First Report of Corynespora cassiicola Causing Target Spot on Cotton (Gossypium hirsutum) in South India.

Abstract

Target spot with unusual symptoms was observed in upland cotton at Regional Agricultural Research Station, Lam, Guntur, Andhra Pradesh, India in 2017. Symptoms appeared in about 60-65 days after planting with minute pinhead size light orange to brick red spots that gradually expanded as circular to oval or irregular concentric spots that finally developed as target spot with yellow halo. These symptoms were also observed during 2019-20 and 2020-21 diagnostic field visits at Krishna zone of Andhra Pradesh, India. Frequent showers had intensified disease severity and defoliation of different cultivars (L 1060, RCH 659 BGII, NDLH-1938 and Jaadoo BG II) examined. Leaf bits containing healthy and diseased portions of the specimens were surface sterilized with 0.1% HgCl2 for a minute and rinsed in three changes of distilled water and were placed on PDA at 27±1˚C for pathogen development. Obtained pathogen was purified using single spore isolation technique. Purified colonies exhibited grey colour on top of culture plate and greyish brown on bottom of culture plate with flat, aerial round colonies having fibrous texture with dense mycelium. Conidiophore were indeterminate septate with intermediate branching. Enteroblastic conidiogenous cells produced conidia subhyaline that were solitary or in chains, straight to curved, 83.43±31.73 x 7.61±1.43 µm with 7.27±2.19 pseudospeta. Morphological characteristics of the isolated fungus were consistent with original description of Corynespora cassiicola on cotton (Jones 1961; Fulmer et al., 2012; Conner et al., 2013) which confirmed it as Corynespora morphologically. Pathogenicity test was conducted on popular hybrid Jaadoo BG II using three isolates Lam (CGGL 19001), Koniki (CPIK 19009) and Venkatapuram (CKMB 19011) obtained during 2019-20 survey. After 50 days of sowing conidial suspension containing 1x104 spores per ml was sprayed on healthy plants maintained in pots under protected conditions. Immediately after spraying, the plants were covered with poly propylene covers for 48 h to prevent cross contamination and to ensure humidity for pathogen establishment. Healthy plants sprayed with sterile water served as control. After seven days of incubation, inoculated plants developed brick red spots initially which later developed into target spot. Absence of symptoms in control plants indicated pathogenic nature of the fungus. Pathogen was re-isolated from diseased tissue to confirm its pathogenicity according to Koch's postulates. The identity of the pathogen was further examined using ITS primers. The DNA was isolated from 10 days old culture using CTAB method. Internal transcribed spacer (ITS) region of three isolates were amplified using ITS1 and ITS4 primers and obtained amplicon (550 bp) were used for sequencing. The sequenced data was deposited in NCBI and accession numbers were obtained as MZ314930, MZ314928 and MZ314927. BLAST search in GeneBank revealed 99% homology of CGGL 19001 isolate with sequences of C. cassiicola (accession number of MN393240) while CPIK 19009 and CKMB 19011 isolates showed 100% homology with C. cassiicola in cotton having accession numbers of MN393238, MN228955, MN733135, MN288934 and MN393238. Based on symptoms, fungal morphology, pathogenicity test and ITS sequences obtained, the fungus was identified as C. cassiicola the pathogen of cotton causing target spot disease. Earlier, Sarma and Nayudu (1970) isolated Corynespora from different host plants including cotton but could not prove its pathogenicity on cotton. Later target spot of cotton associated with C. cassiicola had been reported from Central India (Salunkhe et al., 2019) and this is the first report of C. cassiicola in cotton from South India. Hence, this report of C. cassiicola on cotton is the first report from Souh India. Identification of target spot pathogen is very important as it is emerging as a dominant foliar pathogen for which management strategies are to be worked out.