Italian ryegrass has become a weedy nuisance in California – often found in orchards and vineyards where fruits and nuts are grown. To add to the control challenge, researchers say the weed has now developed resistance to multiple herbicides.

After a paraquat-resistant population of Italian ryegrass was found in a California prune orchard, scientists explored whether the same population might be resistant to other postemergence herbicides, including clethodim, fluazifop-P-butyl, glufosinate, glyphosate, pyroxsulam, rimsulfuron and sethoxydim.

Their study showed that the paraquat-resistant population was also resistant to both glyphosate and clethodim. Though glufosinate, rimsulfuron and sethoxydim controlled the resistant population, the research team cautioned that certain populations of Italian ryegrass have exhibited resistance to those herbicides as well.

So what should growers do? Researchers say to avoid overreliance on postemergence herbicides. A number of preemergence chemicals are effective options for Italian ryegrass control in fruit and nut tree crops, including tank mixes containing indaziflam and flumioxazin.

Want to know more? The article “Multiple Herbicide-Resistant Italian ryegrass (Lolium multiflorum) in California Perennial Crops: Characterization, Mechanism of Resistance and Chemical Management” is available in  Volume 66, Issue 6 of the journal Weed Science.

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