Interactions among Cultivation, Weeds, and a Biofungicide in Organic Vidalia® Sweet Onion

Weed management in the organic Vidalia® sweet onion production system is largely dependent on multiple cultivations with a tine weeder. Earlier research suggested cultivation with a tine weeder did not predispose onion bulbs to infection during storage. Trials were conducted from 2012 through 2014 near Lyons, GA, to determine the interactive effects of cultivation, weed removal, and a biofungicide on weed densities, onion yield, grade, and diseases of stored onion. Cultivation twice or four times at biweekly intervals with a tine weeder reduced densities of cutleaf evening-primrose, lesser swinecress, and henbit compared with the noncultivated control, although weeds surviving cultivation were very large and mature at harvest. Cultivation generally improved onion yields over the noncultivated control, except in 2014, when baseline weed densities were high and weeds surviving cultivation were numerous. Weeds removed by hand weeding improved onion yields, but that effect was independent of cultivation. Four applications of a biofungicide derived from giant knotweed had no effect on onion yield. Cultivation had no effect on incidence of the fungal disease botrytis neck rot, with inconsistent effects on the bacterial diseases center rot and sour skin. Weed removal with hand weeding did not affect diseases of stored onion. The biofungicide had no effect on diseases of stored onion. These results demonstrate the limitations of cultivation when cool-season weed infestations are dense. With no interactions among main effects, weed control and onion yield response to cultivation and hand weeding are independent. Cultivation for weed control is much less costly than hand weeding. With no interaction between the cultivation and weed removal main effects, it is not necessary to supplement tine weeder cultivation with costly hand weeding. Nomenclature: Cutleaf evening-primrose, Oenothera laciniata Hill; giant knotweed, Reynoutria sachalinensis (F. Schm.) Nakai; henbit, Lamium amplexicaule L; lesser swinecress, Coronopus didymus (L.) Sm.; dry-bulb onion, Allium cepa L.