Cruciferous weeds do not act as major reservoirs of inoculum for black rot outbreaks in New York State

Authors: LANGE, HOLLY - Cornell University; Tancos, Matthew; SMART, CHRISTINE - Cornell University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/4/2021
Publication Date: 8/5/2021
Citation: Lange, H.W., Tancos, M.A., Smart, C.D. 2021. Cruciferous weeds do not act as major reservoirs of inoculum for black rot outbreaks in New York State. Plant Disease. 106:174-181. https://doi.org/10.1094/PDIS-05-21-0998-RE.
DOI: https://doi.org/10.1094/PDIS-05-21-0998-RE

Interpretive Summary: Weeds are an annual problem for growers due to their roles in reducing yields, contaminating harvests, and acting as reservoirs for insect and plant pathogen populations. Cruciferous weeds are widely prevalent in Brassica fields (e.g., cabbage, broccoli, cauliflower, etc.) and have been shown to harbor diverse bacterial pathogens, including economically damaging Xanthomonas campestris species that cause black rot and leaf spot of cruciferous vegetables. However, the importance of weeds as inoculum sources for X. campestris outbreaks in New York State remains unknown. Cruciferous weeds that were sampled within/near fields, which previously had severe black rot outbreaks or were rotating between cabbage, possessed genetically diverse strains of X. campestris, but the identification of disease-causing X. campestris weed isolates was rare. There was no evidence of cabbage-infecting X. campestris weed isolates persisting in an infected field by overwintering in weed hosts; however, similar cabbage and weed X. campestris strains were identified over a three-year period. Our findings indicate that while X. campestris isolates can infect both cruciferous weeds and crops, X. campestris weed isolates do not appear to act as primary sources of inoculum in New York.

Technical Abstract: Cruciferous weeds have been shown to harbor diverse Xanthomonas campestris species, including the agronomically-damaging black rot of cabbage pathogen, Xanthomonas campestris pv. campestris. However, the importance of weeds as inoculum sources for X. campestris pv. campestris outbreaks in New York remains unknown. In order to determine if cruciferous weeds act as primary reservoirs for X. campestris pv. campestris, fields that were rotating between cabbage or that had severe black rot outbreaks were chosen for evaluation. Over a consecutive three-year period, 148 cruciferous and non-cruciferous weed samples were collected at 34 unique sites located across five New York counties. Of the 148 weed samples analyzed, 48 X. campestris isolates were identified, with a subset characterized using multilocus sequence analysis. All X. campestris isolates originated from weeds belonging to the Brassicaceae family with predominant weed hosts being shepherd’s purse (Capsella bursa-pastoris), wild mustard (Sinapis arvensis), yellow rocket (Barbarea vulgaris), and pennycress (Thlaspi arvense). Identifying pathogenic X. campestris weed isolates was rare with only eight isolates causing brown necrotic leaf spots or typical V-shaped lesions on cabbage. There was no evidence of cabbage-infecting weed isolates persisting in an infected field by overwintering in weed hosts; however, similar cabbage and weed X. campestris haplotypes were identified in the same field during an active black rot outbreak. Xanthomonas campestris weed isolates are genetically diverse both within and between fields, but our findings indicate that X. campestris weed isolates do not appear to act as primary sources of inoculum in New York.