INFLUENCE OF POSTHARVEST HANDLING PRACTICES AND DIP TREATMENTS ON DEVELOPMENT OF BLACK ROOT-ROT ON FRESH-MARKET CARROTS

Standard postharvest handling practices for fresh market carrots (Daucus carota) grown in organic soils in the Fraser Valley of British Columbia were monitored during 1990-1992 at two grading and packaging operations. Root samples were obtained from 29 loads over the harvest season (July to November) at various stages after carrots were washed, sized, graded, and packaged; and the samples were assessed for the development of black root rot, caused by Chalara elegans (Thielaviopsis basicola). Disease was not detected on carrots harvested by hand from fields infested with the pathogen, and was less than 5% on carrots sampled from the truck following mechanized harvesting. The percentage of carrots that developed disease upon incubation at 25 C increased after each step of the grading and packaging operation (washer, brush rollers, sizer, and grader). The highest disease incidence was observed on carrots that were graded and packaged into polyethylene bags and stored at room temperature; however, no visible disease symptoms developed on these carrots when they were stored at 7-10 C. The pathogen was detected with a carrot root disk baiting assay in more than 60% of the carrot loads, mostly in soil adhering to the roots, and was also found in the wash water and on the surface of the conveyor belts. The inoculum level was generally found to increase with each step of the grading process. A laboratory study was conducted to determine the effects of wounding, time and frequency of inoculation, and incubation temperature on the development of black root rot. Wounding at harvest, a postharvest inoculation treatment, incubation at 30 C for 24 hr, and additional postharvest wounding were all found to significantly (P = 0.05) enhance disease development. When artificially wounded and inoculated carrot roots or root slices were dipped in a 0.05 or 0.1 M solution of either calcium propionate or potassium sorbate for 2 min, disease development was significantly (P = 0,05) reduced compared to the standard sodium hypochlorite treatment (100 mug/ml of chlorine). Treatments applied to carrot tissues within 24 hr after inoculation provided a significantly higher level of disease reduction than those applied just prior to inoculation. The effectiveness of both calcium propionate and sodium hypochlorite was considerably better at low pH than at a higher pH. Ammonium bicarbonate, potassium carbonate, and sodium bicarbonate also reduced disease development compared to the water control; but the level of disease control achieved was not economically acceptable.