First Report of <i>Golovinomyces tabaci</i> Causing Powdery Mildew on <i>Rubia alata</i> in China

Rubia alata, belonging to the genus Rubia, has been widely used as a folk medicine in China (Zhao et al. 2014). In March 2023, typical powdery mildew symptoms were observed on R. alata on Longwen hill, Guizhou Normal University, China. The incidence was ~25% among 60 R. alata plants assessed. Infected leaves presented chlorotic to necrotic phenotypes, with occasional anthocyanin accumulation. Mycelia most commonly occurred on the adaxial surfaces of infected leaves. Hyphae were hyaline and 4–7 μm wide. Hyphal appressoria were slightly lobed and produced singly or in opposite pairs (Fig. S1). Conidiophores were erect, straight to somewhat flexuous, and 60–200 µm long (n = 30). Foot cells were subcylindrical, followed by 2–4 cells. Conidia formed 2–5 in a chain and were ellipsoid to ovoid in shape with dimensions of 22–46 × 11.5–18 µm (n = 50). No fibrosin bodies were observed. Based on these morphological characteristics, the powdery mildew fungus was identified as Golovinomyces tabaci (Qiu et al. 2020). To identify the species, the ITS and LSU regions were amplified with the PM10/PM28R primer pair (Bradshaw and Tobin 2020); the CAM, GAPDH, GS, RPB2, IGS, and TUB regions were amplified using the PMCAM1/PMCAM4R, PMGAPDH1/PMGAPDH3R, GSPM2/GSPM3R, PMRpb2_4/PMRpb2_6R, IGS-12a/NS1R, and BTF5b/BTR7a primer pairs, respectively (Carbone and Kohn 1999; Ellingham et al. 2019; Bradshaw et al. 2022a). The obtained 1311-bp ITS-LSU sequence (GenBank accession no. PV368321), 284-bp CAM sequence (PV384439), 319-bp GAPDH sequence (PV384435), 468-bp GS sequence (PV384436), 758-bp RPB2 sequence (PV384437), 384-bp IGS sequence (PV391934) and 777-bp TUB sequence (PV384438) were deposited in GenBank. Based on the concatenated GAPDH+IGS+ITS+LSU sequences of the Golovinomyces taxa (Bradshaw et al. 2022b), phylogenetic tree was constructed and the isolate GZRA-1 was grouped in the same clade as G. tabaci (Fig. S2). To perform pathogenicity test, leaves of three healthy potted R. alata plants were inoculated by gently pressing with diseased leaves and were incubated in a greenhouse at 25 ± 2°C with 80% relative humidity. Non-inoculated plants served as controls. After 14 days, similar powdery mildew symptoms were observed on inoculated plants, whereas control plants showed no powdery mildew symptoms. Fungal samples from inoculated R. alata plants were morphologically identical to that on originally diseased plants, and ITS-LSU sequences of reisolated fungus shared 100% identity with PV368321. In addition, another powdery mildew isolate (GZRA-2) from naturally diseased R. alata plant was identified. The results revealed that GZRA-2 exhibited comparable morphological and molecular characteristics to GZRA-1. Based on these consistent traits, the powdery mildew fungus infecting R. alata plants was determined to be G. tabaci. Although G. tabaci has been reported to infect R. cordifolia in China (Qiu et al. 2020), this is the first report of powdery mildew caused by G. tabaci on R. alata. This work expands the known host range of G. tabaci on the genus Rubia and also highlights the importance of monitoring and managing this powdery mildew disease.