Evaluation of Chemicals for Control of Citrus Canker

Under Objective 1 (define rates and formulations of copper sprays for more effective control) one trial with Red grapefruit was conducted in Ft. Pierce, FL and one trial with Hamlin orange in Hardee County, each with 14 treatments of formulations, rates and combinations with other chemical treatments of interest. This season, trials had low to moderate disease epidemics as a result of a periodically wet spring, moderately wet summer and an absence of tropical storm events. Evaluations of fruit infection in these trials were made in November 2010 (grapefruit) and January 2011 (Hamlin). Copper formulations containing copper hydroxide or basic copper sulfate (metallic rates of 0.67 to 1.12 kg/ha), varied from low (Hamlin) to moderately effective (grapefruit) for canker control of fruit disease incidence. Copper pentahydrate, at a lower metallic copper rate/ha per application, provided equivalent control to film-forming copper formulations. Greater canker susceptibility of fruit occurred in later season and was likely because of more prolonged opening of stomates in cooler weather and enhanced bacterial entry, coincident with more numerous windblown rain events. Although Hamlin fruit disease incidence was higher, copper protection against early season fruit infection was effective for prevention of premature fruit drop. Under Objective 2 (establish the period of fruit susceptibility, residual activity and phytotoxicity of copper). In 2010, we compared the copper fruit residues from different copper-containing products up to 28 days after application to grapefruit. The copper product used and the number of days after application significantly affected the amount of copper residue. These effects were consistent whether the data were analyzed as copper/fruit or copper/surface area. The rate of copper residue decrease was different for the various products tested. Copper products forming films, Kocide 3000 (copper hydroxide), Cuprofix Ultra 40 Disperss (copper sulfate), and Badge X2 (copper hydroxide and oxycloride) decreased at a similar rate, whereas the non-film forming Magna-Bon CS2005 (copper sulfate pentahydrate) decreased at a higher rate and left about 75% less fruit residue. This was expected as Magna-Bon is applied at approximately 15% of the metallic contained in film-forming coppers. Since Magna-Bon performed as well as film-forming copper products for control of canker on grapefruit the last two seasons, we have hypothesized that the Magna-Bon copper may be locally systemic in fruit rind tissues. This coming season, we will attempt to assay the activity of copper in the rind against Xcc to determine whether the copper is located internally as well as externally. Under Objective 3 ( evaluate the use of streptomycin [Firewall]) As in the past three seasons, applications of Firewall in July and August, were effective for canker control on grapefruit either in combination with a reduced rate of copper or when substituted for copper in the spray program. Based on Firewall’s efficacy and ability to lower the risk of copper phytotoxicity, an application has been submitted by FFVA and FDACS to gain EPA Section 18 emergency registration for use of Firewall against canker on fresh grapefruit. Under Objective 4 (To define risk for development of bacterial resistance to copper and streptomycin in FL citrus groves) a number of factors favorable for the development of copper resistance in Xcc were identified. Findings are being prepared for publication. Under Objective 5 (rapid transfer improved canker management technology to the Florida citrus industry), 2011 canker management recommendations have been submitted for publication in the Florida Citrus Pest Management Guide and to Citrus Industry Magazine. Oral presentations have been delivered at the Florida Citrus Show and at a multi-county extension meeting. Results of the Hamlin trials will be presented at the 2011 FSHS annual meeting.