In vitro testing of apple tetraploids for resistance to fire blight

The most severe bacterial disease of apple tree is the fire blight caused by Erwinia amylovora. Due to the lack of effective disease control methods, it is extremely important to introduce into cultivation apple cultivars with reduced susceptibility to this disease. In our previous studies, several neotetraploids for 6 apple cultivars (‘Free Redstar’, ‘Gala Must’, Co-op 32, ‘Pinova’, ‘Redchief’ and ‘Sander’) have been obtained using in vitro method. Most of the cultivars used for polyploidisation exhibited high or moderate resistance level to E. amylovora. We suppose that due to the resistance gene duplication, some tetraploids with enhanced resistance to E. amylovora could be achieved. The aim of the study was to evaluate the level of E. amylovora resistance in the apple tetraploids compared to their diploid counterparts using in vitro assay. In the first stage of research, an in vitro method for apple resistance to fire blight assessment was developed. Eight apple cultivars including those used for polyploidisation and the reference cultivars highly susceptible to E. amylovora (‘Idared’ and ‘Lobo’) were used for experiments. Shoots from in vitro cultures were inoculated with pathogen (isolate 659) inoculum at various concentrations with different inoculation methods. The best correlation of in vitro test with the assessments performed in natural conditions were obtained using the pathogen inoculation on the cut surface of shoot apex with a scalpel immersed in a bacterial inoculum of 10⁴ cfu mL^‑1. This method was selected for evaluation of resistance level to E. amylovora of the neotetraploids compared to their diploid counterparts. Two to five tetraploid clones of four cultivars were tested. The individual tetraploid clones within the cultivar differed in the resistance level. Most of the tetraploid clones showed similar resistance level to their diploid counterparts and some of tetraploids revealed a significantly greater or lower resistance to the pathogen.