FIRST REPORT OF CITRUS DEPRESSA AS A NEW NATURAL HOST OF XANTHOMONAS CITRI subsp CITRI PATHOTYPE A IN TAIWAN

Abstract

Citrus depressa is an indigenous citrus species in Taiwan. Not much is known about diseases occurring on C. depressa. A leaf spot disease of C. depressa was first observed in Chiayi City, Taiwan, in 2015 and 2016. Symptoms were brownish necrotic spots, surrounded by a yellowish halo. Leaf blight was not observed. Bacteria were isolated from surface-sterilized leaf spot samples, and two Xanthomonas-like yellow isolates, CJHCd003 and CJHCd004, were purified and stored at -80°C in 20% glycerol. Leaves of C. depressa, sweet orange (C. sinensis cv. Liucheng), mandarin (C. reticulata cv. Ponkan), grapefruit (C. paradisi), and Mexican lime (C. aurantiifolia) were infiltrated with bacterial suspensions (1×106 CFU/ml). C. depressa inoculated leaves exhibited necrotic spots surrounded with a yellowish halo ten days post inoculation similar to those observed in natural infections. The same bacterium could be consistently re-isolated from the inoculated leaves and confirmed by PCR with 4/7 primers (Hartung et al., 1993), fulfilling Koch’s postulate. The control leaves remained disease-free. Multilocus sequence analysis of seven housekeeping genes (16S rDNA, fusA, gap-1, gltA, gyrB, lacF, and lepA) (Almeida et al., 2010; Ngoc et al., 2010) fully identified the two isolates from C. depressa as pathotype A strains of X. citri pv. citri with 100% sequence identity (Ngoc et al., 2010; Zhang et al., 2015). Furthermore, these strains caused typical erumpent, callus-like tissue at inoculated sites on the other citrus leaves tested, indicating that these strains belong to pathotype A strains of X. citri subsp. citri. To our knowledge, this is the first report of C. depressa as a newly discovered natural host of X. citri subsp. citri pathotype A in Taiwan.