A Short Summary of AFR
Strawberry is a popular fruit because of its beautiful color, attractive flavor and widely adaptable nature. The United States is one of the top three strawberry- producing countries in the world; California and Florida are the predominant strawberry-producing states, but most other states, including Iowa, have locally based production.
Anthracnose fruit rot is a major disease of strawberry that causes huge financial losses worldwide. Although we have emphasized a single fungal species – Colletotrichum acutatum – in the case study, strawberry is attacked by three Colletotrichum species: Colletotrichum acutatum, C. gloeosporioides and C. fragariae. However, C. acutatum is the dominant species causing anthracnose fruit rot; the other two species attack plant parts other than the fruit (Smith 2008). C. acutatum and C. gloeosporioides are now considered a species complex (Damm et al. 2012, Weir et al. 2012). Pending more biological, ecological and pathological evidence, the anthracnose fruit rot pathogen is referred to as C. acutatum in this case study.
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The Disease Triangle: A Basic Principle of Plant Pathology
The disease triangle (Figure 1) is a simple idea but an important one, because it is the foundation for the study of plant diseases: the existence of a disease caused by a pathogen (whether a fungus, bacterium, virus, etc.) requires the interaction of three things: a susceptible host plant, a pathogen that is capable of attacking that plant, and environmental conditions that favor the development of the particular disease. Each side of the triangle represents one of the disease components. For growers, one of the values of the disease triangle is that it helps them remember that there are three potential ways to manage a crop disease problem: suppressing the pathogen (by spraying fungicides, for example), changing the host (for example, by using varieties of crops that are genetically resistant to the pathogen), or modifying the environment (for example, by increasing plant spacing to reduce leaf wetness duration and minimize spread of the pathogen from one plant to another.)
Background Figure 1. The disease triangle.
Colletotrichum acutatum is one of the most damaging fungal pathogens in agriculture worldwide. It has an exceptionally wide host range, including grape, blueberry, apple, almond, orange, grapefruit, lime, peach, olive, and pine as well as strawberry (Peres et al. 2005). In addition, this fungus is exceptionally adept at remaining hidden on plants for long periods of time before visible symptoms appear. Conidia, the asexual spores of C. acutatum, are too small to be seen individually, but can be seen easily when they are produced by the thousands in sticky, pink to orange masses on diseased fruit (Figure 2) during wet weather. Appressoria are another weapon in Colletotrichum’s arsenal; they are tiny survival structures that form quickly on plant surfaces and enable the fungus to withstand stresses from ultraviolet light, unfavorable temperatures, drying out, and attack by other microorganisms.
Background Figure 2. C. acutatum on strawberry fruit. (Reprinted, by permission, from Louws et al., 2014)
Contaminated nursery plants are the most important source of C. acutatum in commercial strawberry fields. By this means, the fungus can move hundreds or thousands of miles per day, attached to small transplants in air-mailed cartons. When C. acutatum-infected plants are established in a new field, conidia can germinate and infect fruit under warm, rainy conditions. From the infected fruit in the field, abundant conidia can be produced from an acervulus (a specialized spore-producing structure) and spread to healthy plants by splashing water. Even before symptoms show up on the fruit, conidia can be produced on the leaf surface and spread throughout the field, infecting new plants but initially without showing any symptoms. This “stealth” method of invisible spread makes anthracnose fruit rot especially challenging to detect and manage before the problem blows up into a fruit rot epidemic (Figure 3).
Background Figure 3. Disease cycle of anthracnose fruit rot caused by C. acutatum. (Adapted from Louws et al., 2014)
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