First Report of Powdery Mildew Caused by <i>Pleochaeta prosopidis</i> s. lat. on <i>Neltuma juliflora</i> (≡ <i>Prosopis juliflora</i> ) in Mexico

Neltuma juliflora (≡Prosopis juliflora) (Fabaceae) known as mesquite, is a xerophytic, thorny, and deciduous tree native to Mexico, Central America, and northern South America. In July 2022, typical symptoms of powdery mildew were observed in approximately 60% of mesquite trees located in the botanical garden of the Faculty of Agriculture at El Fuerte Valley (FAVF), Juan José Ríos, Sinaloa, Mexico (25°45'21.9"N 108°50'22.0"W). Microscopic analysis revealed superficial, effuse, amphigenous mycelium forming irregular patches or covering the adaxial surface of the leaves. Conidiophores (n = 50) were erect, slender, emerging from external hyphae on the upper leaf surface, and measured 50–130 μm in length. Foot-cells (n = 50) were twisted (33–88 × 4.5–7 μm), followed by 1 to 4 shorter cells producing solitary conidia. Conidia (n = 100) were dimorphic: primary conidia were lanceolate, secondary conidia cylindrical, occasionally centrally constricted, measuring 21–36 × 5–10 μm. Fibrosin bodies were absent, and the germ tube was terminal. Chasmothecia were not observed. These morphological traits matched those described for the genus Pleochaeta as recently circumscribed by Bradshaw et al., (2025). A voucher specimen (FAVF678) was deposited in the Herbarium of the Faculty of the FAVF. To confirm fungal identity, the internal transcribed spacer region (ITS) and the 28S rRNA gene were amplified by PCR using the primers PM5/ITS4 and PM3/TW14, respectively (Mori et al., 2000). Sequences were submitted to GenBank under accession numbers PV871197 (ITS) and PV848594 (28S). Both sequences showed similarity to P. prosopidis in a BLAST search; 92% for ITS (LC108830) and 97% for 28S (OR511962). Maximum Likelihood phylogenetic analysis of the combined ITS+LSU data clustered the specimen FAVF678 with the reference specimen voucher of P. prosopidis (MUMH1691), confirming the affiliation of the Mexican specimen to Pleochaeta as circumscribed in Braun and Cook (2012). The generic affinity of Pleochaeta is currently insufficiently known, and P. prosopidis seems to be a heterogeneous species complex as reflected in the obtained phylogram and its morphology; FAVF678 and the reference sequence LC108830 have a low ITS sequence similarity (92%), and the secondary conidia dimensions of specimen FAVF678 are larger than those reported by Braun and Cook (2012) for P. prosopidis. Pathogenicity was tested by dusting conidia from infected leaves onto healthy leaves of ten mesquite plants, with ten non-inoculated plants serving as controls. All plants were maintained in a greenhouse under temperatures ranging from 25 to 35°C. The experiment was repeated twice. Fourteen days post-inoculation, powdery mildew symptoms appeared on all inoculated plants, while controls remained asymptomatic. The fungal morphology on inoculated plants matched that observed in naturally infected specimens, thereby fulfilling Koch’s postulates. Alvarez (1976), reported Erysiphe taurica on N. juliflora in Mexico; however, it is likely that this specimen was misidentified and it was in fact P. prosopidis s. lat. Although P. prosopidis has previously been reported infecting members of Fabaceae (subfamily Mimosoideae) (Braun and Cook, 2012; Bradshaw et al., 2025; Farr et al., 2025), this study presents the first known and confirmed record of powdery mildew caused by P. prosopidis s. lat. on N. juliflora, both in Mexico and globally. Additional research is required to manage this disease in mesquite.