The effect of organic amendments on the severity of apple replant diseases in subsequent apple orchard replantings

Long-term applied organic amendments have the potential to reduce apple replant disease (ARD).

Semi-selective chemicals applied at planting can serve as a safeguard against nursery trees contaminated with ARD causative agents. The objectives were to (i) investigate the potential of long-term organic amendments (compost and mulch or only mulch) for reducing ARD in subsequent plantings and (ii) optimise a chemical mix to use at planting.

The ARD suppressiveness of organic amendments applied for 5-years or more and an unamended control was investigated in five orchard soils using an apple seedling bioassay. ARD marker pathogens were quantified in orchard tree roots and bioassay seedling roots. Several soil quality parameters (physical, biological and chemical) were investigated. Microbes in the rhizosphere soil of one of the orchards were investigated using next generation sequencing. Various semi-selective chemical mixes containing nematicides (fluensulfone, fluopyram, fenamiphos or iprodione) and oomyceticides (mefenoxam and phosphonates combined), were evaluated using an apple seedling bioassay.

Only the mulch combined with compost resulted in ARD suppression, but only in one of the five soils. For one of the soils, organic amendments increased ARD severity. This was like due to a significant stimulation of Phytopythium vexans and Cylindrocarpon-like species in roots. Microbiome analyses could not identify microbes associated with disease suppression.

Soil health indicators associated with organic amendment practices could only be identified in the three silt loam soils, but not the two sandy soils.

The semi-selective chemical mix glasshouse trials yielded variable results, but altogether suggested that fluopyram was the best replacement for fenamiphos.

Long-term application of organic amendments will not be an option for managing ARD in subsequent crops.

Soil enzymes were the best indicators of organic amendment applications, but this was soil type dependent.

Fluopyram may have potential to replace fenamiphos in a semi-selective mix.

• Organic amendments applied long-term cannot be used for managing ARD.
• Identifying universal soil quality indicators associated with organic amendment applications are difficult to identify since it is soil type dependent.