Occurrence of a citrus canker strain with limited host specificity in South Texas

Abstract

In October 2015, a Mexican lime exhibiting citrus canker symptoms was found by the USDA APHIS PPQ in a residential property in Rancho Viejo, Cameron County, Texas. Real-time PCR analysis detected the presence of Xanthomonas citri subsp. citri; USDA APHIS PPQ in Beltsville, MD confirmed this diagnosis. A delimiting survey was initiated and suspect leaf samples were collected and sent to the PPQ Beltsville lab for analysis. By October 2017, leaf samples from 197 trees were confirmed positive for citrus canker, all within a 5-mile radius; in addition, a further 59 symptomatic trees were found and all 256 infected trees (254 Mexican lime, one makrut lime, and one Ponderosa lemon) were removed. Survey data collected on stem lesions suggested the oldest lesions to be between 4 to 6 years old. A host-range study using 12 citrus varieties, including the major commercial varieties grown in Texas, were inoculated with crude leaf extracts from symptomatic leaves by leaf infiltration method. Mexican limes and alemow were the only citrus plants that developed definitive canker lesions. A combination of host range, serological, and molecular tests suggested that this isolate was different from the typical Asiatic strain and more similar to X. citri subsp. citri AW which was only previously reported from Florida on Mexican lime and alemow.