Occurrence of Colletotrichum truncatum Causing Foliar Spot on Sesame (Sesamum indicum) in Mexico.

Abstract

Sesame (Sesamum indicum L.) is an oilseed crop that present agronomic advantages and nutritional contributions in regions where water and soil fertility are limiting. In September 2020 and October 2022, anthracnose symptoms were observed on sesame fields in Mocorito (25°29'04"N;107°55'03"W) and Guasave (25°45'40"N;108°48'44"W), Sinaloa, Mexico. The disease incidence was estimated at up to 35 % (10 has) in five fields. Twenty samples were collected with symptoms on the leaves. On leaves, lesions were irregular and necrotic. Colletotrichum-like colonies were consistently isolated on PDA medium and five monoconidial isolates were obtained. One isolate was selected as a representative for morphological characterization, multilocus phylogenetic analysis, and pathogenicity tests. The isolate was deposited in the Culture Collection of Phytopathogenic Fungi of the Biotic Product Development Center at the National Polytechnic Institute under the accession number IPN 13.0101. On PDA, colonies were flat with an entire margin, initially white, then dark gray with black acervuli and setae. The growth rate was 9.3 mm/day. Conidia (n=100) on PDA were hyaloamerosporae, 17.5- 22.7 × 3.6-4.5 μm, smooth-walled, falcated and pointed at both ends, with granular content. Acervuli showed setae acicular (2-3 septate setae) tapered to the apex. The mycelial appressoria were brown, obclavate and irregular. Morphological features matched those of the Colletotrichum truncatum species complex (Damm et al. 2009). For molecular identification, total DNA was extracted, and the internal transcribed spacer (ITS) region (White et al. 1990), and partial sequences of actin (ACT), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes were amplified by PCR (Weir et al. 2012) and sequenced. The sequences were deposited in GenBank under accession nos. OQ214919 (ITS), OQ230773 (ACT), and OQ230774 (GAPDH). BLASTn searches in GenBank showed 100%, 100%, and 100% identity to MN842788 (ITS), MG198003 (ACT), and MF682518 (GAPDH) of C. truncatum, respectively. A phylogenetic tree based on the Maximum Likelihood method and Bayesian Inference including published ITS, ACT, and GAPDH sequence data for C. truncatum species complex was generated (Talhinhas and Baroncelli 2021). In the phylogenetic tree, the isolate IPN 13.0101 was placed in the same clade of C. truncatum. Pathogenicity of the isolate IPN 13.0101 was verified on 15 sesame seedlings leaves (Dormilon variety) (15-day-old) disinfected with sodium hypochlorite and sterile water. Each leave was inoculated with 200 µL of a conidial suspension (1 × 106 spores/mL). Five plants non inoculated served as controls. All plants were kept in a moist chamber for two days, and subsequently transferred to a shade house where the temperature ranged from 25 to 30°C. All inoculated leaves developed irregular and necrotic lesions ten days after inoculation, whereas no symptoms were observed on the control leaves. The fungus was consistently re-isolated from the diseased leaves, fulfilling Koch´s postulates. The experiment was conducted twice with similar results. Colletotrichum spp. has been previously reported (Farr and Rossman, 2023) to cause sesame anthracnose in Mexico (Alvarez, 1976), Thailand (Giatgong, 1980) and Cuba (Arnold, 1986), but this is the first report of C. truncatum causing sesame anthracnose in Mexico. This disease is a recurrent problem in sesame fields in Sinaloa, therefore further studies are required to understand its impact.