Luis Espino, farm advisor, UCCE Colusa, Glenn, and Yolo counties

Work continued in 2018 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, research on kernel smut examined methods to rate the severity of infection, to determine effects on rice yield and quality, and to assess the susceptibility of California rice varieties.

Fungicide trials

Five fungicide trials were conducted in 2018. Two Butte County trials focused on stem rot, one Sutter County trial focused on aggregate sheath spot, and two Glenn County trials focused on kernel smut.

In the stem rot and aggregate sheath spot trials, fungicide sprays were applied at midtillering or at heading. In the kernel smut trials, applications were made at the late boot or very early heading stage.

The fungicide azoxystrobin (Quadris® or Cover XL) reduced incidence and severity of stem rot by 23% to 57% and aggregate sheath spot by 73% when applied during the early heading stage. The new product S-2399 (commercial name Indiflin™) and fluxapiroxad (Sercadis®) had good activity against aggregate sheath spot when applied at early heading, reducing the incidence of the disease by 90% and 49%, respectively.

One of the Butte County test sites has a history of severe stem rot problems. Incidence of the disease was 100% in all treatments, meaning all tillers sampled had stem rot lesions. Stem rot severity also was rated high. None of the fungicide treatments significantly reduced the severity of the disease compared to the control. However, different fungicide treatments significantly affected yields. The application of Quadris® at midtillering and Sercadis® at heading resulted in higher yields than the untreated by 8% and 5%, respectively. Levels of disease severity at this site, reduced yield by 540 pounds/acre.

Stem rot incidence and severity at the other Butte County trial (Biggs) was moderate to high. Quadris® and Cover XL applied at heading reduced stem rot severity by 49% and 57%, respectively. Cover XL in combination with ALB 3000 (an experimental product) reduced severity by 43%, while ALB 4003 reduced severity by 56%. Sonata® applied at midtillering and at heading reduced stem rot severity by 23%.

The Sutter County test site has a history of aggregate sheath spot. Strong symptoms were observed in 2017. The disease reached moderate levels in 2018. Several of the treatments significantly reduced disease incidence and severity. Quadris®, Sercadis®, and Indiflin™ applied at heading all gave good control of the disease, ranging from 49% to 90%.

Yield and milling quality were not significantly affected. Nonetheless, disease severity reduced yield by 430 pounds/acre for every unit increase in the severity scale.

Kernel smut

Kernel smut is an emerging problem, especially in the northern part of the Sacramento Valley. In 2017, several reports of affected fields were received, and one organic field was downgraded because of off-color kernels caused by kernel smut spores. In 2018, affected fields were reported in Glenn, Colusa, and Butte counties.

In one Glenn County trial with a history of kernel smut, the fungus was not a problem in 2018. All treatments were done at the late boot, very early heading stage. No significant effects were observed among treatments. However, the treatment with Stratego® had the lowest proportion of infected panicles and number of smutted kernels.

The other Glenn County test site had a severe kernel smut problem in 2017, but levels were very low in 2018. This was an organic field, so all fungicide treatments were with organic fungicides. Treatments were made at the midboot stage.

None of these treatments significantly reduced the proportion of infected panicles compared to the untreated. However, there were differences among some of the treatments. Stargus® and ThermX70® significantly reduced the proportion of panicles infected with kernel smut compared to treatments with Regalia® and ThermX70® at a lower rate. A similar result was observed when evaluating the number of smutted kernels per panicle. Treatment with Stargus® resulted in the lowest number of smutted kernels.

Research also continued to explore methods to assess the level of kernel smut in the field. Determining the number of infected panicles, or the number of infected kernels per panicle, could be used to assess the level of kernel smut in the field. This would allow for a quick evaluation by growers or PCAs without having to collect samples during harvest for processing in the lab.

Three fields affected with kernel smut were selected for sampling: two fields in Butte County (M-105 and M-209) and one in Colusa County (M-206). (See below.) Levels of kernel smut varied among fields. The proportion of infected panicles was similar for M-105 and M-209 but much lower in the M-206 field. A similar pattern was found for the average number of smutted kernels per panicle, with M-209 having higher numbers.

Based on results from 2017, two measures of kernel smut were calculated from field evaluations. Incidence was determined as the proportion of panicles infected with kernel smut, and severity was calculated as the average number of smutted kernels per panicle.

Anecdotal experience indicates that kernel smut incidence is sometimes related to high yields. That’s because high levels of nitrogen fertilizer can increase the incidence and severity of kernel smut. Fields with high levels of nitrogen may also have higher yields and kernel smut. Growers who have seen severe kernel smut infestation in their fields have reported yield losses of up to 10%.

As the proportion of infected panicles increased, milling yield decreased. However, only M-206 showed a strong relationship. Milling yields remained at 72% until the proportion of infected panicles reached 40% to 50%. After that, milling yield decreased. For milling quality to decrease one percentage point, the proportion of infected panicles had to increase by 25%.

Kernel smut was observed in two of the eight statewide variety trials conducted in 2018. (See below.) Smut levels in the Glenn County trial were lower than in the Butte County trial. Long grains CJ-201, L-206, and L-207 had the highest levels of smut. Of the medium grains, M-209 had significantly higher levels.