ESTABLISHMENT OF AN IN VITRO SCREENING SYSTEM OF APPLE PROLIFERATION-RESISTANT ROOTSTOCK GENOTYPES BASED ON MICROGRAFTING

This study was carried out to develop an in vitro system for the rapid identification of genotypes resistant to Candidatus Phytoplasma mali. The basis of this work was a preceding study in which natural resistance to Candidatus Phytoplasma mali was found in the apomictic Malus sieboldii and accessions deriving from crosses between M. sieboldii × M. domestica sp. through in vivo infections. A previously developed in vitro inoculation method based on micrografting was evaluated for its use to speed up the resistance screening process. For this, in vitro cultures of the resistant apomictic genotypes M. sieboldii, H0909, D2212 and the susceptible Malus domestica genotypes ‘Golden Delicious’ and rootstock ‘M.9’ were established as well as in vitro cultures of ‘Golden Delicious’ and ‘Rubinette’ infected with Candidatus Phytoplasma mali subtypes AT2 and AP respectively. Inoculation was made under aseptic conditions by micrografting. Several independent experiments with at least 10 repetitions were done for each combination. After one month graft contact the success of the grafting was recorded. Resistance or susceptibility of the inoculated genotypes was evaluated according to the transmission rate and to the phytoplasma concentration in the inoculated plants as determined by qPCR. Furthermore, the reaction of the inoculated genotype to the phytoplasma infection during the graft contact period as well as the survival of the inoculated plant was monitored. Analysis of the data showed different plant responses between susceptible and resistant genotypes as well as between the different Candidatus Phytoplasma mali subtypes used as inoculum.