A rapid in vitro phenotypic assay of walnut shoots for prescreening resistance of Phytophthora pini

Authors: ZAINI, PAULO - University Of California, Davis; LEE, STEVEN - University Of California, Davis; LESLIE, CHARLES - University Of California, Davis; WALAWAGE, S - University Of California, Davis; Jiang, Cai-Zhong; Browne, Greg; DANDEKAR, ABHAYA - University Of California, Davis; Kasuga, Takao

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/21/2021
Publication Date: 7/28/2021
Citation: Zaini, P.A., Lee, S.H., Leslie, C.A., Walawage, S.L., Jiang, C., Browne, G.T., Dandekar, A.M., Kasuga, T. 2021. A rapid in vitro phenotypic assay of walnut shoots for prescreening resistance of Phytophthora pini. Plant Health Progress. 22(3):235-239. https://doi.org/10.1094/PHP-05-21-0078-FI.
DOI: https://doi.org/10.1094/PHP-05-21-0078-FI

Interpretive Summary: Phytophthora pathogens cause crown and root rot on fruit and nut trees and reduce the yield and quality of crops. Breeding programs are interested in improving tolerance to these pathogens; however, current screening methods are time-consuming, which can span years. Here we present an in vitro assay, which completes within a week for the evaluation of disease symptoms. We showed that in vitro procedure can serve as a good proxy and provides quicker disease tolerance assessment. This quick assay can potentially be extended to other pathosystems for which tissue cultures are available and may significantly accelerate the screening of cultivars for disease tolerance.

Technical Abstract: Phytophthora pathogens causing crown and root rot on fruit and nut trees reduce the yield and quality of crops. Breeding programs are interested in genotypes with improved tolerance to these and other pathogens. Current screening methods are, however, time-consuming, which often span years. Here we present an in vitro assay, that completes within a week from inoculation of tissue culture with a pathogen to evaluate disease phenotypes. Two cultivars, ‘Chandler’ and ‘RX1’, which differ in crown rot susceptibility, were tested in vitro. The observed disease phenotypes corresponded well to those observed under field conditions. Our results thus suggest this in vitro procedure can serve as a good proxy and can provide quicker disease tolerance evaluation. In addition to P. pini (formerly included in P. citricola complex), one of the most serious pathogens causing crown rot in walnuts in California, we also tested a species P. capsici that is non-pathogenic to walnuts, and verified that P. capsici could not infect walnut tissue cultures regardless of their susceptibility to P. citricola. This quick assay can be extended to other pathosystems involving fruit and nut trees for which tissue cultures are available, significantly accelerating the screening of cultivars and lines with increased disease tolerance.