First report of Kuehneola guatemalensis on galls of wild blackberry stems in Mexico

Because blackberries (Rubus spp.) are commercially important in Mexico, monitoring of wild plants (Rydberg 1963) can be valuable for management of disease. In April 14, 2024, and May 19, 2025 galls were observed (~16% incidence) on stems of wild blackberries (Rubus aff. miser Liebm.) near Venta Grande, municipality of Huauchinango, Puebla, Mexico (20.132 N, 98.083 W; 19.994 N, 97.987 W; ca. 2000 m). Galls were rounded, brown to blackish in color, with a woody texture and covered with yellow spores of a rust fungus. Galls on herbaceous stems measured 0.5–4.0 × 0.7–3.5 cm; on woody stems forming dense clusters of up to 31.0 × 7.5 × 5.0 cm. No rust was observed on leaves, petioles, or flowers. Some galls showed up to 90% rust severity. The galls were examined under a stereomicroscope up to 6.3X. Histological sections were prepared, mounted in 85% lactic acid and observed using light, Nomarski, and phase-contrast microscopy (µm measured at 100X); scanning electron microscopy sections were also examined. In all galls examined, uredinoid aecia were produced, but no spermogonia, uredinia, or telia. Sori were superficial, globose to elongated, 1.0–2.0 mm long, pale yellowish to brown, of subepidermal origin, erupting with a powdery appearance, and opening longitudinally with persistent brown epidermal remnants. Aeciospores were hyaline to pale yellow; borne in chains without intercalated cells; some with colorless pedicels, 67.5–100.0 × 5.0 µm long; ellipsoidal to subglobose, occasionally slightly obovoid, globoid, or cylindrical, measuring (17–)20–24(–25) × 22–30(–35) µm (n = 100); spore wall hyaline, 1.0–1.5 (–2.0) µm thick; uniformly echinulate, ornaments 1.15–1.61 × 1.06–1.35 µm. Rarely, spores had two equatorial germ pores. Peridia and paraphyses were not observed. DNA was extracted from spores using the CTAB 2% method (Doyle and Doyle 1987), and a portion of the rDNA repeat spanning the ITS2–28S was amplified and sequenced (Kaishian et al. 2024) with Rust2INV/LR6 and LROR/LR3 primers (Aime 2006). Maximum Likelihood analyses were performed in Iqtree v.1.6.12 and Bayesian inference in MrBayes (Ronquist et al. 2012; Nguyen et al. 2015). The rust was identified as Kuehneola guatemalensis Cummins, a species lacking publicly available sequence data. In Guatemala, K. guatemalensis forms galls on the stems of Rubus sapidus Schltdl. (syn. R. tuerckheimii Rydb., Cummins 1943). The sequence (NCBI PX692954) is congeneric with the gall-forming K. loeseneriana (sharing 95.5% identity) from Rubus humistratus (Hernández and Hennen 2003) which differs from K. guatemalensis in producing aeciospores and urediniospores in the spirally striated arrangement of ornamentation and with K. uredinis (91.5% identity), which only infects leaves and does not form galls (Hernández and Hennen 2003). The specimen was deposited in the CMPH Herbarium (Accessions 3826 and 3827) and the host specimen in the CHAPA Herbarium (Accession 159034). Although we located two herbarium specimens (PUR58566 and PURN6644 from 1963 and 1974, Oaxaca and Veracruz states, on Rubus sp.) of K. guatemalensis from Mexico, this is the first report of K. guatemalensis on R. aff. miser and the first published record in Mexico. This study will aid blackberry producers to proactively monitor for symptoms of K. guatemalensis in their crops.