Colletotrichum brevisporum and C. musicola Causing Leaf Anthracnose of Taro (Colocasia esculenta) in Mexico

Abstract

Taro (Colocasia esculenta var. antiquorum), family Araceae, is an important tropical crop cultivated throughout the world for its edible corms. During the summer of 2017, irregular brown lesions, typical of anthracnose were observed on approx. 40% of taro plants in a commercial orchard located in San Juan Bautista Tuxtepec, Oaxaca, Mexico. Small pieces of the lesions of five symptomatic leaves were surface disinfected with 1% NaOCl for 2 min followed by rinsing with sterile distilled water and plated on potato dextrose agar plates which were then incubated at 25°C for 5 days in darkness. Colletotrichum-like colonies were consistently isolated and 10 monoconidial isolates were obtained. Two isolates were selected as representatives for morphological characterization, multilocus phylogenetic analysis, and pathogenicity tests. The isolates were designated as UACH289 and UACH290 and were deposited in the Culture Collection of Phytopathogenic Fungi at the Chapingo Autonomous University. Conidia (n = 100) of isolate UACH289 were cylindrical, hyaline, aseptate, 15.1 to 18.4 × 4.4 to 5.2 μm, with the apex rounded and the base, rounded to truncate; appressoria (n = 20) were single, dark brown, and with undulate or lobate margin. Whereas conidia (n = 100) of isolate UACH290 were cylindrical to ellipsoidal, hyaline, aseptate, 12.3 to 17.0 × 3.8 to 5.0 μm; appressoria (n = 20) were irregular, bullet-shaped to elliptical, dark brown, with undulate or lobate margin. For molecular identification, the internal transcribed spacer (ITS) region (White et al. 1990), and fragments of actin (ACT), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and β-tubulin (TUB2) genes were amplified by PCR (Weir et al. 2012), and sequenced. A phylogenetic analysis based on Bayesian inference and including published ITS, ACT, GAPDH, and TUB2 data for Colletotrichum species was performed. After phylogenetic analysis, isolates UACH289 and UACH290 clustered with C. brevisporum (Accessions nos. ITS: MK862121; ACT: MK862124; GAPDH: MK862122; and TUB2: MK862123) and C. musicola (Accessions nos. ITS: MK882586; ACT: MK882589; GAPDH: MK882587; and TUB2: MK882588), respectively. Pathogenicity of the fungi was verified by spraying conidial suspensions (1 × 106 spores ml-1) on the upper surface of 10 taro leaves. Ten control leaves were sprayed using sterilized water. All plants were kept under greenhouse conditions at 25°C for 10 days. Anthracnose symptoms were observed on all inoculated leaves after 12 days, whereas control leaves remained symptomless. Koch´s postulates were fulfilled when the fungi were re-isolated 100% from the diseased leaves. The experiment was performed twice. Colletotrichum brevisporum has been previously reported as a pathogen on several crops including Annona muricata, Carica papaya, Sechium edule, Capsicum annuum, Passiflora edulis, Citrus medica, P. edulis, Cucurbita moschata, and C. pepo (Farr and Rossman 2019), whereas Colletotrichum musicola only has been associated with Musa sp. in Mexico (Damm et al. 2019). To our knowledge, this is the first report of C. brevisporum and C. musicola causing leaf anthracnose of taro in Mexico and worldwide.