First Report of Powdery Mildew Caused by Erysiphe cruciferarum on Camelina sativa in Montana

Camelina sativa (L.) Crantz, also known as false flax, is an annual flowering plant in the family Brassicaceae and originated in Europe and Asia. In recent years, it is cultivated as an important biofuel crop in Europe, Canada, and the northwest of the United States. In June of 2021, severe powdery mildew was observed on C. sativa ‘Suneson’ plants under greenhouse conditions (temperature 18.3°C/22.2°C, night/day) in Bozeman, Montana (45°40'N, 111°2'W). The disease incidence was 80.67% (150 pots, one plant per pot). White ectophytic powdery mildew including mycelia and conidia were observed on the upper leaves, usually developed from bottom tissues to top parts, also present on stems and siliques. Mycelia on leaves were amphigenous and in patches, often spreading to become effused. These typical symptoms were similar to a previous report of powdery mildew on Broccoli raab (Koike and Saenz 1997). Appressoria are lobed, and foot cells are cylindrical with size 18 to 26 × 7 to 10 μm. Conidia are cylindrical and produced singly, with a size of 35 to 50 × 12 to 21 μm and a length : width ratio greater than two (Koike and Saenz 1997). No chasmothecia were observed under the greenhouse conditions. The symptoms and fungal microscopic characters are typical of Pseudoidium anamorph of Erysiphe (Braun 1995). The specific measurements and characteristics are consistent with previous records of Erysiphe cruciferarum Opiz ex L. Junell (Braun and Cook 2012; Vellios et al. 2017). To identify the pathogen, the partial internal transcribed spacer (ITS) region of rDNA of sample CPD-1 was amplified using primers ITS1 and ITS4 (White et al. 1990). The amplicons were sequenced, and the resulting 559-bp sequence was deposited in GenBank (CPD-1, Accession number: OK160719). A GenBank BLAST search of the ITS sequences showed an exact match (100% query cover, E-value 0, and 100% identity 559/559 bp) with those of E. cruciferarum on hosts Brassica sp. (KY660929.1), B. juncea from Vietnam (KM260718.1) and China (KT957424.1). A phylogenetic tree was generated with the CPD-1 ITS sequence with several of ITS sequences of close species with different hosts obtained from the GenBank. Isolate CPD-1 was grouped with pathogens from Brassica hosts rather than the holotype strain (KU672364.1) from papaveraceous hosts. To fulfill Koch's postulates, pathogenicity was confirmed through inoculation by dusting conidia onto leaves of seven healthy, potted, 14-day-old C. sativa seedlings (cv. Suneson). Seven non-inoculated plants served as a control treatment. The plants were incubated in a greenhouse with a temperature of 18°C (night) to 22°C (day). The inoculated plants developed similar symptoms after 7 days, whereas the control plants remained symptomless. The fungus on the inoculated plants was morphologically identical to that was originally observed on the diseased plants. Though many Brassica spp. have been known to be infected by E. cruciferarum throughout the world, powdery mildew of C. sativa cultivar Crantz in natural conditions by E. cruciferarum has been reported only in the province of Domokos in Central Greece (Vellios et al. 2017). To our knowledge, this is the first report of powdery mildew caused by E. cruciferarum on C. sativa in Montana. Though the powdery mildew on C. sativa was observed in the greenhouse conditions in this work, it poses a potential threat to the production of this biofuel crop in the northwest of the United States.

First report of Erysiphe cruciferarum causing powdery mildew of Alliaria petiolata in Maryland

Alliaria petiolata (Bieb.) Cavara & Grande (garlic mustard) is a biennial crucifer native to Europe and invasive in North America, where it outcompetes native plants in deciduous forests. In July 2021, powdery mildew was observed on A. petiolata in Frederick County, Maryland. Signs of the disease included white, tomentose mycelium producing abundant conidia (Fig S1). A majority of plants were affected, and severity ranged from the presence of small, discrete infections to complete colonization of leaves, stems, and ripening seed pods. Conidia from field collected leaves were transferred to disease-free A. petiolata for maintenance in a growth chamber at 20°C and 80% RH with a 12 hr photoperiod. Fungal morphology was recorded 30 days after this transfer. Appressoria were irregularly lobed, and conidiophores were straight and composed of 2-3 cells. Cylindrical to oblong conidia were produced singly in pseudochains of 2-6 (x̄ = 3), measured 39-64 by 18-29 (x̄ = 52 by 24) μm, had a length to width ratio greater than two, and germinated at the ends. Fibrosin bodies were absent from conidia, and chasmothecia were not observed in the field or on inoculated plant material. Based on anamorphic characteristics, the pathogen was placed in the genus Erysiphe (Boesewinkel 1980). Species level identity was determined using DNA sequences. Conidia and mycelia were scraped from leaves and used for genomic DNA extraction with the Quick-DNA Fungal/Bacterial Miniprep Kit (Zymo Research, Irvine, CA). A portion of the internal transcribed spacer region of rDNA was amplified using the primers ITS5/ITS4 (White et al. 1990). Purified amplicons (PCR & DNA Cleanup Kit, New England BioLabs Inc., Ipswich, MA) were sequenced at Eurofins Genomics (Louisville, KY). The resulting sequence was compared to those in NCBI GenBank using the blastn algorithm (Altschul et al. 1990). The newly generated sequence (GenBank: OK157430) was identical (599/599 bp) to samples of E. cruciferarum from the United Kingdom (GenBank: KY660931.1, KY660879.1, KY660752.1). Because E. cruciferarum sensu lato is heterogeneous (Pastirčáková et al. 2016), additional sequence comparisons were made to the E. cruciferarum sensu stricto holotype (589/599 bp, GenBank: KU672364) and a vouchered E. cruciferarum s. lat. (596/599 bp, GenBank: LC009943). This supports identification of the pathogen as E. cruciferarum s. lat. and suggests the taxonomy of isolates from A. petiolata should be reassessed following any revision to E. cruciferarum. A modified Koch’s postulates procedure was followed to confirm pathogenicity. Leaves colonized by E. cruciferarum were briefly pressed against the leaves of three disease-free plants grown from seed in a greenhouse. After 14 days, inoculated plants showed signs of powdery mildew similar to those observed in the field, and a control treatment using pathogen-free leaves resulted in no disease. This inoculation experiment was performed twice, and the identity of the pathogen was reconfirmed based on morphology. This is the first report of powdery mildew on A. petiolata in Maryland. Erysiphe cruciferarum s. lat. is widely distributed on other hosts and has been found on A. petiolata throughout Europe and in Ohio and Indiana (Farr and Rossman 2021; Blossey et al. 2001; Enright and Cipollini 2007; Ciola and Cipollini 2011). This pathogen has been proposed as a biological control agent (Cipollini and Enright 2009; Cipollini et al. 2020), and the presence of disease in Maryland suggests the local population of A. petiolata is susceptible to E. cruciferarum and the environment there is favorable to disease development. References: Altschul, S. F., et al. 1990. J. Mol. Biol. 215:403. Blossey, B., et al. 2001. Nat. Areas J. 21:357. Boesewinkel, H. J. 1980. Bot. Rev. 46:167. Ciola, V., and Cipollini, D. 2011. Am. Midl. Nat. 166:40-52. Cipollini, D., and Enright, S. M. 2009. Invasive Plant Sci. Manag. 2:253. Cipollini, D., et al. 2020. Biol. Invasions. 22:1657-1668. Enright, S. M., and Cipollini, D. 2007. Am. J. Bot. 94:1813. Farr, D. F., and Rossman, A. Y. 2021. Fungal Databases, Syst. Mycol. Microbial. Lab., ARS, USDA. https://nt.ars-grin.gov/fungaldatabases/ Pastirčáková, K., et al. 2016. Mycol. Prog. 15:36 White, T. J., et al. 1990. Page 315 in: PCR Protocols. A Guide to Methods and Applications, Academic Press, San Diego. Keywords: Alliaria petiolata, Erysiphe cruciferarum, garlic mustard, powdery mildew, invasive plant, biocontrol Funding and Disclaimer: The author(s) declare no conflict of interest. This work was supported by USDA-ARS Appropriated Project Number 8044-22000-047-000-D. Mention of trade names or commercial products in this report is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity lender, provider, and employer.

Powdery Mildew Caused by Erysiphe cruciferarum on Wild Rocket (Diplotaxis tenuifolia): Hyperspectral Imaging and Machine Learning Modeling for Non-Destructive Disease Detection

Wild rocket is a widely cultivated salad crop. Typical signs and symptoms of powdery mildew were observed on leaves of Diplotaxis tenuifolia, likely favored by climatic conditions occurring in a greenhouse. Based on morphological features and molecular analysis, the disease agent was identified as the fungal pathogen Erysiphe cruciferarum. To the best of our knowledge, this is the first report of E. cruciferarum on D. tenuifolia. Moreover, the present study provides a non-destructive high performing digital approach to efficiently detect the disease. Hyperspectral image analysis allowed to characterize the spectral response of wild rocket affected by powdery mildew and the adopted machine-learning approach (a trained Random Forest model with the four most contributory wavelengths falling in the range 403–446 nm) proved to be able to accurately discriminate between healthy and diseased wild rocket leaves. Shifts in the irradiance absorption by chlorophyll a of diseased leaves in the spectrum blue range seems to be at the base of the hyperspectral imaging detection of wild rocket powdery mildew.

POWDERY MILDEW (ERYSIPHE CRUCIFERARUM) INFECTION ON CAMELINA (CAMELINA SATIVA) UNDER MEDITERRANEAN CONDITIONS AND THE ROLE OF WILD MUSTARD (SINAPIS ARVENSIS) AS ALTERNATIVE HOST OF THIS PATHOGEN

During the growing season 2014-2015, camelina (Camelina sativa (L.) Crantz) plants were naturally infected by powdery mildew at the province of Domokos in Central Greece. Moreover, wild mustard (Sinapis arvensis L.) plants, naturally occurring in camelina field, were infected by the disease. The environmental conditions that influence infection of camelina by powdery mildew were recorded. The anamorph as well as the teleomorph of the fungus pathogen were observed on symptomatic leaves, stems and fruits of camelina and wild mustard plants. Fungal specimens were examined. Conidiophores were cylindrical and comprised 3 cells, while conidia were produced singly; they were oblong to cylindrical and measured 21.8 to 40.5 × 9.4 to 18.4 μm (average 32.1 × 13.9 μm). Chasmothecia were also observed on the upper surface of camelina or wild mustard leaves and on wild mustard siliques. Immature chasmothecia were globose, yellow and turned dark brown as reached maturity. Each chasmothecium contained six asci that measured 47.6 to 74.5 × 29.8 to 44.7 μm. Based on these characteristics the pathogen was identified as Erysiphe cruciferarum Opiz ex L. Junell, being probably the first report of E. cruciferarum on camelina in Greece.