The etiology of apple replant disease

The overall aim of this study was to investigate the etiology of ARD in South Africa in six orchard soils, using a multiphasic approach. This approach first involved determining the ARD status of soils by monitoring apple seedling growth responses in non-treated soil versus growth in pasteurized soil, as well as in 15% non-treated soil that was diluted into pasteurized soil.  Subsequently, the biological agents involved in causing ARD were investigated using (i) biocide applications, (ii) quantification of ARD ‘marker’ microbes (Pythium irregulare, Pythium sylvaticum, Pythium ultimum, Pythium vexans, Rhizoctonia solani AG-5 and the genera Cylindrocarpon and Phytophthora) using quantitative real-time PCR (qPCR), (iii) nematode analyses, (iv) isolations for fungi, oomycetes and actinomycetes, (v) pathogenicity testing of the microorganisms and (iv) pathogenicity testing of actinomycetes when co-inoculated with Pythium irregulare or Cylindrocarpon macrodidymum. The ARD severity of the soils varied from low to severe. Several lines of evidence suggested that actinomycetes are not involved in ARD. The causal biological agents were rather shown to be (i) oomycetes (P. irregulare, P. sylvaticum, P. ultimum,P. vexans and Phytophthora cactorum) that are important based upon their widespread occurrence, high virulence and the fact that metalaxyl applications improved seedling growth in four soils (ii) the genus Cylindrocarpon (four species) that was also widespread, pathogenic and had a synergistic interaction with P. irregulare and (iii) parasitic nematodes that in three orchards at either high or low population numbers could have acted synergistically with other ARD microbes. qPCR analyses of the ARD ‘marker’ microbes showed that R. solani AG-5 is absent from South African orchards, and that P. ultimum is widespread. DNA quantification of the ‘marker’ microbes using qPCR analyses could not be correlated with the severity of ARD.