Interactions among cultivation, weeds, and a bio-fungicide in organic Vidalia sweet onion

Authors: Johnson, Wiley - Carroll; DUTTA, BHABESH - University Of Georgia; SANDERS, F. HUNT - University Of Georgia; LUO, XUELIN - University Of Georgia

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2017
Publication Date: 12/2/2017
Citation: Johnson, W.C., Dutta, B., Sanders, F., Luo, X. 2017. Interactions among cultivation, weeds, and a bio-fungicide in organic Vidalia sweet onion. Weed Technology. 31:890-896.

Interpretive Summary: Weed management in organic Vidalia® sweet onion is largely dependent on multiple cultivations with a tine weeder, but there are concerns that the implement damages onion bulbs possibly causing more diseases in storage. Trials were conducted from 2012 through 2014 near Lyons, GA to determine the interactive effects of cultivation, weed removal, and a bio-fungicide on weed densities, onion yield, and diseases of stored onion. Cultivation 2X or 4X at biweekly intervals with a tine weeder reduced densities of cutleaf eveningprimrose, lesser swinecress, and henbit compared to the non-cultivated control, although weeds surviving cultivation were numerous and were very large at harvest. Cultivation generally improved onion yields over the non-cultivated control, except in 2014 when baseline weed densities were high and weeds surviving cultivation were numerous. Weed removal by handweeding improved onion yields, but that effect was independent of cultivation. Four applications of a bio-fungicide derived from the weed giant knotweed had no effect on onion yield. Cultivation had no effect on the fungal disease Botrytis neck rot, with inconsistent effects on the bacterial diseases center rot and sour skin. Weed removal with handweeding did not affect diseases of stored onion. The bio-fungicide had no effect on diseases of stored onion. These results validate earlier reports of the effectiveness and safety of cultivation with a tine weeder for weed control in organic onion. However, these results also demonstrate the limitations of cultivation when cool-season weed infestations are dense. Cultivation for weed control is much less costly than handweeding. With no interaction between the cultivation and weed removal main effects, there is no reason to supplement cultivation with costly handweeding.

Technical Abstract: Weed management in organic Vidalia® sweet onion (Allium cepa) 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 bio-fungicide on weed densities, onion yield, and diseases of stored onion. Cultivation 2X or 4X at biweekly intervals with a tine weeder reduced densities of cutleaf eveningprimrose (Oenothera laciniata Hill), lesser swinecress (Coronopus didymus (L.) Sm.), and henbit (Lamium amplexicaule L.) compared to the non-cultivated control, although weeds surviving cultivation were numerous and were very large at harvest. Cultivation generally improved onion yields over the non-cultivated control, except in 2014 when baseline weed densities were high and weeds surviving cultivation were numerous. Weeds removed by handweeding improved onion yields, but that effect was independent of cultivation. Four applications of a bio-fungicide derived from the plant Reynoutria sachalinensis (giant knotweed) had no effect on onion yield. Cultivation had no effect on Botrytis neck rot, with inconsistent effects on center rot and sour skin diseases of stored onion. Weed removal with handweeding did not affect diseases of stored onion. The bio-fungicide had no effect on diseases of stored onion. These results validate earlier reports of the effectiveness and safety to organic onion using cultivation with a tine weeder for weed control. However, these results also demonstrate the limitations of cultivation when cool-season weed infestations are dense. Cultivation for weed control is much less costly than handweeding. Therefore, with no interaction between the cultivation and weed removal main effects, there is no reason to supplement cultivation using a tine weeder with costly handweeding.