Alternaria leaf blight (ALB), caused by Alternaria dauci (Kühn) Groves and Skolko, and Cercospora leaf spot (CLS), caused by Cercospora carotae (Passerini) Solheim, are important diseases of carrot crops in Ontario. Alternaria leaf blight symptoms usually appear in early August and the percent of foliage affected increases until harvest. The disease predominates on older and senescing leaves (Rotem, 1994; Soteros, 1979). Cercospora leaf spot symptoms typically appear in early July and continue until harvest. The disease can occur anywhere on carrot foliage but is usually most severe on younger leaves (Kushalappa, 1994). Both diseases begin on leaflets and can spread to petioles. Severe epidemics of either disease can lead to leaf senescence and general weakening of the foliage. This can cause substantial reductions in yield because the foliage may break off during mechanical harvest, leaving marketable carrots unharvested (Langenberg, 1975).
Research has established a relationship between fertilization practices and the severity of ALB alone or the ALB/CLS complex. The severity of ALB was decreased by increasing levels of fertilizer application (nitrogen, phosphorus, and potassium) in Israel (Vintal et al., 1999) and Florida (White et al., 1983), possibly as a result of delayed leaf senescence at higher rates of fertilizer application and, as a result, delayed infection of the foliage by A. dauci (Vintal et al., 1999). Studies examining the ALB/CLS complex showed that severity of the disease was reduced by increasing nitrogen (N) fertilizer application rate in Michigan (Warncke, 1996) and Ontario (Westerveld et al., 2002). However, the response of CLS to N fertilization is not as well defined as for ALB. Cercospora leaf spot severity in the greenhouse increased with increasing N concentration in the nutrient solution (Thomas, 1943), and in a field study, no relationship was found between CLS severity and leaf N concentration (Tremblay and Charbonneau, 1993). To our knowledge, no trials have yet been done to examine the effects of N application rate apart from phosphorus (P) and potassium (K) application on ALB and CLS individually in the field, but this research is necessary to adjust recommended N rates appropriately based on the effect of N rate on ALB and CLS severity.
There is a possibility that a reduction in recommended N rates may be legislated for vegetable crops in Ontario based on the Nutrient Management Act and the Clean Water Act in the province (Government of Ontario, 2002, 2006). This probably will not have much direct effect on carrot yield because there is no yield response of carrot crops to N on organic soils in Ontario, and the response of carrot crops on mineral soil is minimal unless residual soil N concentrations at seeding are very low (Westerveld et al., 2006a). However, the potential interactions between N nutrition of a carrot crop and severity of ALB and CLS could complicate N management of the crop. To develop fertilization strategies that will minimize ALB and CLS severity while optimizing carrot yields, research is required to determine whether N fertilizer apart from P and K application has an effect on either disease. In addition, it is important to determine which of the two diseases is influenced by N nutrition of the crop, because the severity of one disease in relation to the other is dependent on weather and location. Furthermore, studies are required to determine the mechanism of interactions between N fertilization and ALB and CLS severity to identify fertilization practices that may help reduce disease severity. It would also be valuable to determine if the effects of N application rates on disease severity are consistent among carrot cultivars.
The objectives of this study were to identify: 1) the effects of N application rate apart from P and K application on ALB and CLS severity on two carrot cultivars with different levels of susceptibility to the two diseases on each of two soil types; 2) the relationship between ALB and CLS severity and nitrate-N (NO3-N) concentration in carrot leaves; and 3) the nature of the relationship between leaf senescence and ALB severity.
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