First Report of <i>Golovinomyces ambrosiae</i> on <i>Cirsium arvense</i> Worldwide

Cirsium arvense, also known as Canada or creeping thistle, is a noxious perennial weed in North America that severely impacts native and agriculturally significant crops. Recent research has been focusing on the use of the rust Puccinia suaveolens as a biological control agent. In 2025, Canada thistles in a growth chamber on Centennial Campus of North Carolina State University (35.77405°N, 78.67579°W) became infected with powdery mildew. Leaves were completely covered with white mycelial growth. Morphologically, mycelium was amphigenous, effuse; hyphae flexuous, hyaline, septate, 4-6 µm wide; hyphal appressoria indistinct to nipple shaped, 5-7 µm diam.; conidiophores arising from the upper surface of the mother cell, straight, unbranched; foot-cells subcylindrical, (50-)78-94(-112) × 9-11 µm, followed by 1-3 shorter cells after foot cell; conidia hyaline, cylindrical-doliiform, lacking fibrosin bodies, (29-)33-35(-38) × 14-16(-20) µm, length/width ratio 2-2.3; germ tubes ± terminal, 22-32(-73) µm long, conidial appressoria club shaped, Euoidium type; chasmothecia were not observed. The anamorph of the specimen identified in this study was shown to be morphologically identical to Golovinomyces ambrosiae and had significant differences from the closely related G. latisporus in conidial shape and size as well as in the shape of the conidial appressoria. The two species can be differentiated in that G. ambrosiae has narrower and longer cylindrical-doliiform conidia with a length/width ratio that can be >2 versus G. latisporus with shorter and broad ellipsoid-ovoid, doliiform conidia with a length/ratio < 2 on average (1.3-1.9). Additionally, the conidial germ tubes of G. ambrosiae were short with typical Euoidium type, swollen tips while G. latisporus exhibits longer germ tubes often of the longitubus type (Qiu et al. 2020). Golovinomyces montagnei, which is a common pathogen for this host in Europe, has wider and longer conidia; (25-)30-45 × 15-23 μm (Braun and Cook 2012). We sequenced the ITS (GenBank Number: PV581748), IGS (PV584301), GAPDH (PV584324), GS (PV584325), and RPB2 (PV584332) regions with the following primer pairs: PM10 (Bradshaw and Tobin 2020) /PM2 (Cunnington et al. 2003), IGS-12a/NS1R (Carbone and Kohn, 1999), PMGAPDH1/PMGAPDH3R, GSPM2/GSPM3R, and PMRpb2_4/PMRpb2_6 (Bradshaw et al. 2022a). All regions sequenced aligned 100% with other G. ambrosiae specimens including FH00941245 (ON073878, ON361194, ON075631, ON119167, ON075691) and FH00941246 (ON073842, ON361187, ON075629, ON075689, ON119163). The specimen was deposited in the Larry F. Grand Mycological Herbarium (Voucher Number: NCSLG 25310). We confirmed pathogenicity using two uninfected Canada thistle plants. One plant was inoculated by touching diseased leaves to wet healthy leaves and then kept in the same growth chamber as the infected plants. The other plant was not inoculated and placed in a separate growth chamber as a control. Ten days after inoculation, typical powdery mildew symptoms and signs were observed. Powdery mildew was also observed spreading to multiple other Canada thistle plants in the growth chamber that held the inoculated plant. No symptoms were observed on the non-inoculated control plant. Golovinomyces montagnei and G. depressus have been reported to infect C. arvense from Europe (Bradshaw et al. 2024). Multiple reports have identified powdery mildew on C. arvense from Idaho, Washington, and Oregon as Erysiphe cichoracearum or Golovinomyces cichoracearum, but these names have been used broadly for a range of powdery mildew species (Newcombe and Nischwitz 2004; Dungan and Glawe 2007; Bradshaw et al. 2022b); both reports could have been referring to G. ambrosiae, G. montagnei or G. depressus. However, based on the present study, it is highly likely that they were referring to G. ambrosiae. Interestingly, from personal observations, the powdery mildew tended to infect the plant most heavily when the rust, P. suaveolens, was also present. This information is important for the agriculture industry where C. arvense is a major problem.