Non-fumigant management of apple replant disease

Abstract

Field and greenhouse trials examining the response of young apple trees to amendment of apple replant disease (ARD) soils using monoammonium phosphate (MAP), organic matter or replacement soil, are reported. In one greenhouse trial, trees allowed to grow for 4 weeks in a soil-less potting mix before transplanting into ARD soil, pasteurised or amended with various levels of added organic matter, produced markedly longer extension growth than trees planted directly into ARD soil. For the transplanted trees, organic matter amendments generally provided no significant benefit compared with trees transplanted to ARD soil. In a similar greenhouse trial, MAP at 1 and 2 g/l of ARD soil resulted in a significant improvement in first year radial growth, but higher rates of MAP were toxic, producing excessive soil salt levels. MAP also increased growth significantly in steam pasteurised ARD soil, in spite of supplementary applications of nitrogen fertilizer to all treatments. Although overall growth was stronger in the steam pasteurised soil there was no interaction between ARD status of the soil and MAP treatments. In a field trial, MAP, organic matter amendment (with various fertilizer additions) and replacement soil were compared with an untreated control in soil which had been planted to apples for 15 years prior to replanting with this trial. Organic matter incorporated into the top 20 cm of soil was ineffective, but MAP incorporated into the top 20 cm of soil or replacement soil in the planting hole both resulted in near two-fold increases in extension and radial growth measured after two seasons. Leaf analysis for macro- and micronutrients showed most elements in all treatments to be marginal to adequate by accepted standards and there were was no evidence to indicate that the observed growth responses were associated with any nutritional effect.