Diagnostics, Epidemiology, and Phytoremediation of Xylella fastidiosa in Pecan and Related Carya Species

Pecan bacterial leaf scorch (PBLS) is caused by the bacterium, Xylella fastidiosa, and was only previously reported on pecans in Louisiana. Through inter-state collaborative efforts with researchers, PBLS is now considered widespread across the southwestern U.S. pecan growing region. PBLS can lead to premature defoliation, which can cause significant yield losses for pecan growers. Diagnostic methods for determining the presence of X. fastidiosa in pecan had been primarily limited to ELISA and has been problematic due to the host-pathogen relationship. This work led to the optimization of immunological and molecular-based methodology for diagnosing X. fastidiosa within pecan tissues, crude sap, and endosperm. A novel qPCR probe and primer set were developed based on sequence data of X. fastidiosa amplified DNA from 22 geographically distinct species of Carya, including pecan. The new qPCR assay allows for the quantification of X. fastidiosa DNA within total gDNA isolated from pecan tissues. A 21% transmission rate of X. fastidiosa by grafting was demonstrated; thus, novel applications were developed for the phytoremediation of pecan graftwood, or scions, using carbon nanotubes and microwave radiation. A previously reported phytosanitary method using hot-water treatments (HWT) was also validated for its efficacy in the reduction of X. fastidiosa in pecan scions. Both microwave radiation and HWT were effective in reducing the concentration and eliminating X. fastidiosa from scions. Additional efforts were made to investigate the epidemiology from seed to seedling transmission of X. fastidiosa in pecan, the second host crop in which evidence has supported the likelihood of this mode of transmission. X. fastidiosa DNA was quantified in collected nuts and germinated seedlings. The highest presence of X. fastidiosa was found in high-quality nuts with densities ≥ 0.72 g/cm3. Additional tests of HWT on seed were also performed but found to be ineffective. The significance of this work is to provide diagnostic and phytosanitary approaches for extension agents and researchers in order to employ better IPM strategies for the management of PBLS, as well as a greater understanding of the epidemiology of X. fastidiosa in pecan nuts and seedlings