Yields and the extent and causes of damage in cauliflower, bulb onion, and carrot grown under organic or conventional regimes.

Summary

The effect of organic versus conventional growing practise on yield, quality, pests and diseases of three vegetable crops were examined. Cultivars of cauliflower (Brassica oleracea L. convar. Botrytis), bulb onions (Allium cepa L.) and carrots (Daucus carota) were grown organically and conventionally during the years 2000-2003. Differences in cultivar yield, quality and resistance towards pests were determined. However, none of the cultivars were observed to be superior in the conventional compared to the organic growing system. Yields of conventional cauliflower and onions were higher than when grown organically, whereas no differences in carrot yields were observed between the two systems. A larger percentage of the harvested conventional onions were discarded, reducing the difference in net yield between the two systems. Generally, the reasons for discarding varied between the two systems. In organic cauliflower damage by slugs and loose curds were the main reasons for discarding. Loose curds can be explained by nitrogen deficiency while the damage by slugs is ascribed to the warm and humid environment created under insect nets used in the organic growing system. In both systems, the most severe reason for discarding of onions was exterior watery scales. Rot and mold were more pronounced in the conventional system whereas thick-necked onions were more frequent in the organic system due to larger distance between the onions. Conventional carrots were significantly more damaged by carrot flies than organic carrots even though the flies were present in the organic system and pesticides were applied in the conventional system. In contrast, more deform carrots were seen in the organic system.
Generally, less organically crops were discarded due to pests, which presumably can be explained by the resistance of organic crops towards pests due to the increased diversity of microbial biomass and presence of insect natural enemies. On the contrary, the lower yields in organically grown vegetables can mainly be explained by morphological defects probably created by changes in growing system design in order to facilitate weed management.