First Report of Powdery Mildew Caused by <i>Erysiphe alliariicola</i> on Pennycress ( <i>Thlaspi arvense</i> ) in North America

Thlaspi arvense L., or pennycress, is an annual plant in Brassicaceae. It has recently been domesticated as a winter oilseed cover crop for the ultra-low carbon fuel feedstock market (Chopra et al. 2020). In Jan. 2023, severe powdery mildew occurred on greenhouse-grown plants in St. Paul, Minnesota. Disease severity reached 100% on the most susceptible lines. Mycelia and conidia were first observed on the adaxial surface of the lower leaves. Colonies coalesced to form a dense, opaque layer on leaves, stems, pods, and flowers. Infected plant parts senesced prematurely, and seed yield and viability were substantially reduced. Mycelia were unpigmented and smooth. Conidiophores were straight, colorless, erect, cylindrical, 50-93 µm (mean 68.9 µm) long, and composed of 3 to 4 cells. Conidiophore foot cells were cylindrical and 14.3-29.6 x 4.1-6.7 µm (mean 21.8 x 5.9 µm). Conidia were cylindric-doliiform, singly borne, and 22.9-43.1 x 11.3-20.3 µm (mean 34.6 x 15.7 µm). Chasmothecia were observed when plants began to senesce, and were round, 100-120 µm in diameter, dark brown at maturity, and with simple appendages 40-260 µm in length. Asci were ovoid-saccate containing 3-6 colorless ascospores approximately 20-22.5 x 10-12.5 µm in size. To fulfill Koch's postulates and develop an isolate for later investigations, a piece of leaf with a single colony was gently pressed on two leaves of a 3-week-old T. arvense plant "MN106". The inoculated and control plants were incubated in a greenhouse at 22°C (day) and 18°C (night). The reisolated Erysiphe alliariicola was identical in symptoms, morphology, and ITS as the original isolate, thus confirming Koch's postulates. Control plants remained symptomless. The ITS region, and segments of the LSU, CAM, GADPH, GS, and RPB2 loci were amplified by PCR following Bradshaw et al. (2022a, 2022b). Respectively, the GenBank accessions are OR195529, PP499013, PQ213024, PQ213025, PQ213026, and PQ213027, with primers available within accessions. The sample was submitted to the University of Minnesota Bell Museum (MIN984734). Reference sequences from Bradshaw et al. (2022a; 2024) were downloaded from NCBI, and sequences were aligned and concatenated. A maximum likelihood tree was inferred with iqtree2 (Minh et al. 2020) with 1000 bootstraps and the analysis partitioned by locus. A sample of Erysiphe intermedia (HAL 3381F) was selected as an outgroup (Bradshaw et al. 2024). The tree was formatted using the R package ggtree (Yu et al. 2017). Phylogenetic analysis supports the determination of our sample as Erysiphe alliariicola (Fig 1), a recently described species formerly within E. crucifearum sensu lato (Bradshaw et al. 2024) and previously only reported on garlic mustard (Alliaria petiolata) in Europe. Garlic mustard is an invasive weed in North America, where it has been reported diseased by powdery mildew since at least 2004 (Enright and Cipollini, 2007). Notably, the ITS sequence reported from powdery mildew on garlic mustard in Maryland (Fulcher and Smith, 2022) is a 100 % match with OR424895.1 and those from other E. alliariicola samples (Bradshaw et al. 2024), suggesting a possible origin on invasive garlic mustard, common in Minnesota. While many other Brassicaceae host Erysiphaceae in North America, powdery mildew on T. arvense had been documented only in Europe (Braun 1995). To our knowledge, this is the first report of powdery mildew on T. arvense caused by E. alliariicola in North America.