Institute of Plant Sciences -- Pathology, Swiss Federal Institute of Technology, 8092 ETH-Zurich, Switzerland

Background and objectives
In former times, apples were produced mainly for home requirement or sold on local markets with short transport and storage time. Orchards were composed of a large array of cultivars. With the increased industrialisation and development of rational transport systems apple became a general commodity product, which had to be produced in large amounts of equal homogeneous aspect and quality. These requirements slowly led from the sparse meadow orchard with many cultivars to the intensive monocultivar orchard in which "perfect" fruits with long storage and shelf life can be produced. Parallel to this evolution disease problems increased, scab caused by needed more and more intensive chemical control with more and more efficacious fungicides.

Early this century researchers observed that particular cultivar were susceptible in one site and resistant at an other site and the fungus could adapt to cvs which were resistant. By the middle of this century genetic analysis become popular and genes for pathogenicity (what we would call today virulence/avirulence genes) were identified which were able to selectively overcome resistance genes in crab apple and particular selection of American domestic apple. However the notion that all "modern" domestic apple cv. are susceptible to scab and therefore lack resistance genes, found wide consensus. In the last decade researchers rediscovered the ephemeral resistances present in domestic cv. In this paper we try to assign a role in nature to these resistances and to the corresponding virulences.

Material and methods
Monosporic isolates of V. inaequalis were made early season in five orchards across Switzerland (2 monocultivar orchards, 2 with 3 cvs, one with 27 cvs). Each isolate was tested on a range of cvs (Golden Delicious, Ananas Reinette, Boskoop, Glockenapfel, and Jonathan ('standard test set'), Gravenstein, James Grieve, Maigold, Reinette de Champagne, Spartan'and Yellow Transparent (optional cv). Resulting lesions were used to assigne virulence (sporulation present) /avirulence (sporulation absent) toward the test cv. Isolates not able to infect any test cv were considered apatogenic.

Results and conclusions
Avirulent and virulent isolates could be determined for each of the test cv., the pattern corresponded to the classical vertical (differential) resitance scheme. We therefore can conclude that in commercial apple cv functionally different resistances are present and in the pathogen the corresponding virulence/avirulences are present. All isolates not being apathogenic carried the virulence toward their host of origin. Therefore, in the monocultivar orchard, as expected all isolated carried the virulence toward the cultivar present. Other virulences were present at a low frequency, some were absent or bellow the detection level. In the orchard with many cv., frequency of the single virulences in the scab population reflected the quantitative presence of the various cv., no virulence was fixed and the frequency of virulence combination corresponded to the expectation by the Hardy-Weinberg law for unlinked loci.

Apple is an obligate outbreeder (self-incompatible) and therefore in natural ecosystems resistance genes are recombined differently in each tree. This natural variation of resistance gene pattern will give the offspring a better chance to survive scab attack as the pathogen would need the maching combination of virulence genes. The pathogen inoculum at the beginning of the season comes predominantly from ascospores, product of the sexual recombination, which are randomly distributed. Since they cannot choose to land on the tree to which they are best adapted, the best strategy for the pathogen is to create a large array of various pattern of virulences. Even if a large part of the ascospores will not have the maching virulence pattern, some ascospores will have it and guarantee the survival of the pathogen. In monoculture, only one specific virulence is needed for successful reproduction. As all genotypes present will have it, also all ascospores will be capable of infection. If nature used variation in resistance gene composition in each individual to reduce scab incidence, this concept could be used also in designing orchards.