Blackheart injury in 'Golden Delicious', 'Jonagold', 'Empire' and 'Rome Beauty' apple trees on five rootstocks in the 1990 NC-140 cultivar/rootstock trial

Blackheart injury was evaluated at 25 cm above the soil surface on 'Golden Delicious', 'Jonagold', 'Empire', and 'Rome Beauty' apple (Malus X domestica Borkh) trees on M.9 EMLA, B.9, Mark, O.3 and M.26 EMLA rootstocks in the 1990 NC-140 cultivar by rootstock plantings located in Indiana, Iowa, Kentucky, Maine, Massachusetts, Ohio, and Tennessee. Trees grown in Iowa, which recorded the coldest temperatures, exhibited the greatest injury, while trees grown in Tennessee, Massachusetts, and Maine exhibited the least injury. Cultivar susceptibility to blackheart injury was affected by location, with Jonagold trees sustaining a high percentage of injury in Iowa and Indiana, and Rome trees sustaining high injury in Indiana, Kentucky, Maine, and Massachusetts. Trees on Mark rootstock, followed by B.9 and M.9 EMLA, most frequently exhibited a high percentage of blackheart injury, while trees on M.26 EMLA, followed by O.3, most frequently sustained less blackheart injury. Supplemental decline ratings of trees in the Iowa planting in conjunction with the blackheart injury support reports that trees on Mark and Low temperature tolerance of apple trees on dwarfing rootstocks is a concern in colder apple-producing regions. Trees have been killed following test winters, but more often exhibit symptoms of decline from which recovery often occurs. Injury can be to the roots or the above-ground portions of the trees. Root injury typically occurs under conditions where trees are growing on sandy soils and absence of snow cover during a freezing event (16). Depending upon when the freezing event occurs,injury to the above-ground portions of the trees can be to the xylem, bark, or buds. mid-winter, apple vegetative buds and bark are much hardier than living xylem cells, while xylem is hardier than the bark and buds in early autumn and late spring (17, 18). Blackheart is a form of winter injury characterized by the killing ok xylem parenchyma cells and the occlusion of vessel elements (22). The typical symptom of blackheart is oxidative browning of the xylem tissue (20) caused by the supercooled fraction of intercellular water freezing in the xylem ray parenchyma cells (17, 18). Most often, blackheart injury does not cause outright death of the plants with recurring injury being common in nature, and it has been implicated in the decline and reduced productivity of fruit trees (17).