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Alternaria leaf blight (ALB) caused by Alternaria dauci (Kühn) Groves and Skolko and Cercospora leaf spot (CLS) caused by Cercospora carotae (Pass.) Solheim are the major foliar diseases of carrot in Ontario, Canada. In addition to reducing photosynthetic area, the diseases can weaken carrot tops, which can break during mechanical harvesting, reducing harvested yields. Fungicides are commonly used to manage the disease, but there is potential to reduce fungicide applications through nitrogen (N) management. Trials were conducted on mineral soils from 2006 to 2008 to determine the importance of applied N and fungicide applications to control fungal leaf blights of carrot. Three rates of N (0, 110, and 220 kg·ha⁻¹) and 0, 3, or 5 (2006 and 2007) or 6 (2008) fungicide applications were applied. Leaf blight severity was assessed biweekly throughout the season and at harvest. The severity of both ALB and CLS and combined disease severity index at harvest decreased with increasing N and fungicide application. In some cases, disease severity of carrots treated with high N and no fungicides was equivalent to carrots treated with no N and five fungicide sprays. Total and marketable yield increased with increasing number of fungicide sprays in 2006 and 2007, but N application did not affect yield. Results suggest that severity of ALB and CLS can be minimized through a combination of N and fungicide applications, but rates of N higher than 110 kg·ha⁻¹ may reduce marketable yield through a decrease in stand and an increase in oversized roots.

Field trials were conducted to evaluate resistance to clubroot (Plasmodiophora brassicae, pathotype 6) in green cabbage (Brassica oleracea var. capitata) and napa cabbage (Brassica rapa ssp. pekinensis) at sites in southern Ontario in 2009 and 2010. The reaction of green cabbage cultivars Kilaton, Tekila, Kilaxy, and Kilaherb and the commercial standard cultivars, Bronco or Atlantis, were evaluated on organic (two site-years) and mineral soils (two site-years) that were naturally infested with the clubroot pathogen. In addition, fluazinam fungicide was drench applied to one treatment of the commercial standard cultivar immediately after transplanting. The napa cabbage cultivars Yuki, Deneko, Bilko, and Mirako (in 2009) and Emiko, Mirako, Yuki, and China Gold (in 2010) were evaluated only on organic soils (two site-years). At harvest, the roots of each plant were assessed for clubroot incidence and severity. Also, plant and head characteristics of the resistant green cabbage cultivars were evaluated at one site in 2010. The green cabbage cultivars Kilaton, Tekila, Kilaxy, and Kilaherb were resistant to pathotype 6 (0% to 3.8% incidence), but ‘Bronco’ was susceptible (64% to 100% incidence). Application of fluazinam reduced clubroot severity on ‘Bronco’ by 6% at one of three sites. Resistance was more effective in reducing clubroot than application of fluazinam. Plant and head characteristics of the resistant cultivars were similar to those of ‘Bronco’ treated with fluazinam. Napa cabbage cultivars Yuki, Deneko, Bilko, Emiko, and China Gold were resistant to clubroot (0% to 13% incidence), and ‘Mirako’ was highly susceptible (87% to 92% incidence). We conclude that the clubroot resistance available in several cultivars of green and napa cabbage was effective against P. brassicae pathotype 6.