Shanghai pak choy [Brassica rapa L. subsp. chinensis (Rupr.) var. communis Tsen and Lee] and Chinese flowering cabbage [Yow choy; B. rapa L. subsp. chinensis (Rupr.) var. utilis Tsen and Lee] were seeded into organic (muck) soil naturally infested with the clubroot pathogen (Plasmodiophora brassicae Woronin) at the University of Guelph Muck Crops Research Station, Ontario, Canada, in June, July, and Aug. 2001 and May, June, July, and Aug. 2002. At harvest, clubroot incidence and disease severity index (DSI) were assessed. Data from 17 different seedings at the research station over 4 years were used to compare the relationship between disease incidence and DSI and weather conditions during crop development. Clubroot incidence and severity were highest for crops harvested in July and August and lowest for crops harvested in October. Mean air temperatures during crop development ranged from 15 to 22 °C and were positively correlated with clubroot incidence and severity for both pak choy (r = 0.68) and flowering cabbage (r = 0.73). The strongest correlations occurred between air temperatures and disease severity over the final 10 d before harvest (r = 0.82 for pak choy; r = 0.84 for flowering cabbage). The research suggests that clubroot damage in Asian Brassica vegetables could be minimized by seeding in early spring and late summer in areas infested with P. brassicae.
Alternaria leaf blight (ALB) and Cercospora leaf spot (CLS) are economically important diseases of carrot in Ontario. Field experiments were conducted in the Holland Marsh, Ontario, to determine the effect of nitrogen (N) application rates on both diseases. Five rates of N were applied to organic and mineral soils in which two carrot cultivars, Idaho and Fontana, were grown in each of 2002, 2003, and 2004. Both diseases were rated every 2 weeks on a scale of 0 (healthy) to 10 (tops destroyed), and the number of live (green) leaves per plant was assessed at harvest. In addition, three N rates were applied to carrot plants grown in the greenhouse, and the plants were inoculated with Alternaria dauci (Kühn) Groves and Skolko. Disease severity, senescence, and sap nitrate-N concentration were assessed. In the field trials, the response of ALB and CLS to N application rate was relatively consistent across cultivar, soil type, and year. Area-under-the-disease-progress curves typically increased with decreasing N rate for both diseases. In lower N treatments, this resulted in fewer live leaves per plant at harvest. In the greenhouse, ALB severity increased with increasing amount of leaf senescence at final assessment. The results suggest that N application rate could be used to reduce the need for fungicide applications to control these diseases in the field.
Residual nitrogen (N) in agricultural soils is an environmental concern. Nitrogen requirements for carrots were examined over a two-year period in Ontario, Canada. Carrots, cvs. `Idaho' and `Fontana' were seeded into organic and mineral soil in 2002 and 2003 in the same plots. In 2002, N was applied at 0%, 50%, 100%, 150%, and 200% of current Ontario recommendations. Each experimental unit was split in half in 2003 with one half getting the same N rate as in 2002, and the second half getting no applied N. At harvest, total and marketable yield, weight per root, and quality were assessed. Yield and quality were unaffected by N rate in both mineral soil in 2002 and organic soil in both years. On mineral soil in 2003 there was a linear increase in total and marketable yield and weight per root with increasing N rate, with total yield ranging between 36 and 55 t/ha. However, in the treatments given no N in 2003, yield also increased from 36 to 47 t/ha between the 2002 no N and high N treatments. Consequently, a large portion of the increase in yield in 2003 on mineral soil was due to residual N from the previous season. Carrots are capable of taking up very high amounts of N from the soil over a growing season, but may only require additional N for maximum total yield if soil N content is low due to low mineralization rates or depletion by a previous crop. However, low N rates can reduce the ability to harvest the crop due to increased carrot leaf blight severity even when yield is unaffected.
An understanding of nitrogen (N) uptake and the partitioning of N during the season by the carrot crop (Daucus carota subsp. sativus [Hoffm.] Arkang.) is required to develop more efficient N fertilization practices. Experiments were conducted on both organic and mineral soils to track the accumulation of dry matter (DM) and N over the growing season and to develop an N budget of the crop. Treatments included two carrot cultivars (`Idaho' and `Fontana') and 5 N rates ranging from 0% to 200% of the provincial recommendations in Ontario. Foliage and root samples were collected biweekly from selected treatments during the growing season and assessed for total N concentration. Harvest samples were used to calculate N uptake, N in debris, and net N removal values. Accumulation of DM and N in the roots was low until 50 to 60 days after seeding (DAS) and then increased linearly until harvest for all 3 years regardless of the soil type, cultivar, and N rate. Foliage dry weight and N accumulation were more significant by 50 to 60 DAS, increased linearly between 50 and 100 DAS, and reached a maximum or declined slightly beyond 100 DAS in most cases. The N application rates required to maximize yield on mineral soil resulted in a net loss of N from the system, except when sufficient N was available from the soil to produce optimal yield. On organic soil, a net removal of N occurred at all N application rates in all years. Carrots could be used as an N catch crop to reduce N losses in a vegetable rotation in conditions of high soil residual N, thereby improving the N use efficiency (NUE) of the crop rotation.
With the introduction of nutrient management legislation in Ontario, there is a need to improve the efficiency of nitrogen (N) utilization. One possibility is to use critical nutrient concentrations in plant tissue as an indicator of the N nutritional status of the crop. Plant tissue analysis was used to determine the total N and nitrate-N (NO3-N) concentrations of cabbage (Brassica oleracea var. capitata L.), carrots (Daucus carota L.), and onions (Allium cepa L.) grown in Ontario. The tissue samples were collected from plants as part of N fertilization studies from 1999 to 2001 on the organic soils in the Holland/Bradford Marsh area and the mineral soils near Simcoe, Ontario. Yield was assessed at harvest as an indicator of the N requirement of the crop. Testing the usefulness of critical NO3-N concentrations to indicate the N requirement of the crop was problematic because: 1) few published references were available to indicate a critical level of NO3-N in these crops; 2) tissue NO3-N concentrations were highly variable; and 3) field data rarely matched published references. Tissue total N concentrations from the trials corresponded to published critical N concentrations in some cases, however, the use of published critical N concentrations would have resulted in either over or under-application of fertilizer to the crops. Cultivar, soil type, and climate were shown to affect tissue N concentrations. Based on these results it was concluded that local research and field verification is required before tissue N critical nutrient concentrations become useful for determining fertilizer needs of cabbage, carrots, and onions grown in Ontario.