Whitney Brim-DeForest, UCCE Rice Advisor

We talk about herbicide resistance all of the time in California rice. But how does it evolve in a field? Understanding how herbicide management selects for resistant populations is an important part of preventing the problem from occuring in your fields.

We have many weed species in CA rice that are confirmed to be herbicide resistant. The major herbicide-resistant species are: late watergrass, early watergrass, barnyardgrass, smallflower umbrella sedge, ricefield bulrush (roughseed), sprangletop, and redstem. For this illustration of how herbicide resistance evolves in a field, we use redstem as our example. 

Year 1, Beginning of season: A population of redstem is found in a field and are emerging at the beginning of the season. In this illustration, the plants with the blue background are naturally susceptible to an herbicide (Granite SC). The plants with the yellow background are naturally herbicide resistant to Granite SC. There is nothing that the grower has done at thispoint to select for  resistance. The genes that make the plant resistant are naturally found in the redstem population in the field.

Year 1, Mid-season: The grower applies Granite SC.

Year 1, End of season: One herbicide resistant plant survives. This plant goes on to produce seed, and the seeds are deposited onto the soil surface, where they are tilled into the soil seedbank at the end of the season.

Year 2, Beginning of season: The redstem population emerges from the soil at the beginning of the season. Because there are more seeds in the soil seedbank from the resistant plants, more of the emerged plants are resistant to Granite SC this year (yellow background = herbicide resistant).

Year 2, Mid-season: The grower again applies Granite SC or another herbicide with the same mode of action (Regiment, Halomax/Sandea or Londax).

Year 2, End of season: All of the herbicide resistant plants again survive the herbicide application. Again, they go on to produce seed, and the seeds are deposited onto the soil surface, where they are tilled into the soil seedbank.

Year 3, Beginning of season: The redstem population emerges from the soil at the beginning of the season. There are even more herbicide resistant plants than the previous 2 years, as the proportion of herbicide resistant seed in the soil has increased.

Year 3, Mid-season: For the third year,the grower applies Granite SC or another herbicide with the same mode of action (Regiment, Halomax/Sandea or Londax).

Year 3, End of season: All of the herbicide resistant plants again survive the herbicide application. Again, they go on to produce seed, and the seeds are deposited onto the soil surface, where they are tilled into the soil seedbank.

Year 4, Beginning of season: The redstem population emerges from the soil at the beginning of the season. This year, all of the plants are herbicide resistant, as the soil seedbank contains mostly herbicide resistant redstem seed.

The illustrations above are an example of how herbicide resistance evolves and is selected for in a field. A grower may not notice during the first year or two, as there are just a few plants that survive the herbicide applications. However, if the grower continues to use the same herbicide year after year, or the same herbicide mode of action, eventually, the population of redstem (or another weed species) will shift to become composed of only plants that are herbicide resistant.

The best way to prevent the development of herbicide resistance is to rotate herbicide modes of action, both between seasons and within seasons. Refer to the UC Herbicide Susceptibility Chart for CA rice when planning an herbicide program.

The 1st Annual Rice Weed Course will take place: 

Friday, September 16, 2016

from 8:30AM to 4:15PM (Registration begins at 8:00AM)

at the 

Hamilton Road Field (on West Hamilton Rd. between Hwy. 99 & Riceton Hwy.)

and Rice Experiment Station, Biggs, CA

The day will begin with an interactive tour of the Weed Science research plots at Hamilton Road. Participants will also spend time learning about weed identification for important rice weeds both at emergence and at heading. Presentations will cover emerging pests of rice, such as weedy red rice and winged-leaf primrose willow. Attendees will hear about new pesticide registrations and updates to current pesticides, as well as information regarding the new Butte herbicide. This course is also a great opportunity to interact directly with the UCCE and UC Davis Rice Weed Research Team! 

For a full agenda and registration go to: 

http://wric.ucdavis.edu/events/rice_weed_course_2016.html 

For questions, please contact Whitney Brim-DeForest at 530-822-7515, or by email at wbrimdeforest@ucanr.edu 

Credits for PCA, QAC, QAL, Private Applicator, and CA Certified Crop Adviser are pending. 

I have received an update from the Butte County Ag Commissioner's office regarding winged primrose willow. Germinating seedlings have been detected recently. At this point, the seedlings are very small and would be very difficult to notice. Scouts were able to find them in places where the weed had been seen in the past. But this is a good indication that conditions are now good for germination. Growers and PCAs in risk areas should be on the alert. Larger seedlings will probably be visible in 30 days.

For more information and pictures, visit the UC Rice On-line winged primrose willow page.

Photo by Evan Padgett, Butte County Agricultural Commissioner Department

In addition to the well-known rice weeds we deal with every year, there are a couple of weeds I want to bring to your attention this year. One is an invasive and the other has been with us for a while.

The winged primrose willow was identified in 2011 in several rice fields near Richvale in Butte County. Since then it has spread to several other fields, up to 3.5 miles south of Richvale. Winged primrose willow was recently given an “A” pest rating by CDFA. What this means is that this weed is now considered to be of economic importance and may trigger an enforcement action by the state. Currently, no actions are being considered, but its presence will disqualify a seed field. The rice industry is implementing an outreach, monitoring and control program for winged primrose willow to stop its spread and reduce infestations.

Red rice has been found in a few locations in California over the years. In 2003, its presence was confirmed in six fields. In some of these fields, growers were able to eradicate it. However, since then, red rice has been found in other locations. The exact number of locations currently affected is unknown, but estimates are around 15. At this moment, red rice does not represent a threat to the industry. But if this weed spreads, it can severely impact the rice industry, affecting yields, quality and production costs. To get an idea of its potential impact, just ask any southern rice grower about red rice and you'll get an earful.

UCCE will be collaborating with other organizations and agencies to get information on identification and management of these weeds. We will be sending information out through our newsletters and websites, so make sure to be subscribed. In the meantime, here's a list of links with relevant information on these two weeds.

Winged primrose willow:

UC IPM Website (under Emerging Pests in California)

2013 Rice Grower Meeting presentation: Winged Willow Primrose

UC Rice Blog posts: New weed identified in California rice, and Behavior of winged primrose willow and herbicide options for control

June 2012 Rice Briefs newsletter: Behavior of winged primrose willow and herbicide options for control

Red rice:

2016 Rice Grower Meeting presentation: Red Rice Update

UCCE red rice identification guide