Soil amendment by composted plant wastes reduces the Verticillium dahliae abundance and changes soil chemical properties in a bell pepper cropping system

Abstract

Organic amendments can be effective at controlling some soil-borne pathogens. This work entailed two parts; the first involved two field experiments that investigated the effects of four different composts derived from agricultural wastes on soil load of Verticillium dahliae in organic and conventional bell pepper (Capsicum annuum L.) systems. The composts were prepared using dairy and horse manure (DC), grape pomace (GC), olive pomace-dairy manure mix (ODC) or mixed crop residue (CRC). For each system, one 25 t ha−1 compost application was made in May 2018 before bell pepper planting. Soil samples were taken before application and four times after application, approximately every six weeks. Plant growth and yield were also monitored. The second part involved lab experiments that tested the effects of extracts of the above composts on the growth of V. dahliae on selective media in Petri dishes. All four compost treatments significantly suppressed soil pathogen populations in June, two weeks after the application of treatments, with plant-based amendments presenting greater effects than DC. These suppressive effects disappeared within eight weeks (for DC) or 14 weeks (for GC, ODC and CRC) after application. The effects of composts on soil chemical properties, V. dahliae abundance and bell pepper growth and yield parameters were generally similar in both organic and conventional fields. In the Petri dish assays, plant composts reduced V. dahliae growth by 40–60 %, especially during the first week after application.