Rice Disease Research and Management, 2022


Luis Espino, UCCE farm advisor, Butte and Glenn counties

Managing disease in rice fields is the focus of this project. The focus of this project changes with time since different diseases pop up with varying conditions. This year the project focuses on :

Determining the susceptibility of California varieties to stem rot, aggregate sheath spot, and kernel smut, and explore the effect of these diseases on yield and quality
Investigate the relationship between disease ratings at drain time and disease ratings before heading for stem rot and aggregate sheath spot
Investigate the effect on stem rot development when reducing water flow during heading and grain fill
Monitor the response of blast resistant M-210 to blast epidemics in the field

Stem Rot

Two variety trials were conducted in fields with a history of stem rot and aggregate sheath spot (AGSS). Similar to 2021, varieties with longer periods of development showed lower levels of stem rot severity. This translates to varieties S-102, CM- 101 ad M-105 having greater stem rot severity than the lower levels of M-209 and M-211. Application of azoxystrobin reduced stem rot severity by 20%. Yield was not increased, but head rice values increased by an average of 3%.

Aggregate Sheath Spot

The long grain variety, L-208, showed significantly lower levels of AGSS, followed by M-105 and M-206. Azoxystrobin application reduced disease severity by 63% and resulted in a yield increase of 4%. Head rice yield was also increased by 6%. This treatment was effective across nearly all varieties and increased milling yield. The trials show that reducing AGSS levels by applying azoxystrobin resulted in a yield benefit even though disease levels are low.

Disease Ratings at Drain Time

Data on AGSS was not collected for this objective due to low disease levels. For stem rot, disease incidence taken at mid to late boot may predict disease incidence or severity at late maturity. Incidence and severity were related at each of the sampling times. For the mid to late boot stage, incidence and severity were linearly related, with incidence approaching 100% when disease severity was close to 1. At grain maturity, severity increased linearly with incidence until reaching a rating of 2.

Disease incidence and severity at maturity is linearly related to disease incidence at the boot stage until it reaches 30%. At levels above 30%, prediction is variable and difficult. When disease incidence reaches 30% at boot, disease incidence and severity reach approximately 60% and 1.5, respectively.

Reduced Water Flow

Reduced water flow during the heading and grain filling stage did not increase the incidence or severity of stem rot. Holding water after heading, instead of maintaining a continuous water flow until drain time, may not result in increased stem rot levels. Water depth was not manipulated in this trial. Grower may increase their water level at heading and hold the water until it subsides, instead of draining the field. In this case, water depth will be larger and may affect water temperature and stem rot differently than in the trial.


During the season, there were no reports of blast in the Sacramento Valley. Blast was confirmed in two fields in the San Joaquin Valley. This is the first report of blast for rice in this area. Additionally, Nigrospora oryzae was identified causing panicle branch rot in one of the San Joaquin blast affected fields. This pathogen has been identified in 2022 causing collar blight in Butte County and in 2021 causing panicle discoloration in Yolo County. A bacterium, Pantoea ananatis was also identified in 2021 in the San Joaquin Valley. The industry needs to remain vigilant and monitor further developments of these pathogens.