Rice Disease Research and Management, 2017


Project Leader

Luis Espino, farm advisor, UC Cooperative Extension Colusa, Glenn, and Yolo counties

Rice diseases have long been a concern to the California rice industry. A renewed effort to enhance fungicidal control of rice diseases, particularly stem rot and aggregate sheath spot, began in 2017. In addition, research into kernel smut was initiated. Kernel smut is now widespread throughout the Sacramento Valley and seems to be more prevalent in warmer areas. Concern about kernel smut has increased as more grower fields have become infected.

The research objectives for 2017 were to evaluate the timing and potential of fungicide applications for aggregate sheath spot, stem rot, and kernel smut management; to determine the effect of kernel smut on the yield and quality of rice; and to explore the distribution of kernel smut in the rice producing areas of California.

Fungicide trials

The fungicide azoxystrobin (Quadris®) has been used in California rice since blast disease was identified in 1999. Its efficacy on blast—and aggregate sheath spot—has been well established, but positive results in controlling stem rot with Quadris® were not obtained until trials in 2012 and 2013.

In 2017, 11 fungicide trials were conducted in three commercial fields located near Maxwell, Colusa, and Williams, as well as at the Rice Experiment Station. In these trials, stem rot and aggregate sheath spot were present at various levels, with stem rot being the most prevalent. Kernel smut was not found in any of these trials.

One set of trials compared Quadris®, Sercadis®, and Amistar Top®. Other trials compared the two new fungicides, Sonata® and Sercadis®, at different rates and timings.

Application of Quadris® at maximum label rates during the heading stage of rice resulted in significant reductions in stem rot incidence and severity (82% in one trial). However, earlier applications (45 days after seeding) were not effective at controlling stem rot incidence or severity.

Cover XL, a generic azoxystrobin, worked well in reducing stem rot and aggregate sheath spot levels, resulting in a yield advantage of 600 pounds/acre over the untreated.

Amistar Top®, a mix of azoxystrobin and difenoconazole, was marginally effective against stem rot. It did, however, significantly reduce stem rot incidence and severity in one trial. It also reduced incidence and severity of aggregate sheath spot in another trial.

The fungicide Sercadis® was not effective against stem rot or aggregate sheath spot. Testing of Sercadis® under higher aggregate sheath spot pressure may show different results.

The microbial fungicides Serenade® and Sonata® did not reduce disease levels.

Yield increase due to disease control in the trials ranged from 250 to 700 pounds/acre. Milling quality was affected by Quadris® applied at heading in two trials, Amistar Top® in one trial, and Sercadis® in the RES trial. The reason for the increased milling yield in the Sercadis® trial is not clear. Overall, effects on milling quality were small.

The compiled data shows that for every one-unit increase in stem rot severity (measured on a scale of 0 to 4), yields could be reduced by 3%. Milling yield and head rice yield also can be reduced by stem rot. Each unit increase in stem rot severity decreased milling and head rice yield by 0.7% and 2%, respectively.

Kernel smut

Kernel smut is not a new disease in California. It has been present in rice production areas since the early 1990s. For the past decade or so its presence has been considered more of a nuisance, with no reports of problem fields. In 2015, however, instances of higher than usual kernel smut infections were reported. Since then, the incidence of kernel smut seems to be increasing.

A 2014 survey found kernel smut in all rice producing areas except for Sacramento County. It was found more frequently in Butte, Yuba, and Colusa counties. In 2016 and 2017, many instances of severe kernel smut problems were reported. In two of the 2017 statewide variety trials, kernel smut was present at high levels.

For this project, three fields affected by kernel smut were selected for sampling. Two fields planted with M-105 and M-209 were in Glenn County. One field planted with M-206 was in Colusa County. In each field, 30 one-square-foot quadrants were hand harvested. In each quadrant, three samples of 10 tillers were randomly selected and inspected for kernel smut.

The M-105 and M-206 fields had similar levels of kernel smut. The M-209 field had much higher levels. In all three fields, kernel smut was easily noticeable. Incidence was determined as the proportion of panicles infected with kernel smut per sample. Severity was calculated as the average number of smutted kernels per panicle or per infected panicle.

Determining the incidence in the field was quick. Counting the number of infected kernels was time consuming. However, there is a significant statistical relationship between the proportion of infected panicles in a sample and the average number of infected kernels per panicle or per infected panicle. The relationship between the proportion of infected panicles and smutted kernels per panicle was stronger. Therefore, determining the proportion of infected panicles would give a good measure of the severity of kernel smut in a sample. Field evaluation of the proportion of panicles infected could be used to quickly evaluate the level of kernel smut in a field.

The effect of kernel smut on yield is not expected to be significant. In fact, anecdotal experience indicates that high levels of kernel smut are related to high yields. High levels of nitrogen increase the incidence and severity of kernel smut. Since high levels of nitrogen can produce high yields, these fields may also have higher levels of kernel smut.

Results indicate kernel smut can have a negative effect on milling yield. Reductions in milling yield of M-206 and M-209 ranged from 0.15 to 0.4 points for each 0.1-point increase in the proportion of infected panicles. Head rice yield was not significantly affected by kernel smut.