Acquired Tolerance in Apricot Plants that Stably Recovered from European Stone Fruit Yellows

European stone fruit yellows (ESFY) is one of the most destructive phytoplasma diseases of plum, apricot, and peach in Europe. Conventional preventive defense strategies have been ineffective. Because apricot cultivars with innate-constitutive resistance against ESFY are not available, the aim of this more than 20-year-long study was to seek acquired resistance or tolerance. In the first experiment, we surveyed an orchard with seven apricot cultivars for 12 years in an area of northern Italy with a high rate of natural occurrence of ESFY. Of the diseased plants, a few (8.7%) became completely symptomless but retained the phytoplasma, as confirmed by polymerase chain reaction (PCR). In the second experiment, we grafted buds from two stably recovered plants and from two nonrecovered plants onto 'Rubira' peach. Over the next 9 years in an orchard with a high rate of natural infection, 93.0% of the "nonrecovered clones" became diseased but only 1.5% of the plants grafted with the two "recovered clones" developed ESFY symptoms. According to PCR analyses, all of the exposed test plants were ESFY-infected, whether they were derived from recovered or nonrecovered mothers. This could indicate that epigenetic changes occurred in recovered plants due to a graft-transmissible memory. Based on the results attained from the two described experiments, we propose that an acquired tolerance that occurred in stably recovered apricot trees was graft transmitted from two tolerant apricot clones. In contrast, we did not demonstrate a cross-protection process based on protectant avirulent phytoplasma strains that suppress severe strains.