Evaluation of organic amendments from agro-industry waste for the control of verticillium wilt of olive

Biological control of plant diseases using soil amendments such as animal manure and composted materials can minimize organic waste and has been proposed as an effective strategy in crop protection. In this study, 35 organic amendments (OAs) and 16 compost mixtures were evaluated against Verticillium dahliae by assessing both the antagonistic effect on the mycelial growth of two representative isolates of V.dahliae and the effect on the reduction of microsclerotia viability of the pathogen in naturally infested soil. Eleven OAs and five compost mixtures showed a consistent inhibition effect in invitro sensitivity tests, with solid olive-oil waste compost one of the most effective. Therefore, a bioassay with olive plants was conducted to evaluate the suppressive effect against V.dahliae of these selected OAs and compost mixtures. Significant reduction in the severity of the symptoms of V.dahliae indicates the potential use of grape marc compost (100% disease severity reduction) and solid olive-oil waste, combined with other OAs. Microorganism mixtures and dairy waste OAs had a potential suppressive effect when they were combined with compost, showing a 73% and 63% disease severity reduction, respectively. A mixture of agro-industrial waste with other biological control agents is a promising strategy against verticillium wilt of olive. To the authors' knowledge, this is the first report on the effectiveness of compost extracts (compost teas) on the inhibition of natural microsclerotia of V.dahliae, and also on verticillium wilt suppression in olive with solid olive-oil waste.