Understanding the mechanisms involved in pathogen adaptation to quantitative resistance in plants has a key role to play in establishing durable strategies for resistance deployment, especially in perennial crops. The erosion of quantitative resistance has been recently suspected in Cuba and the Dominican Republic for a major fungal pathogen of such a crop: Pseudocercospora fijiensis, causing black leaf streak disease on banana. This study set out to test whether such erosion has resulted from an adaptation of P. fijiensis populations, and to determine whether or not the adaptation is local. Almost 600 P. fijiensis isolates from Cuba and the Dominican Republic were sampled using a paired‐population sampling design on resistant and susceptible banana varieties. A low genetic structure of the P. fijiensis populations was detected in each country using 16 microsatellite markers. Cross‐inoculation experiments using isolates from susceptible and resistant cultivars were carried out, measuring a quantitative trait (the diseased leaf area) related to pathogen fitness on three varieties. A further analysis based on those data suggested the existence of a local pattern of adaptation to resistant cultivars in both of the study countries, due to the existence of specific (or genotype by genotype) host–pathogen interactions. However, neither cost nor benefit effects for adapted populations were found on the widely used “Cavendish” banana group. These results highlight the need to study specific host–pathogen interactions and pathogen adaptation on a wide range of quantitative resistance phenotypes in banana, in order to develop durable strategies for resistance deployment.

Plant pathogens can adapt to quantitative resistance, eroding its effectiveness. The aim of this work was to reveal the genomic basis of adaptation to such a resistance in populations of the fungus Pseudocercospora fijiensis, a major devastating pathogen of banana, by studying convergent adaptation on different cultivars. Samples from P. fijiensis populations showing a local adaptation pattern on new banana hybrids with quantitative resistance were compared, based on a genome scan approach, with samples from traditional and more susceptible cultivars in Cuba and the Dominican Republic. Whole-genome sequencing of pools of P. fijiensis isolates (pool-seq) sampled from three locations per country was conducted according to a paired population design. The findings of different combined analyses highly supported the existence of convergent adaptation on the study cultivars between locations within but not between countries. Five to six genomic regions involved in this adaptation were detected in each country. An annotation analysis and available biological data supported the hypothesis that some genes within the detected genomic regions may play a role in quantitative pathogenicity, including gene regulation. The results suggested that the genetic basis of fungal adaptation to quantitative plant resistance is at least oligogenic, while highlighting the existence of specific host-pathogen interactions for this kind of resistance. IMPORTANCE Understanding the genetic basis of pathogen adaptation to quantitative resistance in plants has a key role to play in establishing durable strategies for resistance deployment. In this context, a population genomic approach was developed for a major plant pathogen (the fungus Pseudocercospora fijiensis causing black leaf streak disease of banana) whereby samples from new resistant banana hybrids were compared with samples from more susceptible conventional cultivars in two countries. A total of 11 genomic regions for which there was strong evidence of selection by quantitative resistance were detected. An annotation analysis and available biological data supported the hypothesis that some of the genes within these regions may play a role in quantitative pathogenicity. These results suggested a polygenic basis of quantitative pathogenicity in this fungal pathogen and complex molecular plant-pathogen interactions in quantitative disease development involving several genes on both sides.

No abstract

During a survey of Streptomyces infecting potato (Solanum tuberosum) in the UK, two strains were isolated which were distinct from S. scabiei, which is already known to occur in the UK. Amplification with speciesspecific PCR primers (Wanner, 2006) identified these strains as S. turgidiscabies (Miyajima et al., 1998) and S. acidiscabies (Lambert & Loria, 1989), and the same PCR assay indicated presence of these species in several fields in England and Scotland. Representative strains were deposited in the UK National Collection of Plant Pathogenic Bacteria as NCPPB4444 and NCPPB4445 respectively. Strain NCPPB4444 was isolated from cv. Desiree tubers, grown in South Yorkshire, England, with large erumpent lesions. A majority of tubers from the same seed stock in the same field exhibited similar symptoms. Strain NCPPB4445 was isolated from cv. Maris Piper, grown in Aberdeenshire, Scotland, on soil of pH 5AE4, where disease incidence was high and dry flaky scab symptoms typically covered almost 100% of the tuber surface. Both strains were isolated on NPPC medium (Schaad et al., 2001), producing small white sporulating colonies.Inoculation of radish seedlings with both strains resulted in severe necrosis within 2 weeks. Potato minitubers (cv. Maris Piper) grown in pots containing sterilized compost were inoculated with 250 mL suspensions of Streptomyces (approx. 10 7 cfu mL )1 ) when leaves first emerged. After 12 weeks, lesions were observed on tubers in each of five replicate pots. Uninoculated controls were free from symptoms. Bacteria recovered from lesions were confirmed by PCR to be the same species as the inocula. To confirm species identity, a portion of the 16S rDNA of each original strain was sequenced. The sequence from strain NCPPB4444 (GenBank Accession No. FJ817424) was 100% identical to that of S. turgidiscabies type strain ATCC700248. The sequence from strain NCPPB4445 (FJ804480) was 100% identical to that of the S. acidiscabies type strain ATCC49003. Although several Streptomyces spp. are known to cause common scab of potato, the species identity is of significance as different diagnosis and control methods may apply. This is the first report in the UK of common scab of potato caused by S. turgidiscabies or S. acidiscabies. Acknowledgements