Alternatives for the management of apple replant disease

The main objectives of the study were to investigate alternatives to soil fumigation for managing apple replant disease (ARD) and to assess the efficacy of a fluopyram-based semi selective mix. ARD is caused by a complex of biological agents that reduce tree growth on replanted apple soils. Organic amendments have potential for managing ARD, metam sodium soil fumigation for reducing soil fumigation costs and semi-selective chemicals for reducing the risk posed by nursery trees infected with root rot pathogens.

Four orchard trails were established from 2017 to 2018 to evaluate various treatments by measuring tree growth and yield along with the quantification of ARD causative agents in tree roots. Two glasshouse trials were conducted to investigate the efficacy of fluopyram by determining the relative percentage increase in seedling growth and ARD causative agents.

Standard soil fumigation only significantly improved tree growth in two of the four orchard trials, i.e. two soils contained ARD. In the third orchard soil ARD symptoms were inconsistent. This was supported by glasshouse pot trials. A compost + mulch + oomyceticides (mefenoxam

+ phosphonate) treatment was able to improve tree growth and yield to a similar level than soil fumigation in the fourth orchard soil that did contain ARD. A semiselective mix (mefenoxam, fluopyram and phosphonates) was ineffective for managing ARD as well as metam sodium+semiselectives. It was unclear from orchard and glasshouse results whether fluopyram has potential for replacing fenamiphos in the semi selective mix.

The fact that only one of the four orchards clearly had ARD, limited inferences from being made as to whether compost + mulch + oomyceticides will be able to provide consistent control of ARD in all ARD orchards. In future, it will be important to test orchard trial sites before trial establishment to ensure that ARD is present. The fluopyram dosages utilized in glasshouse and orchard trials were not optimal and the occurrence of Pratylenchus spp. in experiments were inconsistent, thus requiring further investigations.

Organic amendments (mulch and compost) combined with oomyceticides may have potential for managing ARD, but the treatment will have to be evaluated in more orchard trials. It is well known that the challenge in finding alternatives to soil fumigation for managing ARD is due to the variability in ARD severity and the causative agents across orchard sites. This often results in a management strategy not being effective in all ARD orchards.