Citrus reticulata Blanco cv. Nian Ju, an important ornamental plant, is traditionally displayed during the Chinese Spring Festival because its golden fruits are a symbol of auspiciousness. In the spring of 2012, foliar blight was observed on 10 to 30% of the Nian Ju plants at four nurseries in Yangjiang, Guangdong Province, China. Initial symptoms appeared as brown to black foliar lesions, followed by expansion of spots into blight. Some young branches also had necrosis. During frequent rainfall and prolonged wet periods at 22°C to 30°C, white and dense mycelia and sporangia were observed on the infected seedlings. To isolate the causal organism, leaves and stems were cut into sections. Each section included some partial lesion and adjacent asymptomatic tissues. They were surface-disinfested in 1% sodium hypochlorite for 60 s, rinsed three times with sterile water, and placed on V8 juice agar (V8A) at 25°C. After 3 days, 10 isolates were obtained and purified by single-zoospore method. These isolates were identified to species level by sequencing the rRNA internal transcribed spacer (ITS) region. Four representative isolates had an identical ITS sequence (GenBank Accession No. KF750568), which had 99% homology with Phytophthora capsici sequences in GenBank. In addition, all recovered isolates were identical in morphological characteristics. They produced caducous, papillate, and ovoid to ellipsoid sporangia (Length × width = 46.2 ± 7.7 × 23.6 ± 11.3 μm), often with a tapered base. The average length of pedicels was 33.3 ± 4.5 μm. All isolates are A2 mating type. They produced gametangia when paired with an A1 tester of P. capsici isolated from pepper on V8A. Plerotic oospores were 25.3 ± 2.1 μm in diameter. Amphigynous antheridia were 13.6 ± 2.8 μm long and 11.2 ± 0.9 μm wide. Oogonia were 27.4 ± 3.2 μm in diameter. To determine the pathogenicity, three 3-year-old potted C. reticulata cv. Nian Ju plants were sprayed with 20 ml of zoospore suspension from one representative isolate at 105 per ml. Two control plants were sprayed with 20 ml distilled water. All plants were then maintained at 90% relative humidity at 25°C with a 12-h photoperiod. Symptoms similar to those observed in the nurseries developed on all inoculated plants but not on any control plants after 10 days. The pathogenicity test was repeated once and similar results were obtained. P. capsici was recovered from all inoculated plants and resultant isolates had identical morphology to that of the isolates used for inoculation. P. capsici has a relatively broad host range including pumpkins, cucumbers, peppers, beans, squashes, and spinach (1,2). To our knowledge, this is the first report of foliar blight of C. reticulata cv. Nian Ju caused by P. capsici. This study indicates that P. capsici is potentially an important pathogen of C. reticulata cv. Nian Ju plants and further investigations into its epidemiology and development of site-specific integrated management programs for this new disease are warranted.
References: (1) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. American Phytopathological Society, St. Paul, MN, 1996. (2) D. Tian and M. Babadoost. Plant Dis. 88:485. 2004.