COLD HARDINESS OF MALLING CLONAL APPLE ROOTSTOCKS UNDER DIFFERENT CONDITIONS OF WINTER SOIL COVER

Abstract

Winter survival of Mailing 7, 9, 26, 104, and 106 clonal rootstocks was compared during two winters under conditions of straw mulch with snow, clean cultivation with snow, and clean cultivation with snow removed. Field survival was evaluated by observations of growth during the summers following tests. Survival observations were interpreted in light of soil temperatures recorded under each of the cover treatments. Hardiness of the five rootstock clones was also determined by controlled freezing tests of stem and root tissue in November and May. There were no differences in field survival and growth of rootstock clones under mulch with snow and clean cultivation with snow treatments in either year. Under clean cultivation with snow removed, M 26 was found to be most hardy and M 7 least hardy of the five rootstocks tested. Soil temperature records revealed that all rootstocks survived a brief exposure to − 17.8 C. Controlled freezing tests did not reveal significant differences in stem hardiness levels of the five rootstock clones. November freezing tests of roots confirmed field survival observations that M 26 was significantly more hardy and M 7 significantly less hardy than the other clones.