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Marssonina coronaria is considered a threat to organic apple production in Central Europe. Since the application of fungicides is limited in organic production, breeding for resistance seems to be a promising strategy to manage the disease in the future. In this study, an artificial inoculation method similar to procedures used in apple scab greenhouse screenings was developed using German strains of the pathogen M. coronaria for evaluating 110 Malus domestica cultivars. The strains were morphologically and molecularly characterized and confirmed as M. coronaria. Symptom development was significantly influenced by incubation method, conidia concentration and point of inoculation, but not by leaf position and different water types used for inoculation. The success of inoculation and spread of the fungus on infected leaves were confirmed by conventional PCR. Moreover, there was a difference in development of disease symptoms between inoculations with conidia from in vitro grown strains (in vitro inoculum) and conidia from diseased leaves from the orchard (field inoculum) in a time dependent manner. These differences which were first found with the susceptible cultivar ‘Topaz’ after artificial inoculation with in vitro and field inoculum were confirmed on 20 more apple cultivars inoculated with different in vitro grown strains. In summary, all tested cultivars, including 21 which are scab-resistant, developed symptoms of this disease. Results from three years of investigation indicate a decrease in virulence of M. coronaria strains, when cultivated on artificial culture (growth)-media. Hence field inoculum is recommended for artificial greenhouse screenings for the evaluation of disease resistance in Malus genetic resources.

Raspberry cane disease, caused by a complex of fungi, is amongst the most devastating problems for raspberry production. Using resistant or highly tolerant genotypes is a promising strategy. However, until now, cultivars with good field resistance for German fruit growers as well as the primary causal pathogens of cane disease present in the eastern part of Germany, Saxony state are still unknown. The primary objective of this study was to identify the primary disease causing organisms and resistant Rubus germplasm for future breeding efforts. From 2012 to 2014, we evaluated the degree of susceptibility to cane diseases on 213 raspberry cultivars at two different locations using a rating scale. We also identified the main fungi growing on infected canes. Our results suggest that Fusarium avenaceum is the main fungus causing cane disease in Saxony. The Rubus hybrid cultivars ‘Tayberry’ (2n = 6x = 42), ‘Buckingham Tayberry’ (2n = 6x = 42) and ‘Dorman Red’ (2n = 2x = 14) displayed the highest levels of field resistance at both locations. Moreover, we established an in vitro assay for resistance evaluation which correlates significantly with field data and is particularly suitable for quick assessment of resistance against F. avenaceum in breeding material. Future breeding programs, aiming at cultivars with enhanced resistance towards cane diseases can use the presented data for parental selection and may employ our in vitro F. avenaceum inoculation method for parental testing and progeny selection.

Eighty-two genotypes of Rubus available in germplasm collections, nurseries and home gardens were collected and evaluated using a set of 16 simple sequence repeat (SSR) markers to estimate the level of genetic diversity and relatedness of the germplasm and for testing them on trueness-to-type. Each of the 16 SSRs was successful in amplifying alleles from most genotypes. Fifteen of the markers produced polymorphic bands, whereas marker RhM023 was monomorphic. The polymorphic information content among genotypes varied from 0.056 to 0.83 with an average of 0.348. A neighbor-joining analysis allocated the genotypes to four major clusters containing 11, 24, 39 and eight genotypes, respectively. Cluster I consists of floricane-fruiting cultivars originating from the Scottish and/or British breeding programs or cultivars which have those cultivars in their pedigree. Cluster II included cultivars that have ‘Autumn Bliss’ or ‘Tulameen’ in their pedigree. Cluster III consists of summer-bearing raspberry cultivars, some primocane-fruiting cultivars, and a few intermediate summer-fall-bearing types. Cluster IV consists of the blackberry ‘Navaho’ (R. fruticosus L.), the interspecific hybrid ‘Dorman Red’ and a few other raspberry varieties. A number of yellow fruited varieties was dispersed on three different clusters suggesting a convergent evolution of this trait. The pedigree of several genotypes could be confirmed using a Pedimap based approach, whereas other cultivars were found to be genetically identical. The results disclose the alarming narrow genetic base of Rubus resources in Germany. Broadening of this base is urgently needed.