First Report of Alternaria alternata Causing Leaf Spot Diseases of Cotton in Türkiye.

Abstract

Cotton (Gossypium hirsutum L.) is a crucial crop for the textile industry. Sanliurfa is the major cotton production area in southeast Türkiye (USDA 2021). In the summers of 2021 and 2022, the mid to late season desiccation of leaves, stems and bolls as well as severe defoliation were observed in different cotton fields and cultivars in a 30-ha area centered around 36°51'15.7"N, 39°07'12.2"E. Approximately 45% of the plants were severely affected or completely desiccated. Initially, symptoms were circular, pinhead, necrotic lesions surrounded by a purple halo, scattered all around the infected leaves. As the disease progressed, it spread to bracts, petioles, stem and bolls. The necrotic lesions continued to expand, and formed irregular shapes by coalescing, occupied the whole tissue . Finally, severe infection resulted in premature defoliation. A secondary host (Prosopis farcta) of the inoculum of A. alternata is found in the field where the symptoms of pathogen was seen. The disease symptoms were similar to those described in cotton by Macauley (1982). Infected leaf samples with mycelia were collected (n=35) from 25 diseased plants. The samples derived from lesions on infected leaves were cut into 4- to 5- mm pieces, treated in 2% sodium hypochlorite, dipped in water, plated on potato dextrose agar (PDA) amended with 30 mg/L of streptomycin sulphate, and kept at 27°C in the dark. All the isolated fungal samples formed dark olive-green colonies. For morphological characterization, the colonies were examined under light microscopy at ×400 magnification. Conidia formed both cross or longitudinal septa, and were obclavate to elliptical and measured 16.2 to 30.5 µm long and 7.5 to 10.6 µm wide (n=14). The morphological characters were consistent with the genus Alternaria using a taxonomic key (Barnett and Hunter 1972). For pathogenicity test, healthy cotton plants were grown at 15 to 29°C in greenhouse. Conidial suspension (10 6 per mL) was sprayed on 30-d old plants (n=16) while control plants were sprayed with water. Then, the plants were covered with plastic bags (28x45 cm) at nights, opened in the morning. The disease symptoms were seen 20 days after artificial inoculation. However, the control group showed no symptoms. The pathogen was re-isolated from infected leaves. To confirm the result, the pathogenicity test was conducted twice. Then, DNA was extracted from conidia and mycelia using CTAB method with slight modification (Doyle and Doyle 1990). The nuclear rDNA internal transcribed spacer (ITS) and plasma membrane ATPase regions were (Lawrence et al. 2014; White et al. 1990) amplified, using primers ITS4/ITS5 and ATPDF1/ATPDR1, respectively. The PCR products were Sanger-sequenced and were uploaded to GenBank (accession nos. ITS: OP615138.1, ATPase: OP612816.1). The sequenced parts of the genes were 554 bp and 1025 bp, and showed 100% (ITS) and 97.99% (ATPase) nucleotide identity with the corresponding sequences (MT446176.1, ON442363.1) of the reference strains of A. alternata. To the best of our knowledge, this is the first report of A. alternata causing leaf blight of cotton in Türkiye. In several cotton-growing regions, A. alternata leaf spot epidemics have caused yield loss from 25% (Israel) to 37% (India) (Padaganur et al. 1989; Rotem et al. 1988). Although yield loss caused by the pathogen depends on environmental conditions, observations in Türkiye cotton fields suggest A. alternata has the potential to cause yield loss up to 30% under severe infection.