Biological treatments enhancing the plant immune system of olive (Olea europaea) against Verticillium dahliae

Abstract: The use of biostimulants and biological control agents enhancing the plant immune system results in a potential alternative strategy against Verticillium wilt of Olive (Olea europaea; VWO). To this end, we evaluated a copper phosphite (CoPh), and two microorganisms (Bacillus amyloliquefaciens PAB-024 and Aureobasidium pullulans AP-08) for their effectiveness as resistance host inducers to 6-month-old olive plants (‘Picual’) against VWO. Foliar or irrigation applications were conducted by spraying 15 ml per plant or by irrigation with 350 ml per plant of the dilutions of each product (CoPh: 3 or 10 ml/l, respectively; PAB-024: 108 UFC/ml; AP08: 106 conidia/ml). Treatments were conducted weekly from two weeks before inoculation to 10 days after inoculation moment. Plants were inoculated by means a cornmeal-water-sand mixture (1:2:9; w:w:v) colonized by Verticillium dahliae. Additionally, treated and non-inoculated, non-treated and inoculated, and non-treated and non-inoculated plants were included for comparative purposes. Disease severity progress and shoot fresh weight were assessed. Parameters involved in plant resistance were monitored through determination and quantification of reactive oxygen species (ROS) response (H2O2), and evaluation of hormones by gene expression analysis. A. pullulans and CoPh were the most effective in disease reduction in planta by foliar or irrigation application, respectively. Plants treated with CoPh by irrigation showed significantly highest shoot fresh weight in comparison with that from other treatments. Non-effect on ROS response was observed for any treatments. Concerning hormones evaluation, our results showed that there is a presence of Salicylic acid on leaves; however, none of the observed enhancements was significantly different than the observed in the positive control. Regarding to the gene expression related to salicylic acid, only the WRKY gene has shown a strong enhancement in the treatment with B. amyloliquefaciens. On the other hand, strong accumulation of Jasmonic acid and Jasmonic acid-isoleucine in plants treated with A. pullulans was observed in all the tissues analyzed as well as in roots of plants treated with B. amyloliquefaciens and CoPh.