This article is my annual review of UCCE Delta rice research and seasonal observations.
Variety Trial: Delta rice acreage has been steadily increasing, and yields are comparable with the statewide average (Table 1). I estimate that Delta acreage approached, if not exceeded, 15,000 acres in 2025. To support the growing industry, UCCE collaborates with the California Rice Experiment Station to evaluate commercial varieties and advanced breeding lines. The San Joaquin County Delta was one of eight locations in the 2025 statewide trial. The Delta is the only drill-seeded site and is a test site for very-early maturing varieties because it has cooler growing conditions than other rice growing regions of the state. Variety trial results will be available in the February 2026 newsletter.
Armyworm Monitoring: In 2025, we monitored true armyworms on three Delta farms, and moth catches peaked around June 30th. Figure 1 shows the average catches across the three farms (9 traps total). The peak population number and timing were similar to what we observed in 2024, which was less severe than in years like 2017, 2018, 2022, and 2023. Trap catches varied across the three farms and across fields on the same farm, which is why it is important to scout for field-level impacts.
Disease Observations: I did not hear much about diseases this year. I only submitted one sample to the disease diagnostics lab at UC Davis, and they diagnosed aggregate sheath spot (Figure 2). Aggregate sheath spot is similar to stem rot because both diseases can be made worse by low potassium (K) conditions. Potassium can be limiting in some Delta soils, so proper plant nutrition is a good disease management strategy. UC Rice Specialist, Bruce Linquist, wrote this fact sheet about managing K. If soil extractable K is less than 60 ppm, K is deficient, and if extractable K is between 60 and 120 ppm, K fertilizer would likely be beneficial. Growers who have observed aggregate sheath spot or stem rot may benefit from testing their soil for K. If K fertility is sufficient and aggregate sheath spot or stem rot are still observed, fungicide treatment is also an option. It is important to scout at late-tillering and early-heading because treatment is most effective when applied between late-boot and early-heading.
Herbicide Resistance Testing: Under the direction of UC Weed Management Specialist, Kassim Al-Khatib, UCCE provides herbicide resistance testing for rice growers. Over the years, I have submitted about a half dozen weed seed samples for testing, and while that only represents a few fields on a few farms, the results are interesting. On one farm, barnyardgrass had resistance to Clincher, Regiment, and Loyant. On another farm, barnyardgrass had resistance to SuperWham!, Regiment, and Loyant. On a third farm, redstems had not developed any resistance. I have spoken with some PCAs who have suspected Regiment resistance on farms with a history of rice cultivation, and these limited results corroborate their hunches. The resistance to Loyant was curious to me since that is a newly-registered product and had not yet been used on these farms. I reached out to Kassim to ask how this could be observed. Kassim noted that Postdoctoral Scholar, Deniz Inci, also observed Loyant resistance before commercialization in research trials. His conclusion is that biotypes of barnyardgrass and watergrass species metabolize Loyant differently, and therefore, have a different response to treatment. This is important information for growers to know if they are rotating Loyant into their herbicide programs because efficacy may vary. Overall, what these resistance results indicate is that growers and PCAs should be making observations about whether their herbicide program is still effective. Using the full label rate, spraying at optimum timing (when weeds are small), and herbicide rotation are important resistance management strategies. Please call if you would like to discuss options. The resistance testing program is closed for this year, but in future years, please reach out if you would like to submit samples.
Weedy Rice: Over the years, we have identified weedy rice on a few Delta farms, so we need to stay vigilant in our efforts to prevent the spread and manage weedy rice. This summer, UCCE hosted a meeting in the Delta to extend information to the local industry. Presentations from that meeting are available from my website. In fields where weedy rice has been a problem, post-harvest management should include straw chopping, but not incorporation, and winter flooding. This will keep seed on the soil surface where it can potentially deteriorate over the winter. I have observed that where crop rotations have been implemented, weedy rice pressure decreases in the subsequent rice rotation, but since weedy rice seed can stay dormant for many years, time will tell how well crop rotation works over the long-term.
Cover Cropping: We wrapped up a project funded by the CDFA Healthy Soils Program and CA Rice Research Board where we evaluated different cover crop species and whether cover cropping improves soil carbon and nitrogen dynamics in the rice system. Since rice may be grown over multiple seasons without rotation, cover crops may provide an opportunity to introduce plant diversity, including nitrogen-fixing legumes. Data from the Delta trial will be forthcoming, but overall, we observed that the brassicas emerged quickly and started covering the soil after just one month. They died off, however, when the conditions became wet. In contrast, purple and woollypod vetches and balansa clover started off slowly but had vigorous stands by early spring, despite the wet conditions.
I want to take this opportunity to thank all the growers who collaborated with us on these projects, and I wish everyone a happy harvest season.
