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First report of the occurrence of leaf spot caused by Boeremia exigua on Khaya grandifoliola and K. senegalensis in Brazil

2022; Wiley; Volume: 46; Issue: 1 Linguagem: Inglês

10.1002/ndr2.12109

2044-0588

Lucas Antonio Benso, L. P. Pieroni, Yerly Mira, Andréa Rodrigues Marques, V. Ciriello, Cristiane de Pieri, Bruno César Rossini, Celso Luís Marino, Edson Luiz Furtado,

Plant-Microbe Interactions and Immunity

African mahogany (Khaya spp.) is an arboreal species native to Africa. The economic importance of the genus is due to the value of its wood, used mainly in the furniture industry and in civil construction (Ribeiro et al., 2019). In Brazil, the genus was introduced to supply the market demand for mahogany wood, and currently has an estimated planted area of 37,000ha (Ribeiro et al., 2019). In a tree nursery located in the city of Garça, São Paulo, Brazil, approximately 20% of the seedlings of K. grandifoliola and K. senegalensis showed symptoms of leaf spot in 2020. The spots usually started at the edges of the leaflets and showed a rounded shape. The lesions on K. grandifoliola displayed a purple halo, dark brown edges and a grey centre (Figure 1), whilst on K. senegalensis the lesions were light brown in the centre, with dark brown edges (Figure 2). On both species, premature fall of the affected leaflets was observed. Diseased tissues were sent to the Forest Pathology Laboratory–FCA/UNESP and indirect isolation was done on potato dextrose agar medium (PDA, 39 g/l). The fungal colonies obtained were initially light cream in colour, becoming light brown over time (Figure 3). The morphological characterisation of the spores revealed oval-shaped hyaline conidia measuring 6.3 (4.6–7.6) × 3.6 (2.0–5.2) μm (isolated from K. grandifoliola) and 6.5 (4.1–7.6) × 3.9 (2.3–5.5) μm (isolated from K. senegalensis). The formation of pycnidia was abundant around the lesions of both species of Khaya, however scarce in the colonies formed in PDA culture medium. These characteristics are similar to those of the genus Phoma (Chen et al., 2015). No reaction was observed by the NAOH test, indicating a lack of production of antibiotic E (Boerema et al., 2004). To confirm the identity of the two isolates, we sequenced the internal transcribed spacer (ITS), β-tubulin (Bt) and actin (ACT) regions usingpreviously described methods (White et al., 1990; Glass & Donaldson, 1995; Carbone & Kohn, 1999). The sequences from the three regions were concatenated and analysed with maximum likelihood in RaXML with 1,000 bootstraps and confirmed that the isolates are B. exigua (Figure 4). BLAST analysis resulted in an identity greater than 99.2% for Bt and 100% for ITS and ACT regions to Boeremia exigua. The sequences were deposited in GenBank (Table 1). The pathogenicity test was performed on eight-month-old seedlings using ten seedlings for each species of Khaya. The seedlings were inoculated with 5 mm diameter discs of PDA medium colonised by the previously isolated fungi. This inoculation method was selected due to the lack of spores formed by the fungi in the culture medium. The inoculated seedlings were kept in a humid chamber at 25°C and alternating photoperiod (12 hours of light and dark). After 48 hours, necrosis was observed around the inoculated region (Figure 5). The pathogen was successfully reisolated from inoculated leaflets. The control treatment, which received only fungal-free culture medium, showed no symptoms (Figure 5). To the best of our knowledge, this is the first report on the occurrence of leaf spot on K. grandifoliola and K. senegalensis caused by B. exigua. This report extends the range of known pathogens associated with trees of the genus Khaya and serves as a basis for development and implementing disease management plans. The authors would like to thank CAPES (project number 001) and Futuro Florestal Company for all the support.