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Significance

Apple scab disease has long been a problem on apples; symptoms of the disease can be easily identified on fruit in paintings from the fifteenth and sixteenth centuries.


images courtesy the The National Gallery of Art (NGA). Click the images for larger views and more information.

The frequent depictions of scab-infected apples suggest that the disease was common and that the affected fruit was acceptable in earlier times. All the commonly grown apple varieties were susceptible to the disease, and there were no chemical treatments until the late 1800s. At that time, copper- and sulfur-based fungicides provided preinfection protection, but the treatments caused substantial damage to the apple foliage. Even today, in spite of the highly effective chemicals and the resistant apple varieties that are available, apple scab causes greater economic losses of apples in North and South America, Europe, and Asia than any other apple disease.

Venturia inaequalis was one of the first ascomycetes to be subjected to genetic analysis; the heritability of pathogenicity and of sexual compatibility was investigated as early as the 1930s. It has served as and continues to be a valuable tool for basic genetic research and for studies of the inheritance of pathogenicity. Among the features that make Venturia inaequalis so amenable for genetic research is the fact that it is similar to many obligate parasites which infect young tissues and live in a close association with them, without visibly harming them, for an extended period of time. Yet unlike obligate parasites, it can be cultured on artificial media, and matings can be made in vitro. Other advantages of V. inaequalis for genetic studies include its considerable diversity, with many pathotypes or races occurring in the natural population; the stability of phenotypes and genotypes in culture over many years; the fact that it is haploid, allowing the effects of alleles to be studied directly; the uninucleate conidia which give rise to colonies in which all the nuclei are genetically uniform; and the ability to isolate all eight of the ascospores from a single ascus (Figure 9). The eight spores are the result of a single meiotic event, and so segregation of traits can be studied directly rather than statistically. Furthermore, the eight ascospores are arranged in order, allowing reconstruction of the meiotic divisions and mapping of centromeres as genetic loci. Many of these advantages are shared by other Ascomycetes, but they are uniquely brought together in V. inaequalis.

Figure 9

Many years of study have demonstrated that the genetics of pathogenicity in V. inaequalis are not simple. There are multiple genes involved in pathogenicity to various apple species and cultivars. In several of these interactions, pathogenicity is conditioned by single genes. In some cases, there appears to be a gene-for-gene relationship between the host and the pathogen. In a gene-for-gene interaction, each gene for resistance in the host is matched by a gene for pathogenicity(or virulence) in the pathogen. Resistance will only occur if the host has the dominant allele for the resistance gene and the pathogen has the corresponding dominant allele conditioning non-pathogenicity (or avirulence); all other combinations will result in disease. Gene-for-gene interactions are found in numerous host-pathogen interactions, and are thought to be the consequence of co-evolution of the host and the pathogen.

Because the effects of various environmental parameters on the development of apple scab epidemics are well understood and fairly predictable, this disease serves as a useful model for the teaching of important epidemiological principles. Interactive computer simulators, such as APPLESCAB, have been developed for instructional uses. The APPLESCAB simulator incorporates all the important components of the apple scab disease system, including fungicide treatments, weather conditions, pathogen populations and levels of fungicide resistance, tree growth and development, and economic parameters such as cost of the chemical treatments and market value of the crop (Figures 21). By varying the different components, the simulator can demonstrate their effect on disease development and can predict the economic result of the epidemic. http://www.apsnet.org/education/advancedplantpath/topics/applescab/

Figure 21

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