Phytoplasmas, a group of obligate biotrophic bacteria with extremely small genomes, are pathogens of many agriculturally important crops, including grapevine, coconut and sugarcane, and cause considerable economic damage in several countries. Phytoplasma transmission occurs through grafting of infected plant material or insect vectors, mainly sap-sucking leafhoppers and psyllids. Apple Proliferation (AP), caused by ‘Candidatus Phytoplasma mali’, is a serious disease of apple trees in Central and Southern Europe. Since no direct control methods are available for phytoplasmas, current control of AP consists of the removal of symptomatic plants and insecticide treatments against the vectors. We are interested in studying resistance induction and symptom suppression as possible alternative management options. The effects of four commercially available bio-active compounds on the infection rates, symptom expression and growth rates of apple trees (Malus x domestica) cv. Golden Delicious infected with ‘Ca. P. mali’ were tested over a three-year period under controlled conditions. Post-infection treatments using Bion® (active ingredient: Acibenzolar-S-Methyl), Messenger® (Harpin protein), Regalis® (Prohexadione-Ca) and Dormex® (Cyanamide) had no significant effect on infection rates. 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 non-infected apple trees. 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 compared to the untreated control 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. The resistance inducers and plant growth regulators used in our experiments did not result in significant and permanent recovery of infected apple trees from phytoplasma induced symptoms. However, scientific data from other host systems suggest that induced recovery might be a management option for certain phytoplasma diseases and should be studied in more detail.

Can resistance inducers and plant growth regulators be used to control phytoplasma diseases? A case study of woody plants

254th National Meeting (2017)

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