Abstract
Apple Proliferation (AP), caused by ‘Candidatus Phytoplasma mali’, is a serious disease of apple trees in Europe. Current control of AP consists of the removal of symptomatic plants and insecticide treatments of the vectors. Resistance induction might be an alternative management option. Here we report on the effects of four commercial products on the infection rates, symptom expression and growth rates of potted apple trees (Golden Delicious) infected with the AP phytoplasma. Post-infection treatments using Bion® (active ingredient: Acibenzolar-S-Methyl), Messenger® (Harpin protein), Regalis® (Prohexadione-Ca) and Dormex® (Cyanamid) had no significant effect on infection rates and phytoplasma titres. Terminal growth of apple trees was increased significantly by AP infection; Prohexadione-Ca was the only compound which had a significant inhibiting effect on the growth of both infected and control trees. Expression of typical AP symptoms followed a seasonal cycle with a peak in fall. Acibenzolar-S-Methyl and Harpin had no significant effects on symptom expression. AP symptoms were masked during summer by Prohexadione-Ca, which caused severe growth abnormalities. Cyanamide changed the seasonal appearance of AP symptoms: while symptoms were delayed the first two years (2008 and 2009), symptoms appeared earlier the third year (2010). Differences in symptom expression levelled off later in the vegetative season, and no significant difference was found in October.