Fungicide Chemistry. Advances and Practical Applications

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Plant diseases are best managed by integrating a number of control practices that may include: crop rotation, selection of disease-tolerant or disease-resistant crop cultivars cultivars genetically less susceptible than other cultivars , time of planting, level of fertilization, micro-climate modification, sanitation, and application of fungicides.

Fungicides are often a vital part of disease management as a they control many diseases satisfactorily, b cultural practices often do not provide adequate disease control, c resistant cultivars are not available or not accepted in the marketplace for many diseases, and d certain high value crops have an extremely low tolerance for disease symptoms. In contrast with most human medicines, most fungicides need to be applied before disease occurs or at the first appearance of symptoms to be effective.

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Unlike with many diseases of humans and animals, applying fungicides cannot heal symptoms already present, even if the pathogen is killed. This is because plants grow and develop differently than animals. Fungicides typically only protect new uninfected growth from disease. Few fungicides are effective against pathogens after they have infected a plant. Many fungicides have targeted activity that imparts high efficacy against specific pathogens, which means low potential for toxicity to humans and other organisms, but also results in a high risk of pathogens developing resistance to the fungicide.

A resistant pathogen is less sensitive to the action of the fungicide, which results in the fungicide being less effective or even ineffective. Fungicides that are designed to target specific enzymes or proteins made by fungi do not damage plant tissue, thus they can penetrate and move inside leaves enabling curative properties and increasing the amount of plant tissue protected to more than just where fungicide was deposit when applied.

Since the mode of action of these fungicides is so specific, small genetic changes in fungi can overcome the effectiveness of these fungicides and pathogen populations can become resistant to future applications. Disease management strategies that rely heavily upon curative application of fungicides often lead to more resistance problems due to a the large size of the pathogen population when the application is made from which resistant individuals are being selected and b the difficultly in eradicating a pathogen entirely from inside the plant.

Fungicide resistance is covered in more detail in a separate section. Growers often use disease forecasting systems or action thresholds, when these are available, to ensure fungicides are applied when needed and to avoid the expense and possible environmental impact of unnecessary applications.

Fungicides classification

Forecasting systems have been developed for a number of diseases based on an understanding of the environmental conditions favorable for their development. Typically these are based on temperature and relative humidity or leaf wetness in the area where the crop is grown.

Threshold-based fungicide programs involve routinely scouting the crop for symptoms, then applying fungicides when the amount of symptoms reaches a critical level beyond which the disease cannot be controlled adequately.

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An example of a critical level is one disease spot per five leaves examined. Knowledge of the disease cycle of the pathogen is important when developing and using forecasting systems and thresholds. Economics often influence the choice of fungicide and application timing. Expensive fungicides and numerous applications are used on valuable plantings that might incur substantial economic loss in the absence of treatment, such as fruit trees and golf courses. Recognizing that with some diseases crop yield is not impacted when severity is low, an economic threshold is used to determine when fungicide treatment is needed.

The crop tolerance level, or damage threshold, can vary depending upon the stage of the crop development when attacked, crop management practices, location and climatic conditions. Fungicides are applied as dust, granules, gas, and, most commonly, liquid. They are applied to:. Fungicides are used as a formulated product consisting of an active ingredient plus inert ingredients that improve the performance of the product. Fungicides are typically mixed with water then applied by spraying.

Application equipment ranges from small hand-held and back-pack sprayers to large spray units carried by tractors or aircraft Figures A few fungicides are applied as dusts. Fungicides can also be applied in greenhouses as smoke, mist, fog or aerosol. Coverage of all parts of the plant susceptible to the disease is critical because very few fungicides can move adequately throughout a plant. Advancements are continually being made to nozzles and sprayers to improve coverage Figures 17 and For many diseases, effective control necessitates multiple applications of fungicides, sometimes as frequently as every 5 days.

Repeated applications are needed to protect new growth and to replace fungicide lost from the plant by chemical decomposition, UV-light degradation, and erosion by wind and water. Fungicides are categorized in several ways based on different characteristics. The most common characteristics used and the categories are described below. Table 1 Adobe Acrobat PDF is a list of selected fungicides currently registered in the United States that represent the major fungicide groups and chemistry within these groups.

Fungicide resistance is a stable, heritable trait that results in a reduction in sensitivity to a fungicide by an individual fungus. This ability is obtained through evolutionary processes. Fungicides with single-site mode of action are at relatively high risk for resistance development compared to those with multi-side mode of action. Most fungicides being developed today have a single-site mode of action because this is associated with lower potential for negative impact on the environment, including non-target organisms. When fungicide resistance results from modification of a single major gene, pathogen subpopulations are either sensitive or highly resistant to the pesticide.

Resistance in this case is seen as complete loss of disease control that cannot be regained by using higher rates or more frequent fungicide applications. When fungicide resistance results from modification of several interacting genes, pathogen isolates exhibit a range in sensitivity to the fungicide depending on the number of gene changes.

Variation in sensitivity within the population is continuous.

What Are Fungicides

Resistance in this case is seen as an erosion of disease control that can be regained by using higher rates or more frequent applications. Fungal isolates that are resistant to one fungicide are often also resistant to other closely-related fungicides, even when they have not been exposed to these other fungicides, because these fungicides all have similar mode of action. This is called cross resistance. Fungicides with the same Group Code are likely to exhibit cross resistance. Occasionally negative cross resistance occurs between unrelated fungicides because the genetic change that confers resistance to one fungicide makes the resistant isolate more sensitive to another fungicide.

Managing fungicide resistance is critically important to extend the period of time that an at-risk fungicide is effective. The primary goal of resistance management is to delay its development rather than to manage resistant fungal strains after they have been selected. Therefore, resistance management programs need to be implemented when at-risk fungicides first become available for commercial use.

The objective of resistance management is to minimize use of the at-risk fungicide without sacrificing disease control. This is accomplished by using the at-risk fungicide with other fungicides and with non-chemical control measures, such as disease resistant cultivars, in an integrated disease management program. It is critical to use an effective disease management program to delay the build-up of resistant strains. Using full rates is expected to minimize selection of strains with intermediate fungicide sensitivity when resistance involves several genes quantitative resistance.

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At-risk fungicides should be used in alternation with other at-risk fungicides with different modes of action or different chemical groups, and they should be combined or alternated with fungicides that have a low resistance risk. When one crop could serve as a source of inoculum for a subsequent crop, the alternation scheme among at-risk fungicides should be continued between successive crops such that the first at-risk fungicide applied to a crop belongs to a different cross-resistance group than the last at-risk fungicide applied to the previous crop.

Some at-risk fungicides are formulated as premix products with other fungicides to manage resistance. At-risk fungicides should be used only when needed most. The most critical time to use them for resistance management is early in an epidemic when the pathogen population is small. Multi-site contact fungicides should be used alone late in the growing season, where they have been shown to provide sufficient disease control to protect yield.

Another important component of resistance management is assessing disease control and reporting any loss of efficacy potentially due to resistance. All pesticides must be registered before they can be sold and used in the U. To obtain registration, manufacturers of a pesticide must demonstrate that it does not have the potential to cause an adverse impact on the environment or non-target organisms, including people. This requires conducting several defined toxicology tests and investigating environmental fate.

Additionally, the EPA must ensure that no endangered or threatened species or their habitat are harmed through use of registered pesticides. This assures compliance with the Endangered Species Act ESA of which prohibits any action that can adversely affect these species.

In addition to federal registration with EPA, all pesticides must be registered with appropriate agencies in each state before they can be used. FFDCA regulates the establishment of pesticide tolerances, which are the maximum permissible level of pesticide residues allowed in or on commodities for human food and animal feed. Manufacturers must include residue data in their registration materials.

While well-intended, implementing this amendment became difficult as technology improvements enabled detection of additives at extremely low concentrations that were well below the dose necessary to cause cancer.

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Paradoxically, alternative pesticides could be allowed although they posed higher risks, if these were non-cancer risks. Under the new standard, EPA establishes tolerances by considering a aggregate exposure to a pesticide from food as well as residential and other non-food uses, b cumulative effects to human health from other pesticides with a common mode of toxicity, c potential of increased sensitivity of infants and children as compared to adults, and d effect of the pesticide on estrogen and the endocrine system.

As a consequence of FQPA and stricter EPA standards for pesticide registration, some older pesticides are not being re-registered and it is more difficult to register new products. A pesticide label is a legal document. Therefore it is against federal law to apply a pesticide in a manner other than that described on the label, such as using a higher rate or shorter application interval. Federal law requires specific information be included pep.

Is Fungicide Resistance to Blame?

Labels for fungicides registered in the USA are accessible on-line www. Figure 23 is a fictitious example fungicide label with the type of information found in most labels. Some pesticides are considered restricted and consequently can only be applied by certified applicators who have passed an exam demonstrating an understanding of pesticides and safety www. Additional information on pesticide regulations is available on-line www. Information on potential hazards associated with a pesticide and directions for safe use are provided on the label and in its Material Safety Data Sheet MSDS.

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