Physiological responses of South African avocado rootstocks to Rosellinia necatrix and potential control strategies

Abstract

Rosellinia necatrix Berl. Ex Prill. is a fungal plant pathogen and the causal agent of White Root Rot (WRR), which has caused economic losses in forests and agricultural industries in temperate and subtropical areas. The pathogen was detected and identified in 2016 in South African avocado orchards, causing decline and death of avocado plants. At present, the detection of R. necatrix relies on human visual symptom detection, morphological identification, and real-time PCR confirmation assays. PCR is a molecular tool used for detection of R. necatrix. Still, its reliability is highly dependent on the selection of infected samples for assay, which can be scarce and challenging in newly infected plants. Therefore, early detection of WRR before stress symptoms are evident and rapid response are necessary to limit the spread of the pathogen. Lack of effective post-infection control strategies necessitates early detection of WRR to combat the further spread of the disease.

The study aimed to investigate physiological parameters, including leaf gas exchange and chlorophyll fluorescence, to determine the onset of stress in avocado plants following infection with R. necatrix. Also, to examine the effect of biological agents and chemical products in avocado plants infected with R. necatrix. WRR symptoms were assessed using human visual detection and a disease scoring system. The plants were monitored for signs of stress using the open path LI6400XT photosynthesis machine. PCR confirmation assays were conducted at the end of the trials to confirm the presence of R. necatrix.

WRR symptoms were evident at 35 days post-infection (dpi) in infected plants of all the rootstocks and confirmed death at 60 days, while control plants showed no signs of stress throughout the trial. The effect of WRR caused necrosis and root rot, which was related to plants’ failure to absorb water, therefore, they closed their stomata. Stomatal closure caused a reduction in CO2 availability of the leaf, which affected net CO2 assimilation (AN) and limited carbon fixation. This resulted in a reduction of dry mass production and allocation in infected plants which were significantly different to control plants