The Star fruit (Averrhoa carambola L.) was globally distributed, particularly in countries like China, India, Indonesia and was renowned for its abundant vitamin, mineral and antioxidant content (Reddy et al., 2023). In early February 2023, leaf spot symptoms were observed on A. carambola at 2 hectare model orchard, College farm, Agricultural College, Aswaraopet (17.252038 latitude, 81.109574 longitude) and Horticulture nurseries of Aswaraopet, Bhadradri Kothagudem (Dist), Telangana, India. In the surveyed fields (February-2023 to February 2024), the disease was prevalent year round, with varying incidence i.e., July to February (35% to 40% with a yield loss of 35%) and from March to June (20% to 30% with a yield loss of 20%). The disease was initiated as small reddish spots, which grew over 8-10 days to 1-5 mm spots with a necrotic center, dark reddish brown margin and a prominent yellow halo around them, within 17 to 20 days, all spots coalesced, resulting in leaf yellowing and defoliation (SF 1). To isolate pathogen, diseased leaf tissues (n=20) (5 × 5 mm) were surface sterilized (70% alcohol (30 s), 1% sodium hypochlorite (30 s) and sterile distilled water (3 × 60 s), inoculated to PDA media and incubated at 26 ± 2°C with 12 hours photoperiod for 72 hours (Chi et al. 2022). The emerging hyphae from the diseased tissues were sub cultured and incubated on PDA at 26 ± 2 °C. Initially, the fungal colonies appeared white, later transitioning to light brown and finally developed into olivaceous grey colour (SF 2A). The ascospores (n=20) were lemon shaped, pointed at both ends, with a length of 10.3 μm (9.1 to 12.1 μm) and width of 8.6 μm (7.2 to 10.2 μm) (SF 2B and 2C). For further identification of the pathogen, four fungal isolates were cultured in potato dextrose broth and genomic DNA was extracted using the CTAB method. Identification of the pathogen was confirmed through amplification and sequencing the Internal Transcribed Spacer region (ITS), Translation Elongation Factor 1-α (TEF1) and RNA polymerase subunit (RPB1) genes. The resulting sequences were deposited in Gen Bank with accession numbers (OR337915, OR337916, OR337893 and OR337892 for ITS, OR669280, OR669281, OR669282, and OR669283 for TEF, and PP092153, PP092154, PP092155 and PP092156 for RPB1). To study pathogenicity of fungus, five isolates of C. globosum were isolated from five A. carambola plants, grown in potato dextrose broth. Spore suspension of 1x106 spores/mL were prepared by adjusting with hemacytometer and were sprayed onto the leaves of healthy, surface sterilized (50% ethanol), 3 months old A. carambola plants and incubated in greenhouse (T: 26°C; RH: 80%). For each of the five isolates, the spore suspension from each individual isolate was inoculated into three plants and three control plants were maintained for each isolate. The experiment was replicated thrice. After a period of 10 to 12 days, symptoms appeared on the inoculated leaves in the form of reddish spots, similar to original symptoms (Alam et al. 2021) (SF 1H). The fungus isolated from the inoculated leaves was morphologically similarities to C. globosum. Notably, C. globosum, a widespread leaf spot pathogen in crops like A. hypogaea, C. sativa and Pomegranate (Chaffin et al. 2020). To our knowledge, this is the first report of A. carambola leaf spot caused by C. globosum in India and worldwide. The result will be helpful for providing a basis for further research on the control of the disease.
Keywords: A. carambola; C. globosum; RPB1; TEF; leaf spot.