Stem Rot of Rice

In water-sown rice, stem rot first appears during the tillering stage as small, black lesions on leaf sheaths at the water line. As the disease progresses, infected sheaths die and slough off. The infection may eventually penetrate the culm.

The fungus overwinters as small, black resting structures called sclerotia. Sclerotia may survive free in the soil but are more often associated with infected rice plant residues remaining in the field after harvest. The following season, after the field is flooded, the sclerotia float to the water surface and infect leaf sheaths of young rice plants at the waterline. New infections may continue to occur throughout the growing season.

When infection occurs very early in the season, tillers are either killed or fail to produce panicles. When the culm is infected, grain filling, grain quality, and panicle size are reduced. Additional losses often result from increased lodging of infected plants prior to harvest. Yield losses of 6 to 24% have been documented in California.

Manage stem rot by minimizing carryover inoculum (i.e., the number of viable sclerotia) associated with crop debris and in the soil, and by planting the least susceptible rice varieties available. Surface-applied potassium fertilizer and winter flooding of rice fields are also reported to reduce rice stem rot development. Avoid excess nitrogen, as it increases the severity of stem rot.

Carry-over inoculum produced in residue of the previous crop infects the current year’s crop and any practice that minimizes the amount of inoculum in the seed bed is beneficial in disease management.

Field burning of rice residue in the fall, following harvest, is an effective method of reducing carryover inoculum. Burning destroys the existing sclerotia and residue on which sclerotia form during late fall, winter, and spring. The level of carryover inoculum is determined by the completeness of the residue destruction. Swathing at ground level and removing the straw from the field is nearly as effective as burning. Incorporation of straw and winter flooding has also proven helpful in reducing carry over of sclerotia to the following season. Tillage practices, such as moldboard plowing, that bury sclerotia and prevent them from floating and infecting the plants at the waterline reduce available inoculum levels. The level of disease reduction is dependent on the effectiveness of these practices to reduce the number of viable sclerotia that are free to float to the water surface upon flooding the rice field.

Fallowing fields for a year or two may reduce the number and viability of the sclerotia. Irrigating a fallow field during the summer encourages the decomposition of crop residue, further reducing the production of sclerotia.

In addition to managing inoculum levels, plant the least susceptible varieties of rice available. All public rice varieties currently being grown in California are susceptible to stem rot to some degree. However, longer cycle varieties like M-209 or M-211 are more tolerant than short cycle varieties like CM-101, S-102, or M-105.

Stem rot incidence and severity increase as stand densities increase; thus, avoid rice stands that are too dense. Rice stand density of 25 plants per square foot optimizes yield. A seeding rate of 150 to 175 pounds per acre generally achieves this density.

Excess rates of nitrogen fertilizer increase rice stem rot severity. Apply only the amount of nitrogen fertilizer necessary to optimize yields. Use rice tissue analysis, a leaf color chart, or chlorophyll meter readings to monitor critical nitrogen levels during the growing season to determine if there is a need for supplemental nitrogen.

The stem rot fungus may penetrate the plant directly or invade wounds. Practices that injure or stress rice plants (e.g., herbicide injury) have been shown to increase infection and disease development.

All cultural controls discussed above are acceptable in a certified organic crop.

Currently, there are no guidelines to monitor for stem rot during the growing season; treatment decisions are made according to the disease history of a field. The need for management during the following season can be determined when the field is drained for harvest.

At drain time, examine tillers in several locations throughout the field for the presence of stem rot lesions. If 100% of tillers have lesions, a negative effect on the following year's yield or quality may occur and planning for a fungicide application the following year would be appropriate. Apply azoxystrobin at the early heading stage, when the first panicles start to emerge from the boot and are visible above the canopy. The fungicide azoxystrobin has been shown to reduce stem rot severity by 20 to 30% when applied at the early heading stage. Fungicide applications at propanil timing are not effective in reducing disease severity.