Investigations on biotic factors in root systems of replant diseased (ARD) apple plants

authored by

Tom-Pascal Pielhop

supervised by

Edgar Maiß

Abstract

Apple (Malus x domestica BORKH.) is one of the most cultivated fruit bearing plants worldwide with an enormous relevance to global economics and human nutrition. Apple cultures are susceptible to a high variety of diseases one of which is Apple Replant Disease (ARD). ARD is a worldwide occurring multifactorial phenomenon appearing when apples are planted at the same site repeatedly. Once affected, orchards and nurseries can lose up to 50 % profitability due to growth depression, delayed fruit bearing process, and lowered yield. To contribute to the development of an integrated disease control, the aim of this work was to get a detailed insight to biotic factors of affected root systems by investigating the endophytic fungal microbiome, mycoviruses infecting those fungi, and arbuscular mycorrhizal fungi (AMF). Approaching with ITS amplicon sequencing a shift in the endophytic fungal microbiome of apple roots due to ARD and a strong influence of soil treatments (γ- or heat-irradiation) on the community was detected. The fungal diversity in plants however did not significantly differ between analyzed growing locations or soils. There was also no direct correlation between i) symptom severity and fungal diversity as well as ii) disease symptoms and absolute pathogen load. The fungal families Nectriaceae and Helotiales (f.i.s.) were further confirmed to be crucial for the development of ARD symptoms. Moreover, the investigations on the virome of several ARD-related phytopathogenic fungi resulted in the identification of six new mycoviruses. All viruses were classified taxonomically, and gene functions were deciphered. For the first time, three mycoviruses were found simultaneously in a Rugonectria isolate. They were named rugonectria rugulosa quadrivirus 1 (RrQV1), rugonectria rugulosa mitovirus 1 (RrMV1), and rugonectria rugulosa dsRNA virus 1 (RrV1). A chrysovirus named ilyonectria pseudodestructans chrysovirus 1 (IpCV1) was identified in an Ilyonectria isolate. Further, two new alternaviruses were detected one of which had been identified in the precursor project. They were named dactylonectria torresensis alternavirus 1 (DtAV1), and ilyonectria robusta alternavirus 1 (IrAV1). Furthermore, all alternaviral gene functions were identified and the deciphered genome was adapted for all known viruses of this family for the first time. Moreover, inocula adhering AMF, isolated from ARD-affected apple plants were produced and tested experimentally in a greenhouse approach. Two of the inocula were capable to increase plant performance in ARD soils significantly. The AMF are thus a basis for the development of an integrated plant protection approach. Nevertheless, future studies need to evaluate the interrelationships between the endophytic microbiome and ARD-etiology in a deeper manner. A possible hypovirulent effect of the mycoviruses is to be identified to evaluate their use as biocontrol agent to mitigate ARD as part of inocula mixtures together with AMF, which must be tested in field trails. However, the results of this work contribute to deciphering the multifactorial nature of ARD.

Organisation(s)

Phytomedicine Section

Type

Doctoral thesis

No. of pages

128

Publication date

01.08.2024

Publication status

Published

Sustainable Development Goals

SDG 3 - Good Health and Well-being

Electronic version(s)

https://doi.org/10.15488/17841 (Access: Open)