Observations in controlled field experiments over 5 years indicated that imidacloprid, applied as a soil drench around the trunks of peach (Prunus persica), nectarine (P. persica var. nectarine) and japanese plum (P. salicinia) trees at planting and in the early spring and mid-summer for two subsequent seasons (0.7 g/tree ad.), slowed the development of symptoms of phony peach disease (PPD) and plum leaf scald (PLS) (Xylella fastidiosa) in the trees. After 3.5 years, the percentage of peach trees showing PPD, symptoms was 8.5% for the imidacloprid-treated trees compared to 34.3% for untreated trees. After 4.5 years, the percentage of peach trees showing PPD symptoms was 13.1% in the treated trees and 71.4% in the untreated trees. After 3.5 years, nectarine trees in untreated and treated plots showed PPD symptoms in 8.3% and 0.9% of the trees, respectively. After 4.5 years, PPD symptoms in nectarine were found in 32.3% of the untreated trees and 8.5% of the treated trees. Development of PLS disease in plum was also slowed by the trunk drench with imidacloprid in two japanese plum varieties. After 3.5 years, dieback was observed in 55% of the twigs of untreated and 23% of the twigs of treated trees of 'Au Rosa' plum and 33% of the twigs of untreated and 12% of the twigs of treated trees of 'Santa Rosa' plum.