Integrated systems of weed management in organic "Vidalia" onion

Authors: Johnson, Wiley - Carroll; LANGSTON, D.B - University Of Georgia; TORRANCE, R.L. - University Of Georgia

Submitted to: Proceedings of Southern Weed Science Society
Publication Type: Proceedings
Publication Acceptance Date: 1/31/2010
Publication Date: 12/17/2010
Citation: Johnson, W.C., Langston, D., Torrance, R. 2010. Integrated systems of weed management in organic "Vidalia" onion. Proceedings of Southern Weed Science Society Annual Meeting, January 25-28, 2010, Little Rock, AR. 63:165.

Interpretive Summary:

Technical Abstract: Vidalia® dry-bulb onion are a multi-million dollar crop grown exclusively in a twenty county region in southeastern Georgia, as defined by a Federal Marketing Order issued in 1989. Plantings in 2009 were 11,500 harvested acres, with an estimated gross value of $9,440/A. Acreage of certified organic Vidalia® onion is driven by strong consumer demand and prices received for organic Vidalia® onion are often double the value of conventional Vidalia® onion. Currently, weed control is the primary challenge for organic Vidalia® onion, with producers frequently investing up to $1,000/A for handweeding. Field experiments were initiated in 2007 to develop integrated systems of weed management in organic Vidalia® onion. Trials were conducted at the Vidalia Onion and Vegetable Research Farm near Lyons, GA. Treatments were a factorial arrangement of summer solarization, cultivation, and herbicides appropriate for use in certified organic production systems. Plots were solarized with clear plastic during the summer months prior to transplanting onion, with the mulch removed in October. Cultivation treatments used an Einbock® tine weeder, with cultivation twice at 2-wk intervals, four times at 2-wk intervals, and non-cultivated. Herbicides were clove oil (Matratec®) plus vinegar, clove oil plus a petroleum oil insecticide (Saf-T-Side®) used as an adjuvant, and a non-treated control. Vidalia® onion (‘Savannah Sweet’) were transplanted in December of 2007 and 2008, with cultivation and herbicide application events occurring the following January and February both seasons. Onion were harvested and graded in April each year. Sub-samples of onion bulbs were collected from each plot and stored in a controlled atmosphere storage facility at the University of Georgia Tifton Campus. After 120 days, the sub-samples were rated for fungal and bacterial diseases that developed during storage. Weeds present in the trials were cutleaf eveningprimrose and swinecress. Preplant solarization during the previous summer did not control the cool-season weeds present in these trials, along with no effect on onion yield. It is theorized that dormant, cool-season weed seed are immune from solarization during the previous summer. However, effects of solarization on disease incidence in stored onion are inconclusive. Cultivation with a tine weeder twice at two-week intervals was equally effective in controlling cutleaf eveningprimrose and swinecress as four cultivations, with both cultivation regimes increasing onion yield compared to the non-cultivated control. Cultivating onion with a tine weeder did not injure onion, provided that the cultivator was carefully adjusted prior to each cultivation event. To date, cultivation with a tine weeder neither damaged nor bruised onion bulbs sufficiently to increase incidence of diseases in stored onion. Clove oil treatments provided minimal weed control and had no effect on onion yield. Previous experience with clove oil showed contact activity on warm-season dicot weeds, under summer conditions. This was not the case with cool-season weeds under winter conditions. Efforts were made to apply the clove oil herbicide treatments during periods of mild temperature in January and February. However, visual estimates of weed control and weed counts indicated minimal benefit of clove oil for weed control in transplanted Vidalia® onion. These data conclusively show that cultivation with a tine weeder effectively controlled weeds in transplanted Vidalia® onion and protected onion yields without the need for handweeding. Cultivation with the tine weeder did not cause physical damage to the onion bulbs and there was no increase in diseases of stored onion. A tine weeder has been a proven implement for mechanical weed control in other organic crop production systems and overall results with transplanted onion are