Assessing Malus germplasm for fire blight resistance through floral screening

Apple ( Malus domestica Borkh) is a widely cultivated temperate tree fruit crop. Fire blight, caused by the bacterium Erwinia amylovora (Burrill), severely impacts apple production. Currently the predominant phenotyping method for fire blight resistance is based on shoot inoculation, yet the main mode of entry for E. amylovora is through floral infections. Selected Malus genotypes were phenotyped for fire blight resistance through field floral inoculation and compared with shoot inoculation studies. Observations were analysed for differences in the incidence of floral infection, severity and rate of disease progression, and canker formation. A large variation in the incidence of fire blight infection between the different genotypes was observed. M. baccata ‘Hansen’s baccata #2’, ‘Splendour’, M. ‘Robusta 5’, ‘Red Delicious’, ‘Scired’ and ‘Baujade’ had the lowest incidence of floral infection. Some genotypes showed ‘slow’ infections where the florets were shed before the disease became established in the bourse. Genotypes with the lowest disease severity tended to be genotypes with ‘slow’ infections. ‘Hansen’s baccata #2’, ‘Splendour’ and ‘Baujade’ had a low incidence of floral infections, but many infections were severe. Groupings of genotypes into floral tolerance or resistance/susceptibility classes were proposed. There was no correlation between the glasshouse shoot inoculation studies and artificial floral infection studies in many different genotypes. We have identified potential genotypes for use in studies to elucidate fire blight resistance quantitative trait loci specific to floral infection. This study has observed fire blight tolerance within field floral infections of genotypes with the progenitors M. prunifolia 19651, ‘Red Delicious’, ‘Robusta 5’, ‘Novosibirski Sweet’, M. baccata , M. ×atrosanguinea 804, M. prunifolia pendula , ‘Liestnaya Antonoffku’, and ‘Splendour. ‘Liberty’ scored best overall for combined floral and shoot screening.