The grapefruit (Citrus paradisi) is a citrus hybrid tree (C. maxima & C. sinensis). Due to nutritional value and its bioactive compounds, the fruits are recognized as a functional food, valued as promoting health. French grapefruit production is low (7.5 Kt/year) but is confined to a restricted area in Corsica and benefits from a quality label, the economic impact of its cultivation being therefore locally significant. Since 2015 previously unreported symptoms have been repeatedly observed on grapefruits in more than half of the orchards in Corsica, with an incidence of 30% of fruits altered. Brown to black circular spots were observed on fruits and leaves, surrounded by chlorotic halos on the latter. On the mature fruit, lesions were round, 4 to 10 mm in diameter, brown and dry (e-Xtra 1). Although the lesions are superficial, the fruits cannot be marketed due to constraints linked to the quality label. 75 fungal isolates were obtained from symptomatic fruits or leaves collected in Corsica (in 2016, 2017, and 2021). Cultures obtained after 7 days on PDA at 25°C, were white to light grey in colour, forming concentric rings or dark spots on the agar surface. We did not observe any notable difference among the isolates except some evolved towards a more marked grey. Colonies tend to form a cottony aerial mycelium and orange conidial masses appear with age. The conidia were hyaline, aseptate, cylindrical with ends rounded, and measured 14.9 ± 0.95 µm length and 5.1 ± 0.45 µm width (n = 50). Cultural and morphological characteristics were similar to those described for C. gloeosporioides s. lat. or C. boninense s. lat. (Weir et al. 2012 ; Damm et al. 2012). Total genomic DNA was extracted from all isolates, and the ITS region of rDNA was amplified with ITS 5 & 4 primers, then sequenced (GenBank Accession Nos. OQ509805-808). For 90% of isolates GenBank BLASTn results were 100% identical to C. gloeosporioides isolates sequences, whereas for other isolates the resulting sequences were 100% identical to C. karsti or C. boninense isolates sequences. Four strains (three C. gloeosporioides with light colour differences, in order to see if there was diversity among isolates of C. gloeosporioides s. lato ; and one C. karsti) were further characterized by sequencing partial actin [ACT], calmodulin [CAL], chitin synthase [CHS-1], glyceraldehyde-3-phosphate dehydrogenase [GAPDH], β-tubulin 2 [TUB2], for all strains ; glutamine synthetase [GS], Apn2-Mat1-2-1 intergenic spacer and partial mating type (Mat1-2) gene [ApMAT] for C. gloeosporioides s. lat., and HIS3 for C. boninense s. lat. (Weir et al. 2012 ; Silva et al, 2012) (GenBank Accession Nos. OQ509805-808 & OQ507698-724). Multilocus phylogenetic analyses carried out with the obtained and Genbank available sequences confirmed that 3 isolates (UBOCC-A-116036, -116038, & -116039) clustered within C. gloeosporioides s. s., while the other (UBOCC-A-116037) clustered within C. karsti (e-Xtra 2) 'Star ruby' grapefruits were surface sterilized then wound-inoculated with 20 μl of a conidial suspension (105 conidia ml-1) of UBOCC-A-116036 & 116037 isolates or 20 μl sterile water for control (ten fruits for each isolate or control). After 10 days incubation at 20°C, symptoms, identical to those initially observed, developed around the inoculation point, while controls inoculated with water remained symptomless. Fungal colonies re-isolated from the lesions were morphologically like the original isolates. Recently, various infections caused by some Colletotrichum sp. have strongly compromised citrus production in different Mediterranean countries: ie Italy (Aiello et al. 2015), Portugal (Ramos et al. 2016), Tunisia (Ben Hadj Daoud et al. 2019), Turkey (Uysal et al. 2022). In these studies, C. gloeosporioides s. s. and C. karsti were identified as the causal agents. These two species were the predominant Colletotrichum sp. associated with Citrus and allied genera in Europe (Guarnaccia et al. 2017). To our knowledge, our study is the first report of C. gloeosporioides and C. karsti causing anthracnose on grapefruit in France, which confirms the incidence of these two pathogens on the Mediterranean rim. Given the economic importance of citrus cultivation in the Mediterranean region, the presence of Colletotrichum spp. should deserves to be monitored, and a control strategy should be considered.
Keywords: Causal Agent; Colletotrichum; Crop Type; Epidemiology; Fruit; Fungi; Grapefruit; Subject Areas; disease development and spread; tree fruits.