LOW-TEMPERATURE TOLERANCE OF STARKSPUR SUPREME DELICIOUS ON 9 ROOTSTOCKS IN THE IOWA 1980-1981 NC-140 COOPERATIVE PLANTING

'Starkspur Supreme Delicious' apple trees in the Iowa planting of the 1980-1981 NC-140 cooperative rootstock trial experienced three late-fall/early-winter freezes, and two freezes associated with fluctuating temperatures during four winters of the ten year study. All the trees on MAC.24 were killed following two consecutive winters during which freezes occurred in late-fall or early-winter. Trees on MAC.9 exhibited a high incidence of spur injury, and reduced vigor that may have been confounded by prior injury and poor growth of the cultivar on this rootstock. Shoot injury evaluations following two freezes suggested that trees on MAC.9 acclimate early but do not attain a high tolerance to low temperature stress in mid- or late-winter. Trees on OAR 1 exhibited a similar pattern of hardiness reported for M.7, but were more susceptible to late-fall/early-winter freezes than M.7 EMLA. Trees on O.3 appeared to be similar in hardiness to trees on M.26 EMLA.