Performance evaluation of locally available composts to reduce replant disease in apple orchards of central Europe

A study on locally available composts in Austria, Germany, Italy and Switzerland was conducted to investigate the potential of these non-chemical based tools to increase soil health in orchards afflicted by apple replant disease (ARD). A total of 26 different composts (six to seven per country) were chosen for the study. Composts were divided into ten types according to the waste materials used as substrates in the composting process. Growth reduction is the main symptom associated with replant disease; therefore compost performance was evaluated based on the growth responses of apple rootstock plantlets in compost-amended soils in pots. These greenhouse trials were performed in one research station per country, located in an intensive apple-growing area, and soil was taken from an apple orchard affected by replanting disease. Plant growth response was measured as shoot elongation at the end of each greenhouse trial, and results showed increases in growth compared with the respective controls of 2-26% in 20 out of 26 composts evaluated. The heterogeneous nature of the composts most likely attributed to the finding that similar compost types originating from the different countries had varying effects on plant growth. Overall, no significant changes in chemical and biological properties were observed in amended soils as compared with non-amended controls. The high soil resilience was in part expected given the good organic matter content in the original soils (>2%). The bacterial communities of the composts were investigated using the COMPOCHIP microarray, and analyses showed that differences in plant growth response were mainly attributed to the microbial changes introduced into the soil through composts rather than to changes in soil chemical and biological parameters. However, the bacterial communities of composts appeared to be more influenced by geographical origin than by compost type. The results have shown that soil amendment with composts generated from locally produced wastes have the potential to reduce the effects of ARD, although the effects appear to be both compost and soil specific.