Friday, September 24, 2021

Supervillain grassy weeds vs. ‘Kryptonite’: Who will win?

Vicky Boyd
Vicky Boyd,
Editor

If the weed kingdom had a band of supervillains, barnyardgrass and watergrass would surely be members. These species that belong to the genus Echinochloa wield super powers and are the bane of rice growers.

Herbicides, once the Kryptonite for these grassy invaders, have lost their super powers in many instances as the weeds have developed resistance. With only a few truly new weed-control products on the horizon, growers are left with trying to steward the current ones and slow resistance development.

University of Arkansas weed scientist Jason Norsworthy has been screening barnyardgrass (E. crus-galli) samples sent in for the past several years for herbicide resistance. Between 2017 and 2019, he found populations resistant to propanil, Facet, Newpath/Preface (imazethapyr), Loyant, ACCase-inhibitors (fop and prop) and Command (clomazone). Several had resistance to multiple modes of action, including one with resistance to five MOAs.

Kassim Al-Khatib, a University of California weed scientist, also has been screening weed samples sent in from growers and pest control advisors. He returns a report card to each submitter showing the products to which the weed is resistant as well as ones that still provide control.

During the past few years, Al-Khatib has found populations of late watergrass (E. phyllopogon), early watergrass (E. oryzoides) and barnyardgrass with documented resistance to up to four herbicide sites of action. They include Cerano (clomazone), Clincher (cyhalofop), Abolish/Bolero (thobencarb) and propanil.
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ince 2017, a new grassy member of the Echinochloa genus has been spreading in California rice fields. (See article on page 8.) UC Cooperative Extension rice advisor Whitney Brim-DeForest has taken identification down to the genus level, but she hasn’t been able to nail down whether it’s a new biotype of a resident watergrass species or a watergrass species new to the state. Identifying the newcomer will help formulate a control program.

The new biotype/species is small-seeded and has long purple awns. It grows aggressively, pushing out rice and quickly taking over a field. In initial greenhouse screenings, few products effectively controlled it. But Brim-DeForest said she plans to look at additional products and application timings this season to help develop a battle plan.

Unfortunately, one program growers have used on the newcomer is two sequential propanil applications. With propanil resistance already widespread in other Echinochloa species, several in the California rice industry fear the same could happen with this purple-awned stranger.

And with it, growers lose some of their super powers over these grassy weeds, much like Kryptonite degrades to a piece of worthless iron in the comic books.

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