Pseudomonas syringae pv. actinidiae (Psa) biovar 3, a virulent, canker-inducing pathogen is an economic threat to the kiwifruit (Actinidia spp.) industry worldwide. The commercially grown diploid (2×) A. chinensis var. chinensis is more susceptible to Psa than tetraploid and hexaploid kiwifruit. However information on the genetic loci modulating Psa resistance in kiwifruit is not available. Here we report mapping of quantitative trait loci (QTLs) regulating resistance to Psa in a diploid kiwifruit population, derived from a cross between an elite Psa-susceptible ‘Hort16A’ and a resistant male breeding parent P1. Using high-density genetic maps and intensive phenotyping, we identified a single QTL for Psa resistance on Linkage Group (LG) 27 of ‘Hort16A’ revealing 16–19% phenotypic variance and candidate alleles for susceptibility and resistance at this loci. In addition, six minor QTLs were identified in P1 on distinct LGs, exerting 4–9% variance. Resistance in the F1 population is improved by additive effects from ‘Hort16A’ and P1 QTLs providing evidence that divergent genetic pathways interact to combat the virulent Psa strain. Two different bioassays further identified new QTLs for tissue-specific responses to Psa. The genetic marker at LG27 QTL was further verified for association with Psa resistance in diploid Actinidia chinensis populations. Transcriptome analysis of Psa-resistant and susceptible genotypes in field revealed hallmarks of basal defense and provided candidate RNA-biomarkers for screening for Psa resistance in greenhouse conditions.

Chitosan inhibited growth of Botrytis cinerea in liquid culture and suppressed grey mould on detached grapevine leaves and bunch rot in commercial winegrapes. Germination of B. cinerea was completely inhibited in malt extract broth containing chitosan at concentrations greater than 0AE125 g L )1. However, treated conidia were able to infect detached Chardonnay leaves and pathogenicity was not affected, even after incubation for 24 h in chitosan at 10 g L )1. When added after conidial germination, chitosan inhibited B. cinerea growth and induced morphological changes suggestive of possible curative activity. The effective concentration of chitosan that reduced mycelial growth by 50% (EC 50 ) was 0AE06 g L )1. As a foliar treatment, chitosan protected detached Chardonnay leaves against B. cinerea and reduced lesion diameter by up to 85% compared with untreated controls. Peroxidase and phenylalanine ammonia-lyase activities were also induced in treated leaves. In vineyard studies, Chardonnay winegrapes exhibited 7AE4% botrytis bunch rot severity at harvest in 2007 after treatment with an integrated programme that included chitosan sprays from bunch closure until 2 weeks preharvest, compared with 15AE5% in untreated controls and 5AE9% with fungicide treatment. In the following season, botrytis bunch rot severity was 44% in untreated Chardonnay at harvest and the integrated programme (21%) was less effective than fungicides (13AE8%). However, in Sauvignon blanc winegrapes, the integrated and the fungicide programme each reduced botrytis bunch rot severity to 4% and were significantly different from the untreated control (11AE5%). This study provides evidence that suppression of botrytis in winegrapes by chitosan involves direct and indirect modes of action.

The infection of Pseudomonas syringae pv. actinidiae in kiwifruit is currently assessed by numerous methodologies, each with their own limitations. Most studies are based on either a laborious method of growth quantification of the pathogen or qualitative assessments by visual scoring following stem or cutting inoculation. Additionally, when assessing for resistance against specific pathogen effectors, confounding interactions between multiple genes in the pathogen can make mapping resistance phenotypes nearly impossible. Here we present robust alternative methods to quantify pathogen load based on rapid bacterial DNA quantification by PCR, the use of Pseudomonas fluorescens (Pfo), and a transient reporter eclipse assay, for assessing resistance conferred by isolated bacterial avirulence genes. These assays compare well with bacterial plate counts to assess bacterial colonization as a result of plant resistance activation. The DNA-based quantification, when coupled with the Pfo and reporter eclipse assays to independently identify bacterial avirulence genes, is rapid, highly reproducible, and scalable for high-throughput screens of multiple cultivars or genotypes. Application of these methodologies will allow rapid and high-throughput identification of resistant cultivars and the bacterial avirulence genes they recognize, facilitating resistance gene discovery for plant breeding programs.

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Honey bees (Apis mellifera) have been implicated in the spread of the fire blight pathogen (Erwinia amylovora), and may transmit other bacterial plant pathogens in the process of pollinating crops. Furthermore, the movement of hives from one orchard to another could spread plant diseases over large distances. We investigated whether honey bees might play a role in the transmission of different pathovars of Pseudomonas syringae. We detected live P. syringae pv. actinidiae (Psa), a pathogen of kiwifruit (Actinidia spp.), on caged bees in hives 6 days after the bees were inoculated with Psa, and recorded up to 1.8×10 4 colony forming units of Psa on honey bees foraging naturally on flowers of Psa-infected vines. P. syringae pv. syringae (PssSmr), a pathogen with a wide host range, was spread to untreated bees in a hive within 24 h following the introduction of foragers doused in PssSmr-contaminated pollen and was still detected on bees 9 days later. PssSmr was found on caged bees in hives 6 d after they were inoculated and PssSmr survived in hives for at least 14 days. These results demonstrate that P. syringae can survive in beehives and spread within a hive, which broadens the applicability of results from studies of E. amylovora and supports recommendations for a stand down period before moving beehives from a contaminated to a noncontaminated orchard.