Invasive Species Compendium

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Abstract

Influence of postharvest handling practices and dip treatments on development of black root rot on fresh market carrots.

Abstract

Standard postharvest handling practices for fresh market carrots grown in organic soils in the Fraser Valley of British Columbia, Canada, were monitored during 1990-1992 at 2 grading and packaging operations. Root samples were obtained from 29 loads over the harvest season (Jul.-Nov.) at various stages after carrots were washed, sized, graded and packaged. The samples were assessed for the development of black root rot, caused by Thielaviopsis basicola. The disease was not detected on carrots harvested by hand from fields infested with the pathogen and was <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 temp.; however, no visible disease symptoms developed on these carrots when they were stored at 7-10°. The pathogen was detected with a carrot root disc baiting assay in >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 generally increased with each step of the grading process. A lab. study was conducted to determine the effects of wounding, time and frequency of inoculation, and incubation temp. on the development of black root rot. Wounding at harvest, a postharvest inoculation treatment, incubation at 30° for 24 h 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 smallcap˜M solution of either calcium propionate or potassium sorbate for 2 min., disease development was significantly (P=0.05) reduced compared with the standard sodium hypochlorite treatment (100 -g/ml of chlorine). Treatments applied to carrot tissues within 24 h 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 with the water control; but the level of disease control achieved was not economically acceptable.