Plant Disease Management for Organic Crops
Biorational Control of Crop Diseases
Organic growers have available a large array of biorationals that may be applied for the management of crop diseases. Biorationals typically used to control crop diseases include microbials, minerals, and spray oils. Microbials include live organisms (e.g., beneficial bacteria, fungi, nematodes, and viruses) and/or their fermentation products as the active ingredient. Minerials include sulfur, lime-sulfur, various forms of copper, and potassium bicarbonate. Spray oils include petroleum-derived oils (referred to as “narrow-range oils”); oils derived from plant and fish sources; and essential oils, such as wintergreen, clove and rosemary. Preventive, cultural, mechanical, and physical methods must be the first choice for pest control, and conditions for use of a botanical or synthetic material permitted on the National List must be documented in the organic system plan.
Sulfur and copper are the disease control materials most applied on organic farms. Elemental sulfur may be used for a broad range of diseases in a wide variety of plants. Copper products must be applied in a way that minimizes copper accumulation in the soil. Among the copper products allowed are copper sulfate, copper hydroxide, copper octanoate, copper oxide, and copper oxychloride. Bordeaux mix (copper sulfate combined with hydrated lime) and lime-sulfur are also permitted. Potassium bicarbonate, a relatively new product as a fungicide is also permitted. Each of these products and their uses are summarized in Table 13.1 and described in more detail in the following sections.
Application of Minerals
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. Effective control requires that the application of materials begin prior to conditions favorable for disease development or immediately following the first symptoms of disease.
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 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 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.
Potassium bicarbonate (baking soda) is a contact fungicide killing spores and hyphae on plant surfaces within minutes of application. This is achieved by several modes of action including changing leaf surface pH, fungal spore dehydration, and causing potassium imbalances. Multiple modes of action results in less risk of resistant fungal strains developing. As such, it offers an alternative to sulfur.
Microbials, also called as biological pathogens or biological control agents, contain microbes like bacteria, viruses, or fungi that produce toxins that are harmful to pathogens. While the active ingredients of microbials are generally approved for organic production (Organic Materials Review Institute, OMRI listed) because of their natural origin, certain formulated products are prohibited because the inert ingredients or procedures used in making the product are prohibited. A list of commonly used microbials for controlling plant pathogens can be found in Table 13.2.
Application of Microbials
The successful application of microbials hinges upon the grower having an in-depth understanding of the biology of the target plant pathogen(s). Microbials, also known as biological control agents (BCAs), are registered for use by the Environmental Protection Agency (EPA) and have labels very similar to those for chemical pesticides. BCAs can be hazardous to the applicator and all safety equipment recommended on the label should be used.
Spray oils include certain fractions of petroleum oils (narrow-range oils), which are considered synthetic and allowed for disease control. “Narrow-range oils” are defined as petroleum derivatives—predominately of paraffinic and napthenic fractions. Narrow-range oils are allowed for both dormant and growing season uses for disease control. Allowed oils can also be derived from plant and fish sources. Plant oils are primarily derived from seeds (e.g., soybean, cottonseed, sesame, and canola), while fish oils are by-products of the fish processing industry. Approved products may not contain any prohibited inert components.
Application of Spray Oils
To combat plant fungal pathogens, oils generally must be applied prophylactically prior to infection. Repeated applications of oils may be needed to achieve desired levels of control. Although generally considered safe, oils can injure susceptible plant species. Symptoms of plant injury (phytotoxicity) may be acute or chronic. They can include leaf scorching and browning, defoliation, reduced flowering and stunted growth. Phytotoxicity may be associated with plant stress, ambient temperature and humidity, and application rate. It can vary among plant species and cultivars.
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