Assessment of somaclonal variation in apple. I.Resistance to the fire blight pathogen, Erwinia amylovora

After inoculation of glasshouse-grown somaclones regenerated from apple leaf discs (cv. Greensleeves) 33% of 270 showed an increase in resistance to the fire blight pathogen Erwinia amylovora. In contrast only 21% produced less severe symptoms than parental material after inoculation of plants held in culture in vitro. Sixteen somaclones which showed the highest levels of fire blight resistance were tested intensively as both glasshouse-grown plants and as micropropagated plants in culture. In tests conducted with glasshouse-grown plants up to 60% of plants of the most promising somaclones exhibited minimum symptoms after inoculation compared with 4% of ‘Greensleeves’ parental plants. The comparable figures for inoculations of in vitro cultured plants were 70% and 8%. The growth of E. amylovora was reduced in resistant somaclones compared with parental plants. Pretreating leaf discs with virulent E. amylovora cells prior to somaclone regeneration had no effect on the frequency of regenerated somaclones exhibiting increased resistance to the pathogen.