Apple bitter rot is a globally widespread disease that is observed on fruits both pre-harvest and post-harvest, contributing to considerable economic losses. While the Colletotrichum acutatum species complex are predominant in Europe (Baroncelli et al. 2014; Amaral Carneiro and Baric 2021), in recent years, the Colletotrichum gloeosporioides species complex are emerging, raising many concerns (Amaral Carneiro et al. 2023). Circular, slightly sunken, brown lesions with acervuli produced in concentric spots were observed on 'Story® Inored' cultivar harvested in September 2022 from an organic orchard in Masi (Padova province, Italy), with a disease incidence close to 30%. From ten diseased apples, tissue samples were excised under aseptic conditions from surface-cleaned fruit at the margin between healthy and diseased pulp tissue, transferred to potato dextrose agar medium and incubated in the dark at 25 °C for 7 days, whereafter five single-spore cultures were obtained. Pure colonies grown at 25 °C for 7 days appeared light gray-white on the upper side with floccose aerial mycelium, while the reverse side was dark gray with a distinct margin. Conidia were hyaline, cylindrical in shape with both ends rounded or one end acute and measured 16.6 ± 1.4 × 6.1 ± 0.5 μm [mean ± SD] (n=50) as described by Diao et al. 2017. To identify the species, genomic DNA of a representative isolate (C38) was extracted, beta-tubulin (TUB2), calmodulin (CAL), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), glutamine synthetase (GS), Apn2-Mat1-2 intergenic spacer (ApnMat) genes and the internal transcribed spacer (ITS) region, were amplified by PCR and Sanger sequenced (Rojas et al. 2010; Weir et al. 2012). The obtained DNA sequences of, TUB2, CAL, GAPDH, GS, ApnMat and ITS were submitted to GenBank under the accession numbers OR025589, OR025586, OR025587, OR025588, OR025585 and OR004800, respectively. A MegaBLAST analysis resulted 100 % identity to the epitype CAUG7 of Colletotrichum grossum (Diao et al. 2017) for GAPDH (KP890159), for TUB2 (KP890171), 99.85% for CAL (KP890147) and 99,5 % for ITS (KP890165). The phylogenetic tree constructed by concatenation with the obtained sequences, as well as references, revealed that the C38 isolate clustered within C. grossum, confirming the BLAST approach. Pathogenicity tests were performed on 40 'Story® Inored' apples cleaned and wounded with a sterilized needle and exposed to two different conditions: 20 apples (10 inoculated with 20 μl of spore suspension (105 ml-1) and 10 with sterile water as control) were incubated at 20°C with a 12-hour photoperiod for 14 days, while the remaining 20 apples, prepared with same approach, were placed at 1°C for 3 months, then at room temperature for 14 days. Symptoms appeared after 6 days on apples incubated at 20°C, whereas those stored at 1°C displayed symptoms at 11 days after being placed at room temperature. In both conditions, lesions were similar to those observed on the original fruits; while the controls remained asymptomatic. Identity of reisolated fungal colonies was confirmed by CAL, GAPDH and GS region sequence analysis. C. grossum has been reported rarely: in 2017 on Capsicum annuum var. grossum in China, in 2018 on Mangifera indica leaves in Cuba, and in 2021 on Rhyncospermum jasminoides in Italy (Diao et al. 2017; Manzano León et al. 2018; Guarnaccia et al. 2021). To the best of our knowledge, this is the first report of apple bitter rot caused by Colletotrichum grossum worldwide.
Keywords: Causal Agent; Fungi; Glomerella; Italy; Malus domestica.