Quantitative soil indicators for identifying primary stressors in fruit tree decline: a case study on kiwifruit vine decline syndrome

A multivariate pathway was developed to identify the primary cause of the non-specific decline syndrome of kiwifruit vines (KVDS), which is causing severe economic losses in the main kiwifruit producing region of Italy. A series of physicochemical (21), microbial (6), biochemical (5) and entomological (1) variables were measured in soil samples from striped rows (bulk soil) and planted rows (root explored soil) in four kiwifruit orchards selected according to a gradient of KVDS. This orchard health classification was consistent with three plant stress-related enzyme activities (catalase, phenylalanine ammonia-lyase and proline content), which were subsequently adopted as KVDS markers. The first step of the study, based on the comparison of the root-explored soil with the bulk soil, showed that the main changes concerned the biological soil properties. The analysis was then focused on soil samples from planted rows. A set of eight variables of biotic origin was identified as those with the best linear correlation with plant stress-related enzyme activities, taken as KVDS markers. These variables gave the best performance in discriminating orchards affected by KVDS, supporting the hypothesis that this syndrome is primarily induced by a general alteration in soil biological properties along with a reduction in soil functions. The overall lower abundance of the most important functional microbial populations and the significantly lower nitrogen (N) content in the root zone compared to the bulk soil in the striped rows indicated a reduced ability of the root explored soil to support plant growth and counteract the establishment of root pathogens, which have been variably associated with KVDS by several studies. The soil variables found to discriminate KVDS could be used to detect soil changes associated with KVDS, thus supporting the implementation of cropping practices for early counteraction of this syndrome. The use of plant enzyme activities as markers for early monitoring of non-specific decline in kiwifruit is another interesting result of this study.