INTERNAL MOVEMENT OF ERWINIA-AMYLOVORA THROUGH SYMPTOMLESS APPLE SCION TISSUES INTO THE ROOTSTOCK

Shoot tips of potted Empire and Golden Delicious trees on the suscepti ble dwarfing rootstock M.26 in the greenhouse were injected with inocu lum containing 5 x 10(9) CFU/ml Erwinia amylovora. At intervals after inoculation, trees were sampled at increments between the shoot tip an d the roots by excising stem segments. Segments were ground in phospha te buffer and assayed for the presence of E. amylovora by plating on s emi-selective medium and by a polymerase chain reaction (PCR)-based de tection method. Eleven days after inoculation, E. amylovora was detect ed by PCR in symptomless scion tissue >50 cm below the shoot-tip in Em pire and Golden Delicious, and in 2-year-old tissue in Golden Deliciou s. By 21 days, E. amylovora was detected in the M.26 rootstock of Empi re trees, and by 41 days in the M.26 rootstock of Golden Delicious. In a similar experiment the following year, Empire trees on M.26 rootsto ck were inoculated with E. amylovora at early (16 May), mid-(11 June), and late (6 July) phenophase of shoots. Three weeks after inoculation , E. amylovora was detected by PCR from M.26 rootstocks of five of six plants inoculated at the late phenophase, compared to zero of six pla nts inoculated at the early or mid-phenophase. Late-season fire blight infections of the scion may be particularly hazardous for the health of the rootstock.