Submitted to: Soil Biology and Biochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/20/2011
Publication Date: 8/11/2011
Citation: Tewoldemedhin, Y.T., Mazzola, M., Labuschangne, I., Mcleod, A. 2011. A multi-phasic approach reveals that apple replant disease is caused by multiple biological agents, with some agents acting synergistically. Soil Biology and Biochemistry. 43:1917-1927. 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. Control of soil borne pathogens and parasites is crucial to the successful establishment of new orchards on old orchard sites both in the United States and South Africa. To attain this goal, it is imperative that we possess a clear understanding of the particular pathogen species that contribute to the poor growth of trees newly established on replant sites. Soilborne oomycetes were found to have a dominant role in the development of apple replant disease in the primary apple growing regions of South Africa including Ceres and Grabouw regions of the Western Cape Province. These oomycetes included many different species of Pythium as well as Phytophthora cactorum. Multiple species of Cylindrocarpon were also associated with disease development but these fungi appeared to be of greatest limit to tree growth when acting in concert with Pythium irregulare. Replant orchards in general possessed low populations of plant parasitic nematodes, and these nematodes only occasionally appeared to contribute to disease development. Among these nematodes, those of the genus Pratylenchus were most significant, with multiple species being recovered from diseased trees. Possessing knowledge of the pathogen complex that incites apple replant disease will enable the formulation of non-fumigant strategies for control of this disease syndrome.
Technical Abstract: Apple replant disease (ARD) has been reported from all major fruit-growing regions of the world, and is often caused by a consortium of biological agents. The aim of this study was to investigate the etiology of ARD in South Africa in six orchard soils, using a multiphasic approach under glasshouse conditions. This approach involved determining the ARD status of the soils. Subsequently, potential for specific organisms to function as causal agents of ARD was investigated using (i) biocide applications, (ii) quantification of ARD ‘marker’ microbes using quantitative real-time PCR (qPCR), (iii) nematode analyses, (iv) isolation of actinomycetes and (v) pathogenicity testing of actinomycetes individually, and when co-inoculated with Pythium irregulare or Cylindrocarpon macrodidymum. Analyses showed that two, three and one of the soils had low, moderate and severe ARD, respectively. Several lines of evidence suggested that actinomycetes are not involved in ARD in South Africa. Multiple biological agents were determined to contribute to ARD including (i) oomycetes (Phytophthora and Pythium) that are important based upon their widespread occurrence, and the fact that metalaxyl application improved seedling growth in four soils (ii) the genus Cylindrocarpon that was also widespread, and demonstrated synergistic interaction with P. irregulare and (iii) occasionally parasitic nematodes, mainly Pratylenchus penetrans, P. scribneri and P. delattrei, since fenamiphos application improved seedling growth in two orchards. qPCR analyses showed that R. solani AG-5 is absent from South African orchards, and that P. ultimum is widespread, even though the latter species could not be detected in previous isolation studies. qPCR analyses showed that possible root pruning pathogens such as P. irregulare, P. sylvaticum and P. ultimum had much lower DNA concentrations in seedling roots than P. vexans and the genera Cylindrocarpon and Phytophthora.