Alternaria leaf blight and CLS caused by the mitosporic fungi Alternaria dauci (Kühn) Groves & Skolko and Cercospora carotae (Pass.) Solheim, respectively, are the major foliar diseases of carrot [Daucus carota L. subsp. sativus (Hoffm.) Arcang] in Canada (Carisse and Kushalappa, 1992) and in many other regions of the world (Ben-Noon et al., 2001; Gugino et al., 2004; Pryor and Strandberg, 2002). Both A. dauci and C. carotae infect leaves and petioles of carrot causing damage to the carrot foliage. Alternaria leaf blight symptoms occur on mature or senescing foliage and are characterized by irregularly shaped necrotic lesions along leaf margins (Hooker, 1944). Cercospora leaf spot symptoms occur on both mature and new foliage and include small necrotic lesions surrounded by a chlorotic halo (Thomas, 1943).
In Ontario, the largest carrot producer in Canada, 3683 ha of carrot were grown in 2012 resulting in a farm gate value of $34.5 million [Ontario Ministry of Agriculture and Food (OMAF, formerly the Ontario Ministry of Agriculture, Food and Rural Affairs, OMAFRA), 2013]. Yield losses of carrot resulting from fungal leaf blights can be considerable with as much as 40% to 60% of the yield lost (Vintal et al., 1999). Losses are associated with reduction of the leaf photosynthetic area and weakening of the petioles induced by the leaf blight pathogens. Infected and weakened petioles often break during mechanical harvest, leaving the root in the ground (Langenberg, 1975). These remaining roots cannot be economically harvested. Therefore, vigorous and healthy carrot foliage is essential to maximize carrot yield.
Host nutrition is an important factor affecting the severity of fungal blights of carrot. Fertilization can influence the resistance of plants to pathogens (Huber and Watson, 1974; Marschner, 1995) by delaying leaf senescence. A relationship between fertilization practices with phosphorous (P), potassium (K) and N and leaf blight on tomato (Lycopersicon esculentum Mill.) and potato (Solanum tuberosum L.) has been established, but the reported effects were inconsistent and varied by pathogen, crop evaluated, and type and formulation of the fertilizers applied (Blachinski et al., 1996; MacKenzie, 1981; Soltanpour and Harrison, 1974; Vintal et al., 1999; White et al., 1983). For instance, on carrot, ALB severity was reduced by increasing N application rate in Michigan (Warncke, 1996), Ontario (Westerveld et al., 2002), and Israel (Vintal et al., 1999). For CLS, severity decreased with increasing N application rate in Ontario (Westerveld et al., 2008) but increased with increasing N application rate in controlled environment studies (Thomas, 1943). No relationship was found between CLS and foliar N concentrations in the field in another study, but this study did not examine the direct effect of N application rate on disease severity (Tremblay and Charbonneau, 1993).
Because excessive rates of fertilizers can be harmful to the environment and to subsequent crops, excessive fertilization is not recommended. Research conducted with crops such as tomato, potato, and cotton in which different rates of fertilizers and fungicides were combined to control leaf blight caused by Alternaria spp. showed that crops should be fertilized for optimal yield and ALB should be managed by properly timed applications of fungicides during the growing season (Blachinski et al., 1996; Vintal et al., 1999). In relation to fungal leaf blights of carrot, a recent study conducted in Ontario suggested that N application rate could be used to reduce the need for fungicide applications to control disease in the field (Westerveld et al., 2008). In this study, we intended to investigate if N application could be used to replace some fungicide sprays on carrots, thus reducing grower input costs. Therefore, the main objectives of this study were: 1) to evaluate the effect of N rates and the number of fungicide applications for control of ALB and CLS of carrot; 2) to determine if the N application rates that reduced the severity of fungal leaf blights on carrots could replace some of the fungicide applications; and 3) to determine yield responses as a result of the combined effect of N and fungicide applications.
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