First Report of Colletotrichum siamense Causing Anthracnose on Myrciaria dubia in Brazil

Camu-camu (Myrciaria dubia (Kunth) McVaugh) is an Amazonian fruit with high economic potential due to its reported health-promoting properties, including elevated antioxidant and anti-inflammatory compound levels. Since 2018, anthracnose-like lesions, starting as small dark spots and coalescing into large irregular or circular lesions, were observed in up to 60% of camu-camu plants in an experimental field located in the city of Boa Vista, Roraima, Brazil (2°52'20.7″ N; 60°42'44.2″ W) and in a natural population in Rorainópolis, Roraima, Brazil (0°54'128″ N,60°36'226″ W). Fifty symptomatic leaves were collected from 10 plants. Small (5×5 mm) lesion tissue fragments were surface disinfected in 70% ethanol (1 min), 2.5% sodium hypochlorite (1 min), rinsed in sterile water, and placed on water agar. After 4 days at 25°C, mycelium was transferred to potato dextrose agar (PDA). After 7 days, 10 morphologically similar isolates were obtained, showing grayish-white colonies with orange spore masses. Conidia were hyaline, unicellular, cylindrical, smooth-walled, straight with rounded apices, measuring 15.3 ± 0.8 × 6.95 ± 1.4 μm (n = 30). Appressoria were brown, irregular or ovoid, measuring 6.4 ± 1.5 × 4.9 ± 1.2 μm (n = 30). Single-spore cultures were deposited in the EMBRAPA Roraima Microorganism Culture Collection. Four representative isolates (MTG07, MTG08 from Boa Vista; DXV01, DXV02 from Rorainópolis) were selected for phylogenetic and pathogenicity analysis. Total DNA was extracted and portions of actin (ACT), calmodulin (CAL), chitin synthase 1 (CHS-1), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and β-tubulin 2 (TUB2) genes were amplified (Weir et al. 2012) and Sanger sequenced (GenBank accession numbers ACT: PV463490-PV463493; CAL: PV463498-PV463501; CHS-1: PV055675-PV055678; GAPDH: PV430301-PV430304; TUB2: PV463494-PV463497). Sequences showed 96 to 99% identity with sequences of Colletotrichum siamense published in GenBank. Phylogenetic Bayesian inference analysis based on a combined data set showed that the isolates clustered with the ex-holotype specimen of C. siamense (ICMP 18578) with high support (posterior probability = 0.98). To confirm pathogenicity, five seedlings of camu-camu per isolate were sprayed with 106 conidia/mL spore suspension (15 mL/plant). Control seedlings were sprayed with sterile water. All seedlings were covered with plastic bags 24 h after inoculation and maintained at 27°C in a greenhouse with a 12-h photoperiod. After 6 days, inoculated leaves exhibited small dark brown spots with yellow halo, which enlarged and coalesced into regular or irregular brown necrotic lesions. The control plants showed no symptoms. The pathogenicity test was performed twice with similar results. The fungus was successfully reisolated from the inoculated leaves and its identify was confirmed by cultural morphology and DNA sequence. C. siamense is a known anthracnose agent on diverse hosts (Weir et al. 2012), but in camu-camu, only C. gloeosporioides, C. aeschynomenes, C. tropicale, and C. theobromicola (Perez et al., 2006; Matos et al., 2020; Sachet et al., 2024) have been reported. To our knowledge, this is the first report of C. siamense causing anthracnose in camu-camu. The identification of the pathogen enables future research on management strategies, as this pathogen can cause serious damage to camu-camu production in the region.