In the spring of 2011, leaf and fruit spot symptoms were observed in three orchards planted with Citrus reticulata Blanco cv. Nian Ju in Longmen City, Guangdong Province. The incidence of fruit and leaf spot ranged from of 5 to 25% across all three orchards. Similar symptoms were not observed on the fruit of C. reticulata cv. Shiyue Ju planted at the same orchards. Lesions on the diseased leaves and fruit first emerged on the lower portions of the trees and were initially small, circular, and diffuse in distribution over the leaf surface. Within 2 weeks, the small circular spots on the leaves and fruit enlarged to brown sunken necrotic lesions ranging from 5 to 30 mm in diameter. To isolate the causal organism, symptomatic leaves and fruit peels (from different trees) adjacent to the lesions as well as the lesions themselves were surface-disinfested with 1% sodium hypochlorite for 1 min and rinsed three times with sterile water. The tissues were then plated on potato dextrose agar (PDA) under alternating light and dark conditions at 28°C for 4 days. Sixteen fungal isolates with nearly identical fungal colonies were recovered from all samples. After 7 days of incubation, isolates with grayish white to dark gray mycelium and salmon pink conidia masses were observed. The conidia were hyaline, smooth-walled, aseptate, one-celled, and falcate, with oil droplets in the cytoplasm. The conidia were 19.7 to 22.5 (mean = 21.5) × 2.8 to 3.9 (mean = 3.4) μm. The fungal colonies also produced acervuli, setae, and sclerotia in the culture. The cultural and morphological characteristics of these isolates were consistent with the description of Colletotrichum truncatum (2). The ACT, TUB2, CHS I, GAPDH, and ITS regions of four representative isolates (GenBank Accession Nos. KF591214, KF591213, KF591211, KF591212, and KF011205) were sequenced and identical to those of the type specimen of C. truncatum CBS 151.35. To determine pathogenicity, leaves of three 3-year-old potted plants and ten 8-month-old fruit of C. reticulata Blanco cv. Nian Ju were each wound-inoculated with 20 μl of a suspension of conidia (1 × 105 conidia/ml). Leaves of an additional two potted plants and 10 similarly-aged fruit were wound-inoculated with 20 μl of sterile water as control. The citrus plants and fruit were then maintained at 90% relative humidity at 28°C with a 12-h photoperiod. Symptoms identical to those observed in the orchards in Longmen City developed on all of the inoculated citrus leaves and fruit after 14 days. The controls remained healthy throughout this period. The inoculation experiment was performed a second time with identical results. Isolates of C. truncatum identical to that of the isolate used for inoculation were obtained from all the inoculated symptomatic citrus leaves and fruit. C. truncatum has a broad host range and is known to be pathogenic on several crops including Medicago sativa L., Phaseolus lunatus, and Arachis hypogea (1,2,3,4). To our knowledge, this is the first report of leaf and fruit rot of C. reticulata Blanco cv. Nian Ju caused by C. truncatum in China. The golden-yellow fruit of C. reticulata Blanco cv. Nian Ju are commonly displayed during the Spring Festival of the Chinese New Year, and future epidemics of fruit spot may affect sustainability of this important cultural practice.
References: (1) P. A. Backman et al. Plant Dis. 66:1032, 1982. (2) U. Damm et al. Fungal Divers. 39:45, 2009. (3) C. Eken and E. Demirci. Plant Dis. 84:100, 2000. (4) F. Huang et al. Fungal Divers. 61:61, 2013.