Oakes Irrigation Research
Carrington Research Extension Center * North Dakota State University
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EFFECT OF PREHARVEST SPRAYING OF BENOMYL AND ASTER YELLOWS ON STORAGE LIFE OF CARROTS
Robert W. Stack, Neil C. Gudmestad, and Chiwon Lee. Depts. of Plant Pathology and Plant Sciences, NDSU, Fargo, and Richard Greenland, Oakes Irrigation Research Site
Interest in commercial production of carrots has been increasing in North Dakota for several years. Yield and quality are excellent, comparing favorably with the best crops from states with milder climates. One impediment to wider cultivation of carrot in the north is the need to store the crop for an extended winter period. In 1996, growers reported substantial losses in carrots stored for 3 to 5 months. Losses were due to decay, mainly caused by white mold (Sclerotinia sclerotiorum). White mold rot of carrots in storage generally starts at the crown where the fungus have colonized senescent leaf bases prior to harvest. A pre-harvest spray of benomyl fungicide has been recommended elsewhere for control of this problem.
In 1997, carrots (cv 'Navajo') were grown in beds 4.5 ft wide by 150 ft long as for commercial production. Five beds were chosen for this experiment. One half of each bed was sprayed with benomyl fungicide several days before harvest. Prior to lifting, carrots had been mechanically topped. Only commercially harvestable carrots were retained. Carrots were washed by hand and sorted. Carrots with trace symptoms of aster yellows (fine side roots, multiple crown buds) were separated out and those from the non-benomyl sprayed half-plots were retained as a third treatment. Representative samples of carrots with each symptom were tested by PCR and presence of the aster yellows phytoplasma confirmed. After washing, sound carrots were packed into doubled heavy plastic bags - 10 bags per treatment. Bags held approximately 12 lbs of carrots each. Bags were then placed in crates in a cold storage room at 38 F. Two months after storage a few bags were examined. Symptoms of rot were absent and the study was continued for another 1.5 month.
In February 1998, at 16 weeks after harvest, all carrots were removed from the cooler, bags opened, and carrots examined for symptoms and signs of decay. Carrots were scored individually for white mold, other rots, and where possible for point of entry where decay began. The benomyl preharvest spray increased the proportion of sound carrots remaining after 16 weeks to 56%, up from 36% in the non-sprayed control. Among the carrots from the nonsprayed half plots, those with slight aster yellows symptoms did not store well; the proportion of sound carrots after storage was just 2.5%, compared to 36% for the symptomless carrots.
In 1998, the study was repeated with a few changes from the 1997 study, as follows. Two carrot cultivars were used and entire plots adjacent to each other were either benomyl sprayed or left non-sprayed at 10 days before harvest. In 1998, plots had a more thorough summer treatment to control leafhoppers, and only 2.5% of harvested carrots showed aster yellows symptoms (vs. 20+% at harvest in 1997). There were insufficient numbers of aster yellows infected carrots to fill out a separate third storage treatment in 1998. Carrots were graded, washed and stored as with the 1997 crop. As of this writing, the 1998 carrots are in cold storage. Scoring will be done in February 1999.
Based on the first year's results, a preharvest benomyl spray, combined with more careful pre-storage grading out of roots with even slight aster yellows symptoms, should substantially improve storability of carrots produced in North Dakota.
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