How to Manage Pests
UC Pest Management Guidelines
SYMPTOMS AND SIGNS
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.
COMMENTS ON THE DISEASE
The fungus overwinters as small, black resting structures called sclerotia. Sclerotia can 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, sclerotia float to the water surface and infect leaf sheaths of young rice plants at the water line. New infections continue to occur throughout the growing season.
When the culm is infected, both grain quality and panicle size are reduced. When infection occurs very early in the season, tillers are either killed or fail to produce panicles; additional losses often result from increased lodging of infected plants. Yield losses from 6 to 24% have been measured in California.
Manage stem rot by minimizing the carryover inoculum level (i.e., the number of viable sclerotia) and by using the most resistant varieties available. Surface-applied potassium fertilizer and winter flooding of rice fields are also reported to suppress rice stem rot development. While the protectant fungicide azoxystrobin (Quadris) is labeled for stem rot, data from research trials indicate the effect on disease incidence and severity have been negligible and that applications for this disease are not economically beneficial.
Carry-over inoculum produced in residue from 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. One method of reducing carryover inoculum is field burning of rice residue in fall following harvest. Field burning destroys existing sclerotia and residue upon which sclerotia form during late fall, winter, and spring. The level of carryover inoculum is determined by the completeness of the residue destruction. Another method that is as effective as field burning is cutting rice at ground level and removing the straw from the field in fall. Tillage practices, such as moldboard plowing, that bury sclerotia and prevent it from floating and infecting the plants at the waterline reduce inoculum levels. Again, the level of disease reduction is dependant on the effectiveness of the burying, which could be hindered by soil moisture conditions or a large volume of residue.
Fallowing fields for a year will reduce the number and viability of the sclerotia. If a fallow field is irrigated during summer, the crop residue degrades even faster, thus reducing the production of sclerotia.
In addition to managing inoculum levels, use the most resistant varieties available. All public rice varieties currently being grown in California are susceptible to stem rot to some degree. Average stem rot scores are included in the current list of characteristics of publicly developed rice varieties. Least susceptible of the widely grown varieties include M-205, M-206, and M-402. Most susceptible are M-103, M-104, M-202, M-204, M401, and S-102.
Stem rot incidence and severity increase as stand densities are increased; thus, avoid rice stands that are too dense. In research plots, rice stand densities of 10 to 12 established plants per square foot produced yields equal to stands ranging up to 34 plants per square foot. A seeding rate of 150 pounds per acre is recommended.
Excess rates of nitrogen fertilizer increase rice stem rot severity. Stem rot severity is lower on plants grown at minimum adequate levels of nitrogen. When growing rice at minimum adequate nitrogen levels, use rice tissue analysis, leaf color chart, or chlorophyll meter readings to monitor critical nitrogen levels during the growing season to determine any 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., the use of phenoxy herbicides) have been shown to increase infection and disease development.
Organically Acceptable Control Methods
All the cultural practices discussed above are acceptable for organic growers.
UC IPM Pest Management Guidelines: Rice