Objectives

The long-range objectives of the disease control project are to determine the occurrence, nature, and control of rice diseases in California. The specific objectives for 1983 were continue study of the biology and epidemiology of sheath blight and stem rot diseases of rice; evaluate the effect burning residue of semidwarf rice varieties has on disease control; continue studies of improved methods for disease evaluation and the use of wild species of rice as sources of genetic resistance to diseases; conduct further fungicide screening trials for use in disease control; evaluate semi-dwarf varieties for disease severity at different nitrogen levels, stand densities and MCPA time and rates of application; and, compare disease severity and yield.

The recent increase in the incidence and severity of sheath blight disease has paralleled an increase in the use of semi-dwarf varieties. Symptoms of the disease are very similar to those of sheath blight of rice caused by a related organism in the southern United States. The sheath blight organism common in California has not been reported on rice outside of California, and the organism common in the South has not been found on rice in California. Another organism, Rhizoctonia oryzae, was identified as pathogenic to rice this year, and its role as a rice pathogen in California is being studied.

Dr. Robert Webster examines rice plants for resistance to stemrot.

Stem rot continues to be a rice disease of major concern. Rice plants may be infected by both stem rot and sheath blight. In fields where both diseases occur, one or the other is usually most severe: Methods have been developed to determine- inoculum levels of all three disease organisms. An understanding of the epidemiology of all three diseases is needed as control measures for each disease are developed. Studies will be continued on the biology of all three pathogens to the degree that information regarding control is facilitated.

Studies were continued to determined whether burning of semi-dwarf varieties was as effective in controlling stem rot and sheath blight as was burning for disease control on tall varieties. Preliminary results suggest that the effectiveness of burning for disease control is directly related to the proportion of residue removed by burning. These studies will be continued for one more year.

Screening of weedy Oryzae species and progeny from their crosses with common varieties has been continued for both stem rot and sheath blight. Although progress is slow, results thus far suggest that increased resistance can be incorporated into future varieties. These studies are cooperative with Dr. J. N. Rutger, USDA; and the rice breeders at the Rice Experiment Station.

Tests to evaluate fungicides for potential use in controlling stem rot and sheath blight were continued, but no chemicals as effective as Duter were found. Tilt, a fungicide showing promise last year, gave erratic results this year. However, in one trial with Tilt, highly significant reductions in disease and increases in yield were obtained on three of five varieties tested at two nitrogen levels.

A major effort was made in 1983 to determine the effects of variety grown, nitrogen fertilizer rates, seeding rate, MCPA application rate and time and treatment with fungicide on disease interaction. The results indicate that cultural practices can affect disease reaction and yield. Maximum yield and disease suppression usually occurred at seeding rates and nitrogen levels lower than those used by growers. These studies will be continued and expanded.