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Apple proliferation (AP) disease caused by ‘Candidatus Phytoplasma mali’ is one of the most economical important diseases in the apple industry, causing tasteless undersized fruits. A project to produce AP-resistant rootstocks by crossing the natural resistant wild type M. sieboldii with commercial M. domestica apple rootstock varieties, observed severe decline and death in the progeny associated with latent apple viruses, namely Apple stem grooving virus (ASGV) and Apple stem pitting virus (ASPV). This study aimed to explain this phenomenon by gaining new information on the causative agent(s) as well as to see if there is any interaction between the different apple viruses or between virus and phytoplasma, and to study the effect of these pathogens on M. sieboldii, M. domestica and the hybrid progeny. The genome of the ASGV-AC strain, which was associated with the decline, was determined and variability and evolutionary studies were performed, revealing significant differences in the selection pressure of two variable areas in the genome. An in vitro culture system was established and ASGV, ASPV and ‘Ca. P. mali’ were successfully graft-transmitted to the different Malus genotypes as single infection or as virus-virus and virus-phytoplasma co-infection. The transmission rate was severely affected by the Malus genotype, with each genotype acting differently to the different virus, virus-virus and virus-phytoplasma combinations. A hypersensitive reaction was observed on M. sieboldii and some of the M. sieboldii-derived hybrid progeny depending on the inocula. Some hybrids showed severe resistance reactions while others recovered from the initial reaction and allowed the systemic spread of the virus(es). Quantitative Real-Time PCR was established for ASGV and ASPV and used to determine the mean virus titer per plant cell. ASGV titer was significantly lower compared to ASPV in M. domestica. The ASGV titer was lower in M. domestica compared to M. sieboldii and the hybrid progeny confirming the tolerant nature of M. domestica towards ASGV. An antagonistic interaction was observed between ASGV and ASPV, with the ASGV titer decreasing by half when co-infected with ASPV, while the ASPV titer is unchanged. An antagonistic interaction was also observed between ASGV and ‘Ca. P. mali’. While the ‘Ca. P. mali’ titer was significantly increased, the ASGV titer was decreased compared to the titer values obtained from single infections. This is the first study performed on the possible interaction between phytoplasma and virus.
|Supervisor:||Hell, Prof. Dr. Rüdiger|
|Date of thesis defense:||30 May 2013|
|Date Deposited:||07 Jun 2013 09:12|
|Faculties / Institutes:||Service facilities > Centre for Organismal Studies Heidelberg (COS)|
|Subjects:||500 Natural sciences and mathematics
|Controlled Keywords:||Candidatus Phytoplasma mali, Apple stem grooving virus, Malus|