Eggplant (Solanum melongena L.) is an important vegetable crop that has significant economic value in northern regions of China, especially in Liaoning Province. In April 2013, a leaf spot was discovered on the eggplant cultivar 706 in ten 1-ha commercial greenhouses in Huludao, Liaoning Province, with 30% of the eggplants infected, resulting in reduced eggplant yield and quality. By July 2013, disease incidence was 35%. Spots were found mainly on the leaves. At the early stage of infection, small, chlorotic spots appeared on leaves and gradually expanded into brown, irregular spots with a diameter of 1 to 7 mm. Dark green mold developed in the spots on both sides of the leaves at high humidity, and the spots led to leaf yellowing and defoliation. Conidiophores in the lesions were straight or slightly flexuous with 1 to 7 septa, brown and smooth, with typical swellings at the junction of septa, and 45 to 670 × 3.0 to 5.3 μm. Conidia were oval or obpyriform with a smooth surface, brown or dark brown, with 0 to 2 septa and 5.5 to 14.8 × 2.5 to 4.0 μm. The pathogen was consistent morphologically with Cladosporium oxysporum (1). To identify the pathogen, leaf pieces (3 to 5 mm2) taken from the edge of lesions so that each leaf section included both infected and healthy leaf tissue, were surface-disinfested in 75% ethanol for 30 s, then transferred to a 0.1% aqueous mercuric chloride solution for 30 to 60 s, and rinsed with sterilized water three times. The sections were cultured on potato dextrose agar (PDA) at 25°C in the dark for 7 days. Three pure cultures were obtained from single spores. The conidia on PDA were oval or obpyriform, and 5.4 to 14.7 × 2.4 to 4.2 μm with 0 to 1 septa, and were smaller than the conidia examined directly from infected eggplant leaves. Two isolates were grown on synthetic nutrient agar (SNA) in slide cultures. The conidiophores on SNA were straight or slightly flexuous with swellings at the junctions of septa. On the grounds of these morphological characteristics, the pathogen was identified as C. oxysporum (1,3). For DNA extraction, cultures were grown in potato dextrose broth and the internal transcribed spacer (ITS) region of ribosomal DNA (rNDA) was amplified using primers ITS1 and ITS4 (2). Sequence analysis showed that the ITS sequences of the two isolates were 99% identical to that of C. oxysporum (GenBank Accession No. EF029816). Two isolates were tested for pathogenicity on eggplant using 1 × 107 conidia/ml in sterilized water atomized onto each of six 7-week-old plants of the cultivar Xi'an Green Eggplant. Sterilized water was applied similarly to another six plants as the control treatment. The plants were incubated at 25°C with 85% relative humidity for 8 to 10 days. After 10 days, light brown, irregular spots were found on inoculated leaves, whereas no symptoms were observed on control plants. The pathogen was re-isolated from lesions on inoculated plants but not from control plants. The re-isolates were confirmed to be C. oxysporum based on morphological characteristics. The pathogenicity test was repeated and the same results obtained. Therefore, the pathogen causing leaf spot on eggplant in these greenhouses was identified as C. oxysporum. This is the first report of C. oxysporum causing leaf spot on greenhouse eggplant in China. C. oxysporum is a known pathogen of pepper and tomato. Additional studies are needed to provide management recommendations for this pathogen on Solanaceae crops.
References: (1) K. Bensch et al. Stud. Mycol. 67:1, 2010. (2) Q. Li and G. Wang. Microbiol. Res. 164:233, 2009. (3) W. T. H. Peregrine and K. B. Ahmad. Phytopathol. Pap. 27:1, 1982.
