Rice Disease Research and Management, 2019


Luis Espino, farm advisor, UC Cooperative Extension Butte and Glenn counties

Work continued in 2019 on rice diseases of concern to the California rice industry, including aggregate sheath spot, stem rot, and kernel smut. Research evaluated a number of fungicides for disease management. In addition, a study of the epidemiology of kernel smut began and an extensive survey of rice disease prevalence also took place.

Fungicide trials

Eight fungicide trials were conducted in 2019. Four trials targeted stem rot, two focused on aggregate sheath spot, and two trials focused on kernel smut. Five test sites were in Butte County. The others were in Colusa, Glenn, and Sutter counties.

Researchers evaluated smutted rice kernels after soaking in a potassium hydroxide solution
In the stem rot and aggregate sheath spot trials, fungicide sprays were applied at midtillering, boot and heading. Midboot is the best time to apply fungicides to prevent kernel smut. However, early heading is the recommended application timing to manage stem rot and aggregate sheath spot. If these two diseases could be managed with applications at the midboot stage, then it would be possible to combine their control with control of kernel smut, saving costs.

At the Rice Experiment Station (RES) trial, stem rot incidence and severity was moderate in 2019. Quadris® and Quilt Xcel® applied at the boot stage reduced the severity of stem rot by 35% and 54%, respectively. Application at heading did not result in significant disease reduction most likely because the applications were made too late. In three other stem rot trials, no significant effects were observed on disease incidence or severity.

Growers report that when using Quadris®, rice grains dry down slowly, and the RES trial showed that. Treatment with Quadris® or Quilt Xcel® at the boot or heading stage resulted in one or two percentage points higher moisture, respectively, at harvest than grain in untreated plots. Grain, milling, and head rice yields were not significantly affected by fungicide treatments.

In the Colusa County trial, Quadris®, Quilt Xcel®, and Stratego® reduced the incidence of blast on panicles when applied at the boot stage by 90% and by 47% at heading stage. This is most likely because the heading application was made when 100% of the panicles had emerged. Blast treatment guidelines recommend fungicide application at early heading.

In two other trials, Quadris®, Quilt Xcel®, Excalia®, or Sonata® reduced the severity of aggregate sheath spot when applied at the boot or heading stages. Excalia® was highly effective in reducing the incidence and severity of aggregate sheath spot. At the Sutter County trial, it reduced disease severity by 90% when applied at the boot stage.

No control of stem rot or blast was observed with applications made at 35 days after seeding. For aggregate sheath spot, only Excalia® reduced disease severity when applied 35 days after seeding.

In general, results of the fungicide trials seem to indicate that reduction in stem rot, aggregate sheath spot or blast incidence or severity can be achieved with applications at the midboot stage. More evaluations need to be made to recommend this timing with confidence.

In the Glenn County kernel smut trial, Tilt® and Amistar® Top significantly reduced the number of smutted kernels in grain samples. In a Butte County organic trial targeting kernel smut, a high nitrogen rate resulted in higher levels of smut than a low nitrogen rate. As smut levels increased, kernel whiteness decreased. Organic fungicides had a weak effect on the disease.

Kernel smut

Epidemiological studies to better understand kernel smut distribution, incidence, and severity commenced in 2019. Researchers analyzed 10 samples to determine species identity (Tilletia horrida). Knowing the correct species name is critical in enabling future research efforts to develop accurate detection and quantification methods.

To determine whether rice seed plays a major role in the distribution of kernel smut, researchers tested several detection methods for smutted seed. Twenty-two lots of rice seeds were analyzed. All seed sources had smut-infected kernels, and 80% of all seed lots had one or more smut-infected rice seed. There was significant variability in the number of seeds with smut, ranging from one to 16 infested seeds in 25 grams of seed.

Rice disease survey

In order to comply with the conditional rice straw burning program, every five years the level of rice diseases in the Sacramento Valley needs to be evaluated. This information is used by agricultural commissioners to determine whether rice straw burning should continue in a given county. The last survey was in 2014, so a new assessment was needed to authorize the burning program for 2020–2024.

The survey included the eight Sacramento Valley counties where 99% of the rice is grown. In each county, rice fields were randomly selected for inspection. The number of fields surveyed in each county varied with the size of rice acreage—10, five or two fields in large, medium, and small rice growing counties.

The survey took place between September 12 and October 3. Inspected fields were at the grain-filling and maturity stage during the survey dates. In each field, three or four basins were selected. A total of 162 samples, consisting of 1,649 inspections were taken.

This table shows results of a rice disease survey conducted in 2019 in order to comply with the conditional rice straw burning program.
County Sample size Stem rot (%) Aggregate sheath spot (%) Blast (%) Kernel smut (%)
Butte 40 65.42 36.35 3.85 13.13
Colusa 30 69.00 9.33 2.67 11.00
Glenn 20 68.33 29.00 43.00 13.67
Sutter 37 65.64 56.83 0 19.19
Yolo 15 60.00 32.67 0 11.33
Yuba 10 71.33 84.67 0 55.33
Placer 4 36.67 98.33 0 0
Sacramento 6 51.67 15.00 0 0
All counties 162 65.14 38.50 6.75 15.81

On average, stem rot and aggregate sheath spot were the most common diseases and were found in all counties. Stem rot had a higher incidence than aggregate sheath spot. Blast was found in only three counties. Stem rot was present in every inspected field. For all counties, percentage of inspections with stem rot disease symptoms ranged from 10% to 100%, averaging 65%. Distribution of stem rot is uniform across the Sacramento Valley.

Aggregate sheath spot was present in 51 of 54 inspected fields, with an average incidence of 35% for all counties. A higher incidence was found in Glenn, Butte, Sutter, Placer, and Yuba counties. Incidence and severity of this disease is greater in fields where potassium soil levels are below optimal, which is typically the case in the eastern part of the valley.

Stem rot has been a problem in California since the beginning of rice cultivation. Aggregate sheath spot became a problem after the introduction of high-yielding, semidwarf varieties in the late 1960s. Agronomic practices, especially straw management, have an important influence on these diseases. Rice straw burning in the field reduces the ability of these diseases to survive the winter. Also, excess nitrogen can increase stem rot severity, while potassium deficiency exacerbates aggregate sheath spot. All varieties grown in California are equally susceptible to both pathogens.

Blast was present in only 14 fields in Butte, Glenn, and Colusa counties. More than half the fields inspected in Glenn County showed disease symptoms. Incidence in Butte and Colusa counties was comparatively low. Rice blast was first identified in California in 1996 and is now endemic to Glenn and Colusa counties, showing up at varying levels every year. Blast infections are favored by long periods of leaf wetness, high relative humidity and mild temperatures. Excess nitrogen and straw residue are important considerations in blast disease management. The recently released variety, M-210, is resistant to the blast races present in California.

Kernel smut was present in most counties, but incidence was generally low. Exceptions were north Glenn and Butte counties, and Yuba County, where it was higher. Several factors can affect the development of this disease, such as nitrogen level and variety. Additionally, contaminated seed can spread the disease. Spores of kernel smut can remain in the field after harvest, contaminating the following year’s crop.

Seedling disease and bakanae were not included in the survey.