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New Weed Control Options for Leeks

By Richard Smith
Photo 1. Weeds in untreated
Leeks are in the onion family and are increasingly important crop in Monterey County. In 2017, there were 475 acres produced worth $8.6 million. They are mostly established by transplanting and days to maturity is generally 120+ days, but varies widely depending on the time of year and market demand. Given the long crop cycle and the lack of a dense crop canopy, leeks are susceptible to successive waves of weed emergence. Growers complain that leeks “blow up” weed populations by allowing a significant number of weeds to mature and set seeds.
The preemergent herbicides DCPA and pendimethalin are registered for use and provide good weed control. However, post emergent herbicides such as bromoxynil and oxyfluorfen which provide excellent control of escaped weeds in onion production are not registered on leeks. In Canada prometryn is registered for use on leeks. In 2016, we began studying this material for use on leeks to bringing it into the US market. This project is now supported by the IR4 program and they are evaluating crop safety and developing a tolerance to eventually bring it to market.
Photo 2. Weed control in prometryn at 1.0 pint/A
Prometryn is particularly useful because it controls emerged weeds and has preemergent activity as well, which fills a need for this long-season crop. Table 1 shows weed counts and phytotoxicity ratings for a trial established on August 25, 2017. On the September 20 evaluation date, there was slight phytotoxicity in the prometryn treatments on the leeks, but no difference between the two application timings. Phytotoxicity consisted of a bending down of older leaves (Photo 3). The plants appeared to have recovered for the most part from this issue by the October 10 and November 9 evaluation dates, but bent leaves could still be observed to a small degree. Bromoxynil caused burning on the tips of the leaves that was evident on all evaluation dates. All rates and timings of prometryn eliminated weeds in the plots (Photos 1 & 2). There was no statistical difference in yield, but there is a trend that higher rates of prometryn had lower tonnage than the untreated control.
The potential for prometryn as a preemergent herbicide for use on leeks is quite exciting. It has the potential to provide a level of weed control in this crop that we have not seen. We are exploring its use on other allium crops as well and recently observed good safety on onions in one trial when it was applied at the 5th true leaf stage.
Photo 3. Note bending of leaves with prometryn
Table 1. Trial No. 1. Phytotoxicity ratings, weed counts (No./10 ft2) and yield