Systematic stepwise screening of new microbial antagonists for biological control of European canker

Abstract

Neonectria ditissima is the causal agent of European canker. This pathogen causes cankers on apple branches and the main stem, which may lead to the loss of the whole tree. Chemical control is the essential component in disease management and no suitable biocontrol products are commercially available. This study aimed at selecting potential microbial antagonists against N. ditissima through a systematic stepwise screening program for the development of a new biocontrol product. A total of 520 potential candidates were isolated from apple branches showing canker symptoms. Important characteristics for application of Microbial Biological Control Agents were tested per each candidate: spore production, spore survival during storage, cold tolerance, drought tolerance and UV-B resistance. Isolates able to germinate and grow at human body temperature were excluded. A total of 252 candidates fulfilled the stablished criteria. All 520 candidates belonged to 44 different taxonomic groups, being the most abundant Alternaria spp. (22.2 %), Aureobasidium pullulans (16.1 %), Cladosporium spp. (9.5 %) and Fusarium spp. (9.0 %). Information on possible pathogenicity and toxicity for humans, animals, plants and the environment and on patents in biocontrol use was collected per each identified species. A total of 158 candidates belonging to species that did not show potential risks or patent conflicts were assessed by their antagonistic behaviour against N. ditissima in bioassays in planta. Each candidate was inoculated in ‘Elstar’ apple branches inoculated with N. ditissima 24 h before. Inoculated branches were incubated at 17 °C, 16 h light per day and RH > 90 %. After four weeks, canker symptom expression was visually assessed. The capacity of the candidates to reduce colonisation of N. ditissima in the branches was evaluated by quantifying N. ditissima DNA concentration using qPCR. Four candidates of Clonostachys rosea showed antagonistic properties; after four weeks of treatment, no canker symptoms or small bark cracks were observed in the inoculated branches and N. ditissima DNA was reduced by 90–99 %. Following them, the branches inoculated with one candidate of Akanthomyces muscarius, A. pullulans and Cladosporium europaeum showed small bark cracks and small swollen bark and N. ditissima DNA was reduced by more than 90 %. The systematic stepwise screening approach was a powerful strategy to find new MBCAs against N. ditissima with antagonistic properties that also fulfilled major criteria with regards to commercial production and safety, as well as ecological needs.