Chapter 12

Plant Disease Management for Organic Crops

Synthetic Materials for Disease Management

Organic farmers can use pesticides derived from natural sources (e.g., biopesticides) and also pesticides that include synthetic substances within the regulations of the U.S. Department of Agriculture’s (USDA) National Organic Program (NOP) if other strategies and cultural management practices fail to control pests and diseases. A synthetic substance, as defined by the NOP (Appendix A), is formulated or manufactured by a chemical process or by a process that chemically changes a substance extracted from naturally occurring plant, animal, or mineral sources, except that such term shall not apply to substances created by naturally occurring biological processes. When preventive practices and non-synthetic materials are insufficient to prevent or control diseases, allowed synthetic materials may be used in controlling plant diseases. However, NOP emphasize the use of preventive and cultural methods for disease control. These methods include crop rotation, resistant cultivars, maintaining good air flow in the crop canopy, good soil and nutrient management, and sanitation measures. If these methods are insufficient to manage a disease, NOP-compliant inputs can be applied. All inputs must be included in the organic farm plan provided to the certification agency.

NOP Guidelines in Using Synthetic Materials

The use of allowed synthetic materials is restricted. § 205.601(i) lists synthetic materials allowed for disease control (Appendix A). If a material is listed for disease control it cannot be used for another purpose unless it is also listed for that purpose. For example, hydrated lime can be used for disease control, but not as a soil amendment.

Synthetic Materials for Use in Organic Crop Production

In accordance with restrictions specified in this section, the following synthetic substances may be used in organic crop production:

Application of Synthetic Materials

The frequency of application of any mineral-based pesticides is related to its residual time, or the time required for the product to degrade in the environment. Synthetic materials, approved for organic crop production, in general have a short residual time. On the other hand, this short residual time also limits most concerns about build-up in the soil that are associated with some commercial products.

Mineral-based Pesticides

The mineral-based pesticides that are approved for organic production include sulfur, lime-sulfur, and various forms of copper. Each of these products and their uses are summarized in Table 13.3 and described in more detail in the following sections.

Bordeaux Mixture

Bordeaux mixture is a mixture of copper sulfate (bluestone), calcium hydroxide (hydrated spray lime or slaked lime), and water that can be used as both a bactericide and fungicide on apples, pears, and some stone fruits. Bordeaux mixture prevents pathogen growth by disrupting enzyme function. It works as a preventative measure and has no systemic activity, so applications need to be made prior to infection. Sulfur is only fungicidal, but Bordeaux mixture also is bactericidal, which means that it can be effective against disease caused both by fungi (such as powdery mildew, downy mildew, and various anthracnose pathogens) and by bacteria (such as bacterial leaf spots and fire blight). Bordeaux mixture owes part of its success to its ability to persist through spring rains and adhere to plants. Bordeaux mixture comes in several formulations.

Fixed Copper

Fixed copper is a term that refers to several relatively insoluble formulations of copper that are somewhat less phytotoxic and are more convenient to use on crops than Bordeaux mixture. The term “fixed” copper refers to copper products that are formulated or tank-mixed in such a way as to create relatively insoluble or “fixed” deposits of copper on plants. Fixed copper formulations release less copper ions and are generally less injurious to plant tissues (safer to use) than copper sulfate or Bordeaux mixture. The activity and potential phytotoxicity of these formulations are proportional to the amount of actual metallic copper each contains, the rate and timing of application to the crop, the phenological stage of the plant and pathogen, and the weather conditions after application. Copper-based materials work by disrupting enzyme function after copper ions contact bacterial or fungal cells. Once dried on the plant surface, copper will be reactivated by rain until it is completely washed off. Copper has only preventative or protectant activity, so applications need to be made prior to infection.

Lime Sulfur and Liquid Lime Sulfur

Lime sulfur is a mixture of calcium polysulfides formed by adding elemental sulfur to boiling water slurry of calcium hydroxide. It is usually in a liquid formulation. The active compound, hydrogen sulfide, gives lime sulfur an unpleasant rotten egg smell that may remain in the field for over a week.

Sulfur

Sulfur is a non-systemic contact and protectant fungicide, making it only effective in a protective or preventative schedule based upon predicted infection periods. It also has some secondary acaricidal (mite suppressive) activity. Sulfur becomes toxic to fungal cells by inhibiting respiration, disrupting proteins, and chelating heavy metals.

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