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

Abstract

Apple replant disease (ARD) is a biological phenomenon caused by soilborne agents that include selected species of fungi (Rhizoctonia and ‘Cylindrocarpon’-like), oomycetes (Pythium, Phytopythium and Phytophthora) and nematodes (Pratylenchus). Old 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 potential external ARD inoculum sources in South Africa. Investigations conducted at five nurseries over two years revealed that roots of nursery trees were infested by several ARD agents. In the 2013 sampling year, Pythium irregulare and Pythium ultimum were obtained from trees in five and two of the nurseries, respectively, using isolation studies. 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 (qPCR) analyses of root tissue from individual trees demonstrated that all of the nurseries and 95% of trees contained P. irregulare, whereas three nurseries and 41% of trees contained P. utlimum. The only other oomycete pathogens that were detected in nursery tree roots were Pythium spp. complex B2A and Pythium sylvaticum and each only occurred in one nursery in one of the sampling years. 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 5 g 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 two years on a monthly basis. The ARD pathogens P. ultimum, Phytopythium litorale and Pythium spp. complex B2A were rarely identified in irrigation water, along with five other non-pathogenic Pythium and Phytopythium species. The association of ARD causative agents with nursery trees and irrigation water suggests that these could function as potential ARD inoculum sources that might limit post-plant tree growth.