Phytophthora nicotianae (P. parasitica), P. citrophthora, and P. syringae, are some of the most destructive Phytophthora species known to California citrus production, causing crop losses and in some instances, quarantine trade restrictions. The currently limited number of fungicides for the control of Phytophthora root rot and brown rot outbreaks has led to the over-use of available treatments and resulted in resistance development. In response, we sought to determine the suitability of four new fungicides (i.e. ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin) being proposed for registration on citrus in California as possible treatments to control Phytophthora diseases of citrus.
The results of this dissertation are presented in studies on 1) the in vitro sensitivity of selected life stages of several species of Phytophthora including P. nicotianae, P. citrophthora, P. syringae, and P. hibernalis collected from selected citrus production regions in California; 2) greenhouse and field studies on the efficacy of the newly identified fungicides for managing Phytophthora root rot of citrus; and 3) mobility of oxathiapiprolin and mefenoxam in citrus seedlings after root applications.
In vitro studies established ranges of EC50 values for 31, 62, 71, and 2 isolates for P. nicotianae, P. citrophthora, P. syringae, and P. hibernalis, respectively that can be used as baselines for future comparisons to determine shifts in isolate sensitivity or resistance once the fungicides are used extensively. Most of the isolates were within a narrow range of sensitivity and therefore resistance was not identified to any of the new fungicides. Oxathiapiprolin was the most efficacious fungicide tested in vitro, inhibiting all life stages evaluated including mycelial growth, sporangium and oospore formation, as well as zoospore cyst germination. Ethaboxam, fluopicolide, and mandipropamid were also shown to be highly effective in inhibiting mycelial growth.
In greenhouse and field studies with the new fungicides, efficacy was demonstrated by in reduced root rot incidence and recovery of P. nicotianae and P. citrophthora from collected soil and root samples. In field trials, positive correlations were also observed in growth trends of treated trees such as increased canopies, trunk diameters, and fruit production following fungicide applications. Fluopicolide and oxathiapiprolin treatments had the greatest effect in promoting the recovery of infected trees and increasing fruit yields.
Studies to determine the systemic nature of oxathiapiprolin in citrus plants were done using bioassay screenings and HPLC-MS/MS analysis of tissue extracts of plants treated with soil applications of oxathiapiprolin or mefenoxam. Oxathiapiprolin and mefenoxam were detected in leaf, stem, and root tissues indicating that both compounds can be translocated acropetally.
In summary, the information obtained from these studies will help in the registration of new fungicide treatments for managing Phytophthora diseases of citrus, provide four new modes of action with no cross resistance between the different fungicide resistance action committee (FRAC) groups, and ultimately provide a high level of efficacy in the management of Phytophthora root rot and brown rot of citrus in California with a minimal risk of resistance development through the rotational use of these fungicides.