First report of <i>Lasiodiplodia brasiliense</i> causing mango dieback in Côte d'Ivoire

Abstract

In 2020, surveys of mango (Mangifera indica) orchards across the departments of Boundiali (9°36′34.668″N, 6°41′16.1628″W), Korhogo (9°17′11.1228″N, 5°37′6.8016″W) and Odienné (9°31′16.3992″N, 7°36′15.93″W) revealed the presence of mango dieback symptoms, such as leaf, twig and branch drying from the top of the tree downwards as well as death of the whole tree (Fig. 1). The disease incidence varied from 20 to 100%. Isolation was performed on samples of diseased organs (leaf, twig, branch and trunk) on potato dextrose agar (PDA) medium and plates were incubated at 25°C in darkness for one week. A representative isolate YANRSYA5 was selected and transferred onto 2% water agar (WA) containing sterilised Pinus nigra needles. Plates were incubated at 25°C with an eight-hour photoperiod for four weeks to induce the formation of fruiting bodies and sporulation. Colonies produced dense aerial mycelium, initially white, which turned to dark brown and black (Fig. 2). At this stage, dark coloured pycnidia were formed (Fig. 3a). Conidiogenous cells were hyaline, thin-walled or smooth. Conidia were subovoid to ellipsoidovoid, with the apex broadly rounded, thick-walled, and contents granular. Initially the conidia were hyaline and aseptate, then dark brown and one-septate with longitudinal striations (Fig. 3b-e). Conidia were 16.23-30.53 × 11.45-16.51 µm (mean ±S.D = 24.08 ±3.25 × 13.55 ±1.2 µm, L/W ratio = 1.77; n = 50). The cultural and morphological characteristics of the isolate were similar to those reported for the genus Lasiodiplodia (Phillips et al., 2013). Genomic DNA was extracted and the internal transcribed spacer (ITS) region of ribosomal DNA (rDNA) and translation elongation factor (tef1-α) were amplified by PCR and sequenced with ITS1/ITS4 and EF1-688F/1251R primers, respectively. A BLASTn search revealed 100 % identity with GenBank Accession Nos. JX464063.1 (ITS) and JX464049 (tef1-α) in GenBank corresponding to Lasiodiplodia brasiliense (Fig. 4). The ITS and tef1-α sequences of isolate YANRSYA5 were deposited in GenBank (OQ067216 and OQ269653 for ITS and tef-1α, respectively). A pathogenicity test was performed according to Ismail et al. (2013). Plants were maintained subsequently in a greenhouse at 30 to 35°C for 60 days. The leaves of inoculated plants became withered and desiccated, whereas the control plants remained asymptomatic (Fig. 5). Lasiodiplodia brasiliense was re-isolated from all the inoculated plants, fulfilling Koch's postulates. Lasiodiplodia brasiliense has been reported in several major mango-production regions, such as Mexico (Bautista-Cruz et al., 2019) and Peru (Rodríguez-Gálvez et al., 2017) causing canker and dieback symptoms. This study represents the first report of mango dieback disease caused by Lasiodiplodia brasiliense in Côte d'Ivoire. Further studies should be undertaken to assess variation among the isolates and whether other species of Lasiodiplodia are associated with the disease in order to elaborate control strategies. This research was funded by Partnership for Skills in Applied Sciences, Engineering, and Technology - Regional Scholarship and Innovation Fund (PASET-RSIF). The authors acknowledge the support of the Fondation Louis Omer DECUGIS and the Société Internationale d'Importation (SIIM, France)