Effect of Compost Particle Size on Suppression of Plant Diseases

Predictability of compost-induced suppression of soil-borne plant diseases is poor. Part of the variability in disease suppression could be due to the heterogeneity of a given compost. Therefore, the disease suppressive properties of different wet-sieved fractions of two composts against three soil-borne plant pathogens were studied. The ability of a green waste and a yard waste compost to suppress the soil-borne plant pathogens Fusarium oxysporum f.sp. lini (host: flax), Phytophthora cinnamomi (host: lupin), and Meloidogyne hapla (host: tomato) was determined. The following compost fractions were prepared: 2-4 mm (40% v/v with peat-based substrate), 1-2 mm (35 or 45% v/v; comparable number of particles and comparable organic matter content as the 2-4 mm fraction amendment respectively), and 1-2 mm (35% v/v) obtained from the 2-4 mm fraction by a series of dry-sieving, crushing, and wet-sieving. The 2-4 mm compost fraction of both composts showed significantly higher disease suppression for the three pathosystems, except for P. cinnamomi with one compost, in which there was no effect. For both composts, oxygen uptake rate showed a significant positive correlation with disease suppression of all pathogens except for P. cinnamomi. For the composts studied, substrate quality as expressed by oxygen uptake rate, seems to be of greater importance for disease suppression than compost particle size per se.