Composition of the sooty blotch and flyspeck complex on apple in Norway is influenced by location and management practices

Abstract

Sooty blotch and flyspeck (SBFS) is a complex of at least 80 fungal species in 24 genera within the Dothideomycetes order Capnodiales. These epiphytes embed in the waxy epicuticle of apple fruit and colonies become visible in mid- to late summer. The blemished fruit are not marketable as fresh produce. Previous surveys of orchards in central and eastern USA and southern Europe found SBFS assemblages that were unique to geographic region. In the USA, SBFS species diversity was higher in orchards that received no fungicide sprays than those receiving fungicide sprays, suggesting that orchard management was an important determinant of SBFS species distribution and prevalence. To test the hypothesis that the composition of the SBFS complex is affected by geographic region and orchard management practices in Norway, 35 sites with a range of different management practices and cultivars were sampled. Colonies on each apple were counted according to the mycelial type, and subsamples of each mycelial type were amplified and sequenced using either primers specific to the rDNA of Capnodiales or a universal fungal primer pair for the large subunit of rDNA. Results indicated that a single genus was associated with each mycelial type. Seven genera known to cause SBFS were identified: Microcyclospora, Dissoconium, Peltaster, Microcyclosporella, Phaeothecoidiella, Schizothyrium, and Geastrumia. The percent composition of each site’s SBFS assemblage was estimated to genus. Significant differences were found in SBFS assemblages among geographic districts and management practices, and there was a significant interaction of district and management. Organic orchards tended to have the least SBFS severity and genus diversity, whereas higher SBFS diversity tended to occur in abandoned than managed orchards. This is the first survey of SBFS fungi in apple orchards in Scandinavia.