Black Leg and Chlorotic Leaf Spot Occurrence on Brassicaceae Crop and Weed Hosts

Black leg (caused by Plenodomus lingam and P. biglobosus) and chlorotic leaf spot (caused by Pyrenopeziza brassicae) are economically important fungal diseases of Brassicaceae crops. Surveys of seed fields and weed hosts were conducted to understand the distribution and prevalence of these diseases in Oregon after black leg and chlorotic leaf spot outbreaks occurred in Brassicaceae crops in 2014. Postharvest black leg ratings were conducted in seed fields of canola, forage rape, and turnip in 2015 and 2016. The incidence of black leg was greater for turnip (51%) than for canola (29%) and forage rape (25%). The overall average disease incidence was greater for seed crops harvested in 2015 (46%) than for crops harvested in 2016 (28%). A disease survey of wild Brassicaceae plants was conducted along Interstate 5 in Oregon. Brassicaceae weed population sites were identified and 40 sites were sampled for these diseases. Black leg and chlorotic leaf spot were present in 60 and 45%, respectively, of the sampled sites. Both species of Plenodomus were detected in weed populations, with P. lingam being the predominant species recovered (95%). The northernmost sample site with black leg was <32 km from the Oregon-Washington border, and the southernmost site with black leg was within 32 km of the Oregon-California border. Chlorotic leaf spot was detected <32 km from the Oregon-Washington border, whereas the southernmost site where it was detected was approximately 164 km from the Oregon-California border. Based on this study, infected crop residues and weed hosts may facilitate the persistence and spread of these pathogens.