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

Authors: Fulcher, Michael; Owen Smith, Paul

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/23/2021
Publication Date: 3/29/2022
Citation: Fulcher, M.R., Owen Smith, P.C. 2022. First report of Erysiphe cruciferarum causing powdery mildew of Alliaria petiolata in Maryland. Plant Disease. 106(5):1532. https://doi.org/10.1094/PDIS-09-21-2060-PDN.
DOI: https://doi.org/10.1094/PDIS-09-21-2060-PDN

Interpretive Summary: Invasive plants introduced to the United States from other countries can disrupt natural ecosystems and compete with desirable native plant species. Plant diseases can influence the success of invasive plants, and plant pathogens (fungi, bacteria, and viruses) may be used to manage the spread of invasive plants. Garlic mustard is a common invasive plant in the eastern United States, where it covers forest floors and excludes native vegetation, but there is very little information available about the diseases affecting garlic mustard. In this research project, a disease of garlic mustard found in Frederick County, Maryland was identified and described. The disease is called powdery mildew and is caused by a fungus. This fungus has been found previously in Europe and some other U.S. states. The fungus could potentially be used to slow the spread of garlic mustard or prevent it from outcompeting native plants. This research is valuable to scientists studying whether plant diseases can control garlic mustard in different environments and benefits forest and land managers who are looking for new options to control garlic mustard.

Technical Abstract: 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 (x¯ = 3), measured 39-64 by 18-29 (x¯ = 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) and 99.64% similar (596/599 bp) to a vouchered specimen of E. cruciferarum from Korea (GenBank: FJ548627.1). Based on morphological and molecular evidence, the pathogen was determined to be E. cruciferarum Opiz ex. L. Junell (Junell 1967). 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 is widely distributed on other hosts and has been found on A. petiolata throughout Europe and in Ohio and Indiana, though previous North American incidences have not been confirmed with molecular evidence (Farr and Rossman 2021; Blossey et al. 2001; Enright and Cipollini 2007). This pathogen has been proposed as a biological control agent (Cipollini and Enright 2009), and the presence of disease in Maryland in addition to other states suggests the North American A. petiolata population is susceptible to E. cruciferarum and the environment across the weed’s introduced range may be favorable to disease development.