- Author: Michelle Leinfelder-Miles
- Author: Rachael Long
- Author: Rob Wilson
- Author: Ian Grettenberger
I have received a couple calls this year from growers and a PCA about root scarring in alfalfa fields. These observations were a bit of a “head scratcher” for all of us. At one of the fields, I noticed what looked to be a wireworm on the root, so that was throwing me off (Figure 1). Cutworms can also feed on plant roots. The larger instars have been found below the crowns under plant debris, but we didn't find any at this site. I reached out to Farm Advisor Rachael Long to get her thoughts. I had once heard Rachael talk about clover root curculio, and I wondered if it was causing the problem. While we have not confirmed that clover root curculio is present and causing damage in San Joaquin County alfalfa fields, it can occasionally be found in the Central Valley. It is, however, generally found in low numbers because it favors cooler climates. The purpose of this article is to bring awareness for this pest and to summarize some research that Rachael did in collaboration with Rob Wilson, Ian Grettenberger, and graduate student Jasmin Ramirez Bonilla.
Clover root curculio is a pest of alfalfa, clovers, and other legumes throughout the United States, often in cool climates. We are not clear on the distribution of it in California, but it has been observed in the Intermountain Region. The larvae are white and feed on the roots, in contrast to the green larvae of the alfalfa weevil that feed on leaves. The adults are mottled gray-brown and lack a pattern on their backs. Their damage is usually observed as patches of poor growth or stand decline because the larvae gouge, even girdle, the taproots (Figure 2). The gouges can serve as a point of entry for diseases.
The life cycle of the clover root curculio is shown in Figure 3. There is one generation per year. Adult weevils may be observed in just about every season, and the larvae are present during the spring. The larvae can be difficult to find, however, because they are in the soil and often may be inside the root nodules during early instars.
Unfortunately, management practices are limited. Cultural practices include rotating out of alfalfa and other legumes for at least one year, avoiding planting alfalfa next to infested fields (since seedling roots are weaker and would be more susceptible to feeding damage), good irrigation practices and nutrition to reduce plant stress that might make stands more susceptible to decline, and cleaning equipment when moving between fields.
There are no insecticides registered for clover root curculio larvae, and our understanding of how to control adults is limited. In the Klamath Basin research, spring-applied insecticide trials did not have efficacy because adults lay eggs from spring to summer and can escape the treatment, and eggs overwinter in the soil. Fall treatments were also evaluated, after adults had migrated back into the field. Warrior and Warrior plus Entrust were effective at reducing the adult population compared to an untreated control, and reduced the number of eggs the following spring. Insecticide treatments might not be economical, however, because there was no yield benefit the following year. So overall, this pest remains a “head scratcher”.
Over the last few years, I have been working on a project to characterize a suite of soil health properties in alfalfa receiving full and deficit irrigation. Soil health has been described as the ability of soil to function and is characterized by biological, chemical, and physical soil properties that are sensitive to changes in management. The idea for this project developed after the 2012-2015 drought when water shortages and regulatory curtailments meant that growers had to make tough decisions on how to apply scarce water resources. Some growers opted to cut irrigation to alfalfa since it is a deep-rooted crop that can scavenge water and nutrients from the soil profile. (See this recent blog post by UC Alfalfa and Forage Specialist Dan Putnam, and Farm Advisor Rachael Long on the resiliency of alfalfa during drought.) I had a hunch, however, that while alfalfa may be adapted to survive drought conditions, soil health properties might be negatively impacted because water is essential to life in the soil, facilitates nutrient movement and availability, and influences soil physical characteristics, among other things. Fortunate for me, there was a research trial at UC Davis where I could test this idea.
I view alfalfa as a model crop for studying soil health under restricted water conditions because practices like crop rotation and tillage do not occur over the four or more years of an alfalfa stand. Therefore, those practices would not confound the results. From this experiment, we are learning how imposing varying levels of deficit at different stages of the cropping season impact soil properties, which will help us optimize deficit irrigation strategies for alfalfa. Additionally, the deficit treatments serve as a proxy for drought and could potentially demonstrate how prioritization of water uses during drought may impact soil conservation outcomes.
Data analysis is ongoing, but preliminary results suggest that soil health may not be resilient under deficit irrigation or drought, even if alfalfa is. When the trial began in Spring 2019, there were no differences in rootzone salinity among treatments, which averaged 0.41 dS/m. After two cropping seasons where deficits were imposed, the 60 percent ETc treatment with the water cut-off toward the end of the season (CT) resulted in significantly higher rootzone salinity down to the 36-inch depth (Figure 2). The salinity in that treatment was higher than even the 40 percent ETc treatment that had the sustained deficit (SD) throughout the entire season. In other words, it appears that the timing of the deficit is more important than the amount of deficit, and applying water throughout the season – even if the amount is severely reduced – appears to mitigate salinity build-up in the rootzone. Of note, salinity is not high enough to be problematic at this site. The overall ECe of the soil is low, and water quality is generally good at this location. I would expect, however, that in locations where soil and/or water has higher salinity to begin with, then deficit irrigation that includes a water cut-off could be problematic.
There will be a lot more information to come about this project in the near future, but the salinity information seemed timely to share given our current water year. In addition to Dan, Isaya, and Umair, I want to acknowledge Daniel Geisseler (UC Nutrient Management Specialist), Will Horwath (Professor of Soil Biogeochemistry), and graduate student Veronica Suarez Romero who have helped on soil nitrogen and carbon testing. I also want to acknowledge the South Delta Water Agency for financial support of the project.
It feels like we were cheated out of our rainy season since we beat dry records for the months since January. October through December seem like the distant past, but those were wet months! (Depending on location, over 5 inches fell in late October, and over 7 inches in late December.) On fine textured and/or low permeability soils, the high amount of rainfall that fell in short windows of time may have impacted alfalfa roots. In one field that I visited (Figure 1), notice how the borders are greener, and I suspect, better drained. We dug up plants from this field, and the taproots looked pretty healthy. There were no obvious signs of root rot, but there were almost no fine roots coming off the taproots (Figure 2). The fine roots are where we would expect to see root nodules, and without root nodules, the plant will not fix nitrogen the way it should. This can cause lower nitrogen availability for the crop and reduce growth. While we generally do not recommend applying nitrogen to alfalfa fields, there are a few rare situations where it may be beneficial, as noted in this article by Farm Advisor Rachael Long. A field with poor nodulation may be one situation where starter N (11-52-0) can help to regrow roots and reestablish nodulation.
Because of the wet conditions, winter herbicide sprays were delayed into mid- to late January, and even early February. On top of that, temperatures were fairly mild this winter, and many alfalfa fields continued growing. Early February is a bit late for applying herbicides. Even mid- to late January is not ideal, particularly if early April is targeted for the first cutting. Some burn-down herbicides are pretty ‘hot', especially at the high label rate, so it's important to spray early enough that the alfalfa has time to regrow. Some fields still looked pretty burned in mid-March, especially where there was a lot of growth at the time of spraying. I have observed, however, that new stems were emerging from the crown (Figure 3), at least in the fields I visited. So, my hunch is that the fields will snap out of this, but probably not in time for an early April cutting.
As I was writing this article, I came to realize that I wrote an article a few years ago with this same title. The reasons for slow spring growth in 2019, however, were a bit different from the reasons in 2022. So, it would seem that no two years are ever the same.
Please reach out if you would like to discuss this information further, and good luck this season!
UC Cooperative Extension will host a Weedy Rice Meeting on Thursday, March 31, 2022 from 10:00am to 11:30am. The meeting location is the Cabral Agricultural Center in Stockton (2101 E. Earhart Ave., Stockton, CA 95206). A printable version of the agenda is attached at the bottom of this post. The agenda is as follows:
10:00am Weedy Rice Identification Refresher/Review, Luis Espino, Whitney Brim-DeForest, UCCE
10:15am Current Status of Weedy Rice – Acreage Surveys, Luis Espino, Whitney Brim-DeForest, Michelle Leinfelder-Miles, UCCE
10:30am Seed Certification Process/Rules and 2021 Observations, Timothy Blank, CA Crop Improvement Association
10:45am Best Management Practices, Luis Espino, Whitney Brim-DeForest, Michelle Leinfelder-Miles, UCCE
11:00am Research Updates, Whitney Brim-DeForest, UCCE
11:15am Open Discussion – Q & A
We have submitted applications for continuing education, including credits for pesticide licensing (1.5 of Other) and certified crop advisers (1.5 of PM). Refreshments will be provided. If you require special accommodations, please contact UCCE San Joaquin County at 209-953-6100. Thank you, and hope to see you at the meeting!
Statewide results will be available from the UC Rice webpage (https://rice.ucanr.edu/Reports-Publications/Agronomy_Papers/). Table 1 (below) shows the San Joaquin County results. Among the entries, M-206 is the most commonly planted variety in the Delta, followed by M-105. The average yield across all commercial varieties and advanced lines was 10,129 lb/ac. When interpreting the results, the CV, or coefficient of variation, is a measure of variability in the data in relation to the mean. The LSD (.05), or least significant difference at 95%, is used to compare means of different varieties. When the difference between two varieties exceeds the LSD value, we are 95% certain that the two varieties performed differently; the results are not due to random chance.
This trial helps to support the Delta rice industry, which has a different production system from the Sacramento Valley. We plan to continue trialing rice varieties in the Delta again in 2022. Please don't hesitate to reach out if you have questions about Delta rice production, and good luck this season!
Table 1. 2021 San Joaquin County very early rice variety test (advanced lines and varieties).