First Report of Anthracnose Crown Rot of Strawberry Caused by Colletotrichum siamense in Rajshahi District of Bangladesh

Abstract

Strawberry (Fragaria × ananassa) was introduced to Bangladesh in 1996 and has gained remarkable acceptance. However, with the expansion of acreage, anthracnose crown rot (ACR) is posing a challenge to strawberry growers. In March 2016, typical ACR symptoms were observed on cultivar Strawberry Festival in a farmer’s field in Rajshahi District. Symptoms were evident on approximately 10% of plants and included flagging of young leaves, mimicking lack of water, and wilting of the entire plant in daytime heat. A lengthwise cut through the crown of diseased plants showed characteristic shades of red and white marbling similar to that described by Weir et al. (2012). Crowns of seven symptomatic strawberry plants were obtained after excising foliage and roots and washed under running water for 1 h. Crowns were then surface disinfested with 70% ethanol followed by 1% NaClO for 2 and 3 min, respectively. Small pieces (2 × 2 mm) of the internal infected marbled red tissues were cut with a sterile scalpel, placed on corn meal agar, and incubated for 7 days at 25°C. Conidia grown on tissue pieces and hyphal mats from the tissues were streaked on PDA plates to obtain single conidial colonies. Two of the single-conidial isolates (BRSP08 and BRSP09) that had white margins with circular, dull orange centers were selected to prove Koch’s postulates. The average mycelial growth rate was 9.8 mm/day at 25°C on PDA with cottony aerial growth and pale yellowish to pinkish color. Conidia were hyaline, cylindrical, with round ends, and measuring 14.90 to 18.19 × 4.05 to 6.20 μm (average ± SD, 16.22 ± 1.20 × 5.90 ± 0.84 μm) (n = 30). Appressoria were brown, globose to cylindrical, lobbed, and measuring 7.50 to 12.70 × 6.08 to 8.4 μm (10.72 ± 1.30 × 7.20 ± 0.80 μm) (n = 20). The morphological characteristics of these isolates are consistent with some Colletotrichum species within the C. gloeosporioides complex including C. siamense (Afanador-Kafuri et al. 2003; Weir et al. 2012). BRSP09 was used for molecular identification. Internal transcribed spacer regions and intervening 5.8S nrRNA gene (ITS) and genes encoding chitin synthase (CHS-1), actin (ACT), β-tubulin (TUB2), calmodulin (CAL), and the Apn2–Mat1–2 intergenic spacer and partial mating type (Mat1–2) gene (ApMat) of the selected isolate were amplified using primers ITS1F/ITS4, Bt2a/Bt-2b, CHS-79F/CHS-354R, ACT512F/ACT783R, CL1C/CL2C, and AM-F/AM-R, respectively (Silva et al. 2012; Weir et al. 2012). Sequencing of the purified polymerase chain reaction products was performed by Macrogen (Korea), and sequences were deposited in GenBank (LC420017 to LC420022). Neighbor-joining and maximum likelihood phylogenetic trees were inferred using MEGA 6.0 (Tamura et al. 2013) based on the multilocus alignment (ITS, TUB2, ACT, CHS-1, CAL, and ApMat) of the present isolate and related Colletotrichum spp. sequences previously deposited in GenBank. BLAST search and phylogenetic analysis revealed that BRSP09 is C. siamense (Mills et al. 1992). To confirm the pathogenicity of the fungus, ten 3-month-old tissue-cultured Strawberry Festival plants were inoculated by dipping the roots and crowns into either 300 ml of conidial suspension (8 × 10⁶ conidia/ml) of the isolates BRSP08 and BRSP09 or sterile distilled water (control) for 30 min. The inoculated plants were transplanted into plastic pots with artificial peat-based soil mix and maintained in the greenhouse with 28/20°C day/night temperature. The experiment was conducted twice. After 4 weeks, all plants inoculated with BRSP08 or BRSP09 were stunted and developed typical wilt symptoms similar to those observed in the field, whereas the control plants remained healthy. C. siamense was reisolated from crown tissues with typical ACR symptoms and identified using morphological and molecular methods mentioned above, fulfilling Koch’s postulates. This is the first report of ACR of strawberry caused by C. siamense and the members of C. siamense complex in Rajshahi District.