Characterisation of apple replant disease under South African conditions and potential biological management strategies

Abstract

A possible biological origin of apple replant disease (ARD) in South Africa was investigated. Elements responsible for stunted growth and root discolouration could not be reduced to a level having no negative effect on apple seedlings by dilution of the original ARD field soils, with increased proportions of fumigated soil, from 100% to 25%. Pythium, Fusarium and Cylindrocarpon spp. were consistently isolated from all replant soils forming part of this study and Rhizoctonia spp. to a lesser degree, indicating that these fungi may play a role in ARD etiology in South Africa. Further research is needed, since identifications were not made to species level and no pathogenicity tests were performed. Nematodes implicated in ARD were inconsistently associated with the ARD soils tested, indicating only a secondary role. Furthermore, studies were conducted to investigate more environmentally friendly disease control alternatives to replace methyl bromide in ARD management. Applications of compost and compost extracts were identified as promising, practical tools for managing ARD, especially under the marginal production conditions of South African apple producing regions. In pot trials, the application of compost, as well as sterilised and unsterilised compost extracts, significantly increased growth of apple seedlings in ARD soils. Although results varied with addition of a slow release fertiliser, compost and compost extracts still significantly increased seedling growth parameters for several of the ARD soils tested, suggesting that they can ameliorate the effects of ARD, in addition to supplying nutrients. Three field trials were conducted in commercial orchards to verify pot trial results and to compare the impact of organic amendments on ARD severity with the standard fumigation control methods. Compost and mulch consistently increased shoot growth to the same or greater extent as the standard chemical treatments, for the 2-year trial period.