Involvement of Dactylonectria and Ilyonectria spp. in tree decline affecting multigeneration apple orchards

Authors: MANICI, L - Cra-Plant Pathology Research Institute; CAPUTO, F - Cra-Plant Pathology Research Institute; SACCA, M - Cra-Plant Pathology Research Institute; KELDERER, M - Research Centre For Agriculture And Forestry Laimburg; NICOLETTI, F - Cra-Plant Pathology Research Institute; TOPP, A - Research Centre For Agriculture And Forestry Laimburg; Mazzola, Mark

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/10/2018
Publication Date: 2/7/2018
Citation: Manici, L.M., Caputo, F., Sacca, M.L., Kelderer, M., Nicoletti, F., Topp, A.R., Mazzola, M. 2018. Involvement of Dactylonectria and Ilyonectria spp. in tree decline affecting multigeneration apple orchards. Plant and Soil. https://doi.org/10.1007/s11104-018-3571-3.
DOI: https://doi.org/10.1007/s11104-018-3571-3

Interpretive Summary: Replant disease is well established as an impediment to apple orchard productivity when replanting a new orchard on an old orchard site. Recently, replant type symptoms were observed within small defined sites within productive established orchards in the South Tyrol region of northern Italy. Studies were conducted using soils collected from the rhizosphere of symptomatic trees, the rhizosphere of healthy trees, and from the orchard aisle where tree roots had not occupied soil. When apple rootstocks were grown in the greenhouse in soil collected in the orchard from symptomatic trees, the young rootstocks grew poorly and exhibited replant disease symptoms comparable to those observed in the orchard. Rootstocks grown in soils from the orchard aisle or collected from the root zone of healthy trees was similar in all orchards examined and superior to that obtained in soil collected from symptomatic trees. Disease development was not associated with the presence of plant parasitic nematodes; however fungi belonging to the genera Dactylonectria, Ilyonectria and Rhizoctonia were isolated more frequently from rootstock plants grown in soil initially obtained from the root zone of trees demonstrating disease symptoms in the field. Although certain of these fungi could not induce disease symptoms independently, they did produce compounds when grown in culture that caused damage to plant roots. It is possible that these compounds aid the fungus in colonizing plant roots and are important in the establishment of host root infection.

Technical Abstract: This study examined the etiology of typical replant disease symptom development, including growth reduction and leaf chlorosis, that was limited to specific foci within three multi-generation apple orchards that overall demonstrated optimal growth and good quality standard production. In bioassays conducted using soils collected from these orchards, growth of apple rootstock plantlets in soil obtained from the root zone of symptomatic trees was significantly lower than in soils collected from asymptomatic plants and from strip-rows with permanent vegetation. Cylindrocarpon-like species (represented by Dactylonectria torresensis and, to a lesser extent, by Ilyonectria robusta) and binucleate (bn) Rhizoctonia spp. (anastomosis groups -AG- A, K and G) were recovered from plants grown in soil collected from the rhizosphere of apple trees, whilst Fusarium spp. dominated in plantlets grown in soil from the orchard strip-rows. Although rootstock plantlets were highly colonized by Dactylonectria and Ilyonectria spp. in native soil, when re-inoculated, the same isolates showed low or null pathogenicity and the lowest ability to colonize apple roots as compared to Rhizoctonia sp. AG-A and Fusarium oxysporum. Phytotoxicity and lytic activity of sterile crude culture filtrates of Dactylonectria and Ilyonectria suggested that fungal metabolites released into the rhizosphere may favor their root infection. However, variable phytotoxicity of culture filtrates of bnRhizoctonia, increased the hypothesis that the pathogenic role of those soilborne fungi seems linked to a specific physiological state of plants or environmental factors responsible for switching the relationship between plant and fungi from neutral to pathogenic.