvant, but on the other hand, it has been suggested that some products have been negatively affected by the adjuvant used. Laboratory testing provides an indication of product activity, but these results alone are not sufficient for predicting field efficacy because of the many environmental factors that can affect performance. Finally, most efficacy studies have not been conducted in organic systems, where healthy soils, crop rotations, and biodiverse agroecological environments may effect outcomes.

For additional information, see the related eOrganic afticles Efficacy of Biochemical Biopesticides that may be used in Organic Farming, and Efficacy of Microbial Biopesticides that may be used in Organic Farming.

References and Citations

• Biopesticide active ingredient fact sheets [Online]. 2009. United States Environmental Protection Agency. Available at: http://www.epa.gov/oppbppd1/biopesticides/ingredients/index.htm (verified 31 March 2010).
• Minimum risk pesticides under FIFRA Section 25 (b) [Online] 2009. Available at: http://www.epa.gov/oppbppd1/biopesticides/regtools/25b_list.htm (verified 31 March 2010).
• The IR-4 Project: Biopesticides [Online]. 2010. Rutgers, The State University of New Jersey. Available at: http://ir4.rutgers.edu/biopesticides.html (verified 31 March, 2010).