Apple nursery trees and irrigation water as potential external inoculum sources of apple replant disease in South Africa

Authors: MOEIN, S - University Of Stellenbosch; Mazzola, Mark; NTUSHELO, N - Agricultural Research Council Of South Africa; MCLEOD, A - University Of Stellenbosch

Submitted to: European Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/17/2018
Publication Date: 4/1/2019
Citation: Moein, S., Mazzola, M., Ntushelo, N.S., McLeod, A. 2019. Apple nursery trees and irrigation water as potential external inoculum sources of apple replant disease in South Africa. European Journal of Plant Pathology. 153(4):1131-1147. https://doi.org/10.1007/s10658-018-01631-9.
DOI: https://doi.org/10.1007/s10658-018-01631-9

Interpretive Summary: Replant disease of apple is a major limiting factor to the development of an economically viable orchard on sites previously planted to this crop. In general, it is believed that the disease is caused by a biological diverse pathogen complex that includes fungi, oomycetes and parasitic nematodes. Soil populations of these pathogens, which build up during the life history of the previous orchard, is believed to be the major source of inoculum for infection of trees planted on old orchard sites. However, other sources of pathogen inoculum may exist. This study was conducted to examine orchard irrigation water and nursery planting stock as potential sources of replant pathogen inoculum in South Africa. Nursery tree root systems were repeatedly shown to harbor various apple replant pathogens including oomycetes (e.g.Pythium spp.), fungi (Cylindrocarpon spp.) and the plant parasitic nematode Pratylenchus penetrans. Studies indicated that, depending upon survey year, up to 95% and 41% of trees from a nursery were infested with Pythium irregulare and P. ultimum, respectively. Nursery trees were consistently infested with Cylindrocarpon spp. Almost 1/4th of the nursery sourced apple trees surveyed possessed high root lesion nematode densities. Orchard irrigation water also proved to be a significant inoculum source of the pathogen Pythium irregulare. These studies demonstrate that external sources of replant pathogen inoculum are likely introduced into orchard soils through planting stock and irrigation water in South Africa. This is of particular significance due to the effort and cost associated with treating soils by fumigation prior to re-planting sites as a means to eliminate pathogen inoculum. Fumigated soils are not resilient to pathogen infestation as biological properties are diminished. Thus, this inadvertent introduction of pathogen inoculum has the potential to limit the growth and production of new trees on fumigated orchard replant sites.

Technical Abstract: Apple replant disease (ARD)is caused by soilborne agents that include select species of fungi (Rhizoctonia and ‘Cylindrocarpon’-like), oomycetes (Pythium, Phytopythium and Phytophthora) and nematodes (Pratylenchus). Orchard soils previously planted to apple or related species are a primary inoculum source of ARD pathogens. In the current study, nursery trees and irrigation water were investigated as external ARD inoculum sources in South Africa. Investigations conducted at five nurseries over 2-years revealed that roots of nursery trees were infested by several ARD agents. Pythium irregulare and Pythium ultimum were obtained from trees in five and two of the nurseries, respectively. Based on isolation studies conducted on individual trees in 2013, 47% and 4% of all surveyed trees contained P. irregulare and P. ultimum, respectively. In the 2014 season, real-time quantitative PCR analyses demonstrated that all of the nurseries and 95% of trees were infested with P. irregulare, whereas three nurseries and 41% of trees contained P. ultimum. Other oomycete pathogens that were detected in nursery tree roots were Pythium spp. complex B2A and Pythium sylvaticum. For all nurseries in both years, trees were consistently infected with ‘Cylindrocarpon’-like spp. Pratylenchus spp., which were only analysed in 2013, was present in all five nurseries and in 29% of the trees. Infestation levels were high, with 22% of trees having Pratylenchus root densities that exceeded 100 per 5g of roots. Pythium irregulare was the dominant oomycete pathogen detected in irrigation water samples (31% to 76% of the samples) obtained from 13 orchards over 2-years. The ARD pathogens P. ultimum, Phytopythium litorale and Pythium spp. complex B2A were rarely identified in irrigation water. Nursery trees and irrigation water are thus potential ARD inoculum sources that could limit post-plant tree growth.