Contributed by Marcelo L. Moretti and David R. King, Oregon State University, Department of Horticulture
Pyroxasulfone, marketed as Zidua SC and sold by BASF, is a preemergence herbicide in WSSA group 15, a very-long-chain fatty acid inhibitor. Although pyroxasulfone is registered in many annual crops and some perennials, nonbearing hazelnuts are the only tree crop with a registered use in Oregon. Hazelnuts are very tolerant to pyroxasulfone, with registered rates up to twice those for other crops, raising questions about how best to integrate Zidua SC into your weed management program.
Label recommendations and weed control
Zidua SC can be applied as a single application of 4 to 8.25 fl oz/A to nonbearing hazelnuts grown in Oregon (EPA SLN No. OR-230008). However, it can only be applied to crops grown in medium and fine soil with at least 1 % organic matter. Major weeds controlled by Zidua SC include annual grasses such as annual bluegrass, Italian ryegrass, crabgrass, and barnyard grass. It also controls broad leaves like pigweeds, nightshades, and lambsquarters. Zidua SC can suppress yellow nutsedge. Wild carrot is an important weed not controlled by Zidua SC.
Our experience in hazelnuts and areas to expand the label
We applied Zidua SC in the fall of 2023, alone or in combination with other preemergence herbicides. We observed complete control of annual bluegrass, northern willowherb, common groundsel, and chickweed for the first three months after treatment. Weed emergence was initiated in early March, four months after treatment.
Zidua SC performed best in mixtures with other preemergence herbicides (Figure 1). When used alone, Zidua SC at 4 to 8.25 fl oz/A provided up to 4 months of weed control and was followed by the emergence of annual bluegrass, northern willowherb, and common groundsel. However, mixtures of Zidua SC provided near complete control at five months.
We cannot overstate the importance of selecting proper herbicide partners for Zidua SC. Mixtures of Zidua SC with Chateau (flumioxazin) or Prowl H2O (pendimethalin) provided the best results after five months (Figure 2). A mixture of Zidua SC with Matrix was effective when both herbicides were used at the maximum allowable rate. Performance of these mixtures of Zidua SC were comparable to indaziflam (Alion) at 5 fl oz, which was included as a reference. Two important messages must be emphasized here:
- Zidua SC (pyroxasulfone) plus Prowl H2O (pendimethalin) is a highly effective treatment that can be used after hazelnut transplanting.
- Zidua SC mixed with Chateau (flumioxazin), Prowl H2O (pendimethalin), or Matrix are effective options to manage herbicide resistance. These herbicides have different modes of action than indaziflam, which has been used year after year in some orchards, increasing the risk of resistance and loss of the indaziflam tool.
Acknowledgment: This work was funded by the Oregon Hazelnut Commission, the Ferrero Hazelnut Company, and BASF.