Diseases are a common occurrence on plants, often having a significant economic impact on yield and quality, thus managing diseases is an essential component of production for most crops. Broadly, there are three main reasons fungicides are used: (a) To control a disease during the establishment and development of a crop. (b) To increase productivity of a crop and to reduce blemishes. Diseased food crops may produce less because their leaves, which are needed for photosynthesis, are affected by the disease (Figures 8 - 11). Blemishes can affect the edible part of the crop (Figures 6 and 14) or, in the case of ornamentals, their attractiveness (Figures 12 - 13), which both can affect the market value of the crop. (c) To improve the storage life and quality of harvested plants and produce. Some of the greatest disease losses occur post-harvest (Figures 14 and 15). Fungi often spoil (render unusable) stored fruits, vegetables, tubers, and seeds. A few which infect grains produce toxins (mycotoxins) capable of causing severe illness or even death in humans and animals when consumed. Fungicides have been used to reduce mycotoxin contamination in wheat affected by Fusarium head blight, but most fungicides developed so far have not been sufficiently effective to be useful for managing mycotoxins associated with other diseases.
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. 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. Those that do have “curative” properties, which means they are active against pathogens that have already infected the plant, have limited ability to do so, often only being active on a pathogen within a few days of infection.
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. 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. 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. Important aspects of the disease cycle include whether the disease is monocyclic (one generation per year) or polycyclic (multiple generations) and latent period (time between infection and production of new inoculum).
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:
(a) Seed, bulbs, roots of transplants, and other propagative organs. These treatments are usually done by the seed company. Some treatments need to be done by the grower on-site at the time of planting. The goal is to kill pathogens that are on the planting material or to protect the young plant from pathogens in the soil.
(b) Soil either in-furrow at planting, after planting as a soil drench (including through drip irrigation), or as a directed spray around the base of the plant.
(c) Foliage and other aboveground parts of plants by means of a sprayer.
(d) Inside of trees via trunk injection.(f) Harvested produce, as a dip or spray in the packinghouse.
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 16-22). 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 19).
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.
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