Expansion of the specific replant disease biotest

The reliability of the apple replant disease (ARD) biotest was improved by refining the methodology and by revising the current sampling protocol for ARD soil samples.  From field trials established at ARD tested sites, it was observed that a good correlation exists between the results of the biotest and those of field trials, and that the biotest successfully predicted ARD in 75% of cases investigated.  Furthermore, it seems that not only apple planted on apple is affected negatively, but also apple planted on pear and vice versa, indicating that the disease is affecting pome fruit in general, although the effect may be less severe when not replanting the same fruit kind.

Furthermore, it was clear that 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 soils from 100 to 25%, indicating that ARD in South Africa is primarily of a biological nature.  Furthermore, Pythium, Fusarium and Cylindrocarpon spp. were consistently isolated from all replant soils forming part of this study, indicating that these fungi may have a role in ARD etiology in South Africa.  However, further research on the soil microbial profile associated with apple roots is essential.  Nematodes implicated in ARD were inconsistently associated with the ARD soils tested.

Results from this study indicate that the application of organic amendments (compost and straw mulch) could possibly substitute for methyl bromide (MeBr) fumigation in replanted orchards. However, compost quality standards need to be implemented and different types of compost need to be compared in specific soil environments.  As a short term alternative Telopic has been identified as a possible chemical to replace MeBr in controlling ARD.