Response of apple (Malus domestica) accessions to UK Phytophthora cactorum isolates in cut-shoot tests

Phytophthora cactorum is a water-borne oomycete pathogen responsible for economically-significant losses in the commercial production of apple and strawberry. In cultivated apple (Malus domestica), P. cactorum causes bark rots on the scion (collar rot) and rootstock (crown rot), as well as necrosis of the fine root system (root rot) and fruit rots. Reproducibly characterizing plant genetic resistance in controlled environments can be difficult; most reports of inheritance in apple have looked at segregations following inoculation of young seedlings while cultivar performance is often confirmed in field plantings. This study aimed to test the usefulness of inoculating detached shoots to determine the response of apple accessions to two UK P. cactorum isolates. Twenty-nine apple accessions were tested with the intention of determining the feasibility of employing this method to optimise large scale phenotyping of germplasm, breeding lines and mapping populations for UK material. Isolate P295 was markedly less virulent than the recently isolated R36/14. Variation in susceptibility was observed in apple and nine accessions were found to be very resistant to both isolates, with no lesion development recorded. These results highlight useful material for future resistance breeding to UK isolates.