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Combining Scouting and Tolerant Cultivars for the Effective Management of Leaf Blight Diseases of Carrots 2000

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Project Leaders: George Abawi and John Ludwig, Dept. of Plant Pathology. NYSAES, Cornell University, Geneva, NY 14456

Cooperators: Carol MacNeil, Coop. Ext., Ontario CCE, Canandaiqua; Chuck Bornt, Coop. Ext., Lake Plains Region, Albion; Gib Scott, Agrilink Foods, Phelps; Nellie Call, Private Consultant, Batavia; Don Sweet, Private Consultant, Batavia; Peter Call, Grower, Batavia; Edward Hansen, Grower, Stanley; Larry Christenson, Grower, Hall; Bob McCarthy, Private Consultant

Assistants: Christine Vogel, Rebecca Gerhart, Scott VanNostrand

Abstract:

Leaf blight diseases continue to be a major factor in the production of carrots in New York. Alternaria dauci has been considered as the principal pathogen of carrot blight in New York, whereas only a low incidence of leaf blight caused by Cercospora carotae was observed previously. Yield losses can be significant if blight development occurs early in the season. In addition, the development of severe blight diseases will result in pre-mature defoliation of carrots, thus reducing the efficiency of harvesting the crop and increasing yield losses. Management of these diseases is based primarily on the use of fungicide applications, usually starting in late June and continuing throughout the long carrot season. Currently, Bravo 720 is the most often used fungicide by carrot growers and is applied at the rate of 2 pts./A and as many as 8 times/season. Results of extensive trials conducted during 1997-199 have validated the scouting program and the critical threshold level of 25% infected leaves as the trigger for the first fungicide application. These trials also documented that available carrot varieties differed greatly in their tolerance to Alternaria-leaf blight. In 1999, the carrot cultivars Carson, Bolero, Ithaca, Calgary, Fullback and Neal were among the most tolerant to leaf blight. Thus, the objectives of the IPM program in 2000 were to re-confirm the reaction of carrot varieties to leaf blight diseases and to demonstrate the cost-benefit of combining the scouting program with the use of tolerant varieties in the management of these diseases.

The unusual wet and rather cool weather conditions, specially the low night temperatures, that prevailed during the 2000 growing season greatly affected the development and prevailing pathogen of leaf blight d. In contrast to previous years, Cercospora carotae was the predominate pathogen causing leaf blight during this growing season. Alternaria-leaf blight appeared late and was only light in incidence and severity. Alternaria-leaf blight appeared late and was only light in incidence and severity. It is known that the development of Alternaria-leaf blight is favored by warm night temperatures.

Results of the replicated variety trial showed that the severity of Cercospora-leaf blight development on the 21 varieties included in the test varied significantly and ranged from moderate to very severe. Bergen, Bristol, Neal, Bolero, Eagle and Fullback exhibited lower severity of Cercospora-leaf blight, whereas Napa, Fontana, Canada, Goliath, Calgary and Ithaca were among the most severely affected varieties. Not surprisingly, the reaction of a number of the carrot variety to Cercospora-leaf blight differed from their reactions to Alternaria-leaf blight that was determined in previous years. Interestingly, Bolero, Neal, and Fullback appear to be tolerant to both leaf blight diseases caused by either Cercospora or Alternaria.

Three carrots fields of collaborating growers were selected to demonstrate the combined impact of scouting and the use of tolerant varieties in the management of leaf blight diseases. In each field, leaf blight development on Carson was monitored and compared to its development on Eagle, Bergen, or Canada. Based on previous data, Carson was selected as the tolerant variety, whereas Eagle, Bergen and Canada were selected as the susceptible varieties to Alternaria-leaf blight. Results obtained again demonstrated the effectiveness and practical use of the 25% infected leaves as the trigger for the first fungicide application. It was encouraging that the first spray was not applied until July 17, July 19 and August 11 in the selected fields. The latter is specially significant since unusually wet weather conditions that are favorable to leaf blight development occurred earlier than normal during the growing season. Thus, the monitoring program for timing the first spray greatly contribute to reducing the number of fungicide applications, even during weather conditions that favor disease development. Also, the epidemic development of Cercospora-leaf blight rather than the usual Alternaria-leaf blight in 2000 made it difficult to accurately assess the impact of combining the scouting program with the use of tolerant varieties in the control of these diseases. However, a knowledge was gained on the reaction of the carrot varieties to the Cercospora-leaf blight pathogen. Actually, three varieties (Bolero, Neal and Fullback) have exhibited tolerant reaction to the two fungal pathogens of leaf blight diseases. In 2000, Bergen (which is tolerant to Cercospora, but susceptible to Alternaria) was sprayed only 4 times, whereas Carson (Tolerant to Alternaria, but moderately susceptible to Cercospora) was sprayed 6 or 7 times, depending of the location.

The on-going IPM project on carrots has demonstrated the available options and strategies for the effective management of leaf blight diseases, reduced the number of needed fungicide applications, thus it is contributing to improving the economics of carrot production and its IPM labeling.