A Rapid In Vitro Phenotypic Assay of Walnut Shoots for Prescreening Resistance toPhytophthora pini

Abstract

Phytophthora species cause crown, root, and aerial canker diseases in diverse horticultural crops, resulting in a loss of yield, loss of quality, and plant death. Breeding programs are interested in developing genotypes resistant to these and other pathogens, but current screening methods for tree crops are time-consuming, potentially spanning many years. Here we present a rapid in vitro assay in which walnut shoots obtained from tissue culture can be challenged with Phytophthora zoospores and monitored for symptom development within a week. As a proof of concept, two cultivars, scion variety ‘Chandler’ and clonal rootstock ‘RX1’, with known different degrees of crown rot susceptibility, were inoculated in vitro with P. pini (formerly included in P. citricola complex), and results of the assay reproducibly matched the differences in disease susceptibility generally observed for these two walnut selections under greenhouse and orchard conditions. In addition, when the in vitro assay was used for inoculations with P. capsici, which is not pathogenic on walnut, no disease development was observable, indicating that the assay can discriminate between species of Phytophthora that are aggressive on walnut and those not known as walnut pathogens. Our results suggest that this in vitro shoot inoculation procedure may provide rapid assessments useful for prescreening resistance to Phytophthora in walnuts. Further testing of the assay is justified to determine whether it can resolve more subtle differences in resistance and whether it can be useful with other perennial hosts of Phytophthora.