The use of biological liquid fertilizers against oak decline associated with Phytophthora spp.

Abstract

Worldwide, multiple ecosystems are currently affected by global change because of important anthropogenic disturbances which have quickly increased levels of abiotic (e.g. climate) and biotic stress (e.g. diseases, pests, herbivory). In particular, oak-dominated systems have experienced a general decline as a consequence of habitat destruction, mismanagement and extreme climate events along with the aggressive root pathogen Phytophthora spp. Here, we investigated the effect of soil improvement through soil biological fertilizers on oak defense against tree decline symptoms associated with Phytophthora for two ontogenetic stages of oak trees: adults and recruits. We examined oak survival and crown defoliation status on 60 pairs of adult trees (a treated tree with biological fertilizers vs. non-treated tree) as well as the survival, growth and herbivory on 30 quartets of treated and non-treated recruits, with and without protection against large herbivores. We used two different liquid fertilizers: OptiPlus (a biological fertilizer with organic N, P, S, Ca, Mg, trace elements (Fe, Mn, B, Zn) and humic fulvic acids) and OptiFer (a biological trace element fertilizer with only Fe, Mn and Mg). Treated oak trees with fertilizers showed lower oak defoliation (0.33-fold difference) than non-treated trees. In addition, trees treated with OptiPlus liquid showed a significant reduction of crown defoliation compared to those trees that received OptiFer treatment (0.68-fold difference). Interestingly, this crown amelioration was more effective in steep slopes, revealing that OptiPlus may buffer the stress generated by shallower and poorer soils. Overall, fertilized plants (particularly OptiPlus) were more attractive to herbivores, probably as a result of the greater content in deficient minerals, increasing their palatability. This study has slightly visualized the importance of soil improvement against oak decline through biological fertilizers and the need to further explore different times and techniques of application. Managers should be cautious with the use of fertilizers as they may increase the browsing impact of livestock and wild herbivores on tree regeneration. It is therefore imperative to improve the resilience of oak trees against oak decline while minimizing the herbivory impact on oak recruits.