Dr. Michelle M Leinfelder-Miles / UCCE
This article is my annual review of UCCE Delta rice research and seasonal observations.
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…