Stem rot is a common rice pathogen present in all fields. The pathogen produces resting structures called sclerotia that survive in the soil after harvest. The sclerotia germinate in the spring and the pathogen infects tillers at the water level approximately when plants reach panicle initiation (45 to 50 days after seeding). The pathogen develops in the tiller, causing lesions that penetrate the tiller, reducing the movement of nutrients to the grains and increasing lodging. As sclerotia accumulate over time, the disease becomes more severe.
Several years of trials have shown that when the disease is severe, yield losses can be as high as 12-14%. Managing stem rot requires an integrated approach: burning, managing straw residue to maximize decomposition, using an appropriate N rate, addressing K deficiency, and, if necessary, incorporating a fungicide in your program.
The challenge with stem rot is that it is a difficult disease to evaluate. Yield losses can be masked by other problems such as weeds, weather, etc. Because tiller lesions are at the water level, symptoms are not evident until the disease level is very severe. To aid in the monitoring of stem rot, guidelines for the mid boot stage were developed. By knowing the level of stem rot at mid boot, a manager can make the determination to make a fungicide application to help reduce the severity of stem rot. However, remember that an integrated approach is needed to reduce the severity of stem rot over time.
Another factor that needs to be discussed here is variety. Trials have shown that varieties that have thicker tillers, such as M-209 and M-211, tend to develop less stem rot than varieties with thinner tillers like M-105. This should be taken in consideration when monitoring for stem rot.
Based on the information generated by the trials mentioned above, a fungicide application would be beneficial when the incidence of tillers with stem rot at the mid boot stage is:
To determine incidence, cut a handful of tillers at the soil level in several areas of a check when plants are at the mid boot stage. The more handfuls the better, but no less than three. Avoid N overlaps or skips. Combine the tillers and randomly select 30 to 50. Inspect these tillers and determine the percentage that have stem rot lesions. Use the guideline above to determine if a fungicide application would be of benefit. Remember that a fungicide typically reduces the severity of stem rot by 30%. Do not expect stem rot to disappear from the field after a fungicide is used. To further reduce the severity, implement other management actions such as straw burning, incorporation, reducing N rate, and addressing K deficiency.