The 2023 Rice Production Manual is available for download here. The Manual was produced in association with the 2023 Rice Production Workshop; it describes the principles of rice production based on research conducted in the rice production areas of California.

I recently visited a field that had quite a bit of rice seed midge injury. In this case, the seedlings that were not injured were almost at the one leaf stage, and one could observe quite a few seeds still in the soil surface. Close inspection of these seeds showed that they had been hollowed out by rice seed midge. Some midge casings were observed as well. Later, I got some reports of more fields injured by rice seed midge in the Colusa area.

Seed injured by rice seed midge

Rice seed midge casings and injured seed

Rice seed midge has been considered a pest of rice for many years. However, it has been an infrequent pest. The thought was that pyrethroid insecticides controlled it. Nevertheless, in the past three years, I have seen considerable injury in fields that had been treated with a pyrethroid.

Ian Grettenberger, UC Davis Entomology Specialist, collected some midges from the field I mentioned above and exposed them to a full rate of a pyrethroid or an insect growth regulator for 48 hours. The larvae survived. We are not sure if the survival is due to low susceptibility or resistance build-up to the pesticides. Ian started doing some pesticide work last year but the results were not clear cut. More work is going to be necessary to improve on the management of this pest. Let us know if you have seen issues with seed midge recently so we can have a better idea of the current importance of this insect.

Coming out of 2022, where roughly half of all California rice fields were left fallowed last year, and others may have been fallowed for two seasons, many of us have questions about what weed management will look like in 2023. While we do not have data on what a 1- or 2-year fallow does to all of our major weed species, we have some preliminary and anecdotal data that might lend some insight.

For small-seeded weeds such as smallflower umbrella sedge, redstem, and ricefield bulrush, the fallow period will likely have no effect. A good anecdotal example is from a field in Davis that was planted for a long time in rice, followed by nothing being planted for over 10 years. Once the field went back into rice, ricefield bulrush was widely distributed throughout the field, and readily emerged. Ricefield bulrush seeds have a thick seed coat, making it easy for them to survive in the soil for a long period of time, and they have a high level of dormancy, increasing their persistence. In an unpublished overwintering experiment with smallflower umbrella sedge to determine if decomposition occurred over the winter in flooded conditions, the smallflower seeds did not decompose. Seeds of smallflower umbrella sedge are also found in the 1,000s per square foot of soil, so once established, they are difficult to get rid of. 

For larger seeded weeds including watergrass, a fallow may have an impact, but that impact will likely only be on those weed seeds near the surface of the soil, where they may decompose, and are at increased chance of predation. In a preliminary study conducted last year, we found that in fields where tillage occurred in the spring post-fallow, watergrass emerged, as seeds were brought up from deeper in the soil profile, where they would have remained dormant if tillage did not occur. In fields that were no-till in the spring, in at least one instance, the watergrass did not emerge at the same rate as in the tilled field. Similar results were seen with arrowhead, a perennial aquatic species. The effect of the tillage was to bring up the seeds from deeper in the soil profile, increasing emergence in the tilled area of the field. 

At this time, no anecdotal or preliminary data exists for sprangletop. However, due to the small seed size and high dormancy period, it is likely that sprangletop would not be greatly affected by a fallow period and would emerge as normal. 

Preliminary data from grower fields that were monitored over three years shows a reduction in weedy rice seed viability in the top 12 inches of soil. However, these fields have not been planted back into rice, so it is unknown whether the reduction of viable seed in the soil translated to reduced emergence. 

To summarize, it is unlikely the 1- to 2-year fallows will cause a great reduction in weed emergence, especially for small-seeded species. For larger-seeded species, there may be an impact, which can be increased by use of zero- or reduced tillage in the spring following a fallow period. 

"Originally published in Rice Farming, February 2023".

In general, rice diseases are not a big problem in California. However, in some fields, disease pressure can be high and can result in yield losses. One disease that can be found in practically every rice field is stem rot. This fungus causes the tillers to rot at the water level and, when severe, it causes blanking and lodging. In our fungicide trials, we rate the severity of stem rot at drain time using a scale that goes from 0 to 4, where 0 is no disease and 4 is when the tiller is completely rotted. In my experience, when a field approaches an average severity of 2, a yield loss occurs. The problem with this rating system is that it is very time consuming. However, there is a good relationship between severity and incidence, the percentage of tillers that have stem rot symptoms. As severity increases, so does incidence, and when severity reaches a level of 2, incidence is 100%. This means that if you monitor your field at drain time and find that in a random sample of tillers all show symptoms of stem rot, the field is likely at severity level 2 or above and a yield loss may occur.

While this method of evaluating the level of stem rot in your field may help you explain blanking, lodging, and lower yields, it does not help you with the decision to take any corrective action that year. At drain time, it is too late to use a fungicide. I'm currently working on a project trying to determine if we can evaluate stem rot earlier, around the boot stage. In this way, a grower or PCA can decide if a fungicide treatment for stem rot is really needed before reaching the heading stage. Last year was the first year of this project. More years will be needed to provide a recommendation.

A project I have conducted for the past two years looked at the differences between varieties in their susceptibility to stem rot. All our varieties are susceptible, but there are some differences. Varieties with a short cycle, like S-102, CM-101, or M-105 are more susceptible than varieties with longer cycles, like M-209 and M-211. This information might help growers and PCAs improve their disease management program.

As you know, azoxystrobin (the active ingredient in Quadris) is the main fungicide used in rice in California. Many trials have confirmed that we can obtain stem rot severity reduction when applying this fungicide at the late boot to early heading stage. We also tested applying the fungicide at propanil time, but we did not see any benefits of this application timing. In the variety testing mentioned above, stem rot severity was reduced 20-30% when azoxystrobin was applied at early heading (when the first few heads were visible above the canopy). In 2021 this disease reduction resulted on a 4% yield increase. While in 2022 there was no yield increase, we did see increased milling quality in fungicide treated plots.