Colletotrichum coffeae-arabicae from leaf necrosis of sweet orange in Brazil induces postbloom fruit drop symptoms

Brazilian citriculture is concentrated in São Paulo and Minas Gerais states, accounting for about 70% of national production. In this region, postbloom fruit drop (PFD) is an important disease, causing severe fruitlet abscission and yield losses that may exceed 80% under favorable conditions (Gama et al. 2019). PFD is associated with species in the Colletotrichum acutatum (CASC) and C. gloeosporioides (CGSC) complexes (Brown et al. 1996; Lima et al. 2011). Species from CASC, CGSC, C. boninense and C. truncatum complexes also cause citrus anthracnose (Huang et al. 2013; Guarnaccia et al. 2017). In 2023, anthracnose-like symptoms were observed on leaves of sweet orange (Citrus sinensis) in orchards in Pedregulho, São Paulo, Brazil (-20.3191ºS, -47.4825ºW). Leaves showed necrotic lesions, chlorotic halos, and deformation (Fig. 1A). Symptomatic tissues were disinfected, plated on potato dextrose agar (PDA), and incubated at 25 °C, resulting in 20 Colletotrichum isolates. DNA was extracted (Brandão et al. 2024), and partial sequences of ITS, GAPDH, CHS-1 and TUB2 loci were amplified and sequenced (Weir et al. 2012). Multilocus phylogenetic analyses using Maximum Likelihood and Bayesian Inference were performed using IQ-TREE and MrBayes, respectively, and assigned all isolates to the CGSC. Nineteen isolates were identified as C. gloeosporioides sensu stricto (100% identity with strain CBS 953.97). One isolate (FDC14/23.2) was identified as C. coffeae-arabicae, a CGSC species distinct from C. kahawae, the causal agent of coffee berry disease, with 99% identity to the ex-holotype strain PPDU26B across all loci (GenBank Accession Nos. PZ342067 [ITS], PZ349759 [GAPDH], PZ349760 [CHS-1], and PZ349758 [TUB2]). This isolate clustered with PPDU26B (1/100, BI/ML), while the remaining isolates were identified as C. gloeosporioides sensu stricto and clustered within the corresponding clade (PP = 0.95; BS = 93) (Fig. 2A). Morphological evidence showed isolate FDC14/23.2 produced grayish-white, cottony colonies with irregular margins on PDA after 7 days (Fig. 1B) and showed low conidial production (3×10 ⁴ conidia mL ^-1 ), compared with a representative C. gloeosporioides isolate (7×10 ⁶ conidia mL ^-1 ). Conidia were hyaline, smooth-walled, oval to slightly elongated (41.6 × 12.9 µm) (Fig. 1C). Pathogenicity assays were conducted using elongated flower buds inoculated with a conidial suspension (10 ⁵ conidia mL ^-1 ). Typical PFD symptoms developed on petals within five days after inoculation, whereas non-inoculated flowers remained healthy. The fungus was re-isolated from symptomatic petals but not controls. Reisolated colonies matched the original morphology, fulfilling Koch’s postulates (Fig. 1D). Young citrus leaves (V3–V4) inoculated with the same suspension and kept in ahumid chamber for 24-72 h remained symptomless. Wounded and unwounded mature fruit inoculated with the conidial suspension and mycelial plug remained symptomless during the 7-day assessment period. These findings suggest that infection depends on specific environmental conditions and/or host phenological stages. C. coffeae-arabicae has recently been reported causing anthracnose in coffee (Coffea arabica L.) in Saudi Arabia (Alhudaib et al. 2023). The surveyed orchard is surrounded by coffee plantations (Fig. 2B), suggesting cross-host interactions. This finding expands the host range of C. coffeae-arabicae and supports disease monitoring and epidemiological studies.