The fungus Aureobasidium pullulans is ubiquitous and can cause russet of fruit in New York orchards. The details of russet induction by this fungus are not well known. We inoculated `McIntosh' apple fruits with a suspension of A. pullulans spores (10 million colony-forming units/mL) 1–2 weeks postbloom or later at about 30 days postbloom. We dropped inoculum into plastic “microwells” attached to the fruit surface. The cuticle of uninoculated fruit (wells filled with water only) had no russet by autumn. Skin susceptibility to russet diminished with fruit age. The cuticle of inoculated young fruit began to break down in a few days, likely through direct cuticular digestion. Further erosion and breaching of the protective cuticle caused underlying epidermal cells to die. Within 1–2 weeks, cuticle disruption and epidermal cell death were widespread. This stimulated the fruit to initiate a repair process that involved periderm formation (russet), where many rows of cells were produced in nearby tissue to seal off the injury. This type of repair is not stretchable, so as young fruit expanded, additional skin splits and checks developed. This breakdown–repair process repeated itself, which created a scurfy skin. Older fruit did not expand as much after inoculation as did young fruit, and so they developed few obvious leathery patches of periderm. Older cuticle also resisted digestion better than did the young fruit cuticle, but we do not know if resistance resulted from increased cuticle thickness in older fruit or a change in cuticular compounds during fruit growth. Regardless, A. pullulans applied to older fruit did not progress beyond the early phase of cuticle digestion, even after 3 weeks postinoculation.
Martin C. Goffinet, Thomas J. Burr, Mary Catherine Heidenreich, and Mary Jean Welser
William C. Johnson, William C. Johnson, Martin Goffinet, Mary J. Welser, Terence L. Robinson, H. Todd Holleran, Karl J. Niklas, and Steve A. Hoying
An increased incidence of graft union failure of apple trees during high wind events has been noted by researchers participating in the NC-140 regional rootstock testing project for certain rootstock-scion combinations. By measuring the strength of graft unions in a survey of mature apple trees in multiple stock-scion combinations, we have determined that there are significant differences. These differences may be attributable to genotype specific characteristics of rootstocks, scions, and/or rootstock-scion interactions. We are presently exploring potential biophysical and anatomical differences related to weak graft unions of apple rootstock and scion varieties. As traits correlated with weak graft unions are identified, they will be useful to help growers avoid the rootstock-scion combinations that are particularly susceptible to tree failure.