Objectives

Long range objectives of this project are to determine the occurrence, nature and control of rice diseases. The specific objectives for 1985 were to:

• continue studies of the biology of bordered sheath spot caused by Rhizoctonia oryzae and aggregate sheath spot caused by Rhizoctonia oryzae-sativae;
• evaluate the occurrence of aggregate sheath spot and stem rot in minimum tillage systems;
• evaluate fungicide use for control of aggregate sheath spot, stem rot and kernel smut;
• complete studies of the effects of nitrogen fertilization on the severity of aggregate sheath spot and stem rot;
• continue studies of water management effects on these two diseases; screen rice germplasm for resistance to diseases;
• determine the efficacy of seed treatment for control of seed and seedling diseases in drill seeded rice;
• continue studies on the distribution, epidemiology, biology and control of kernel smut.

Biology of Boardered Sheath Spot and Aggregate Sheath Spot

Bordered sheath spot was found in several fields in Butte County but is less prevalent than stem roc or aggregate sheath spot. However, it appears to be a more aggressive disease than aggregate sheath spot and could become a more serious problem.

Diseases in Drill Seeded Minimum Tillage Rice

• Indications from 1985 field trials are chat diseases in minimum tillage systems are no greater and, perhaps, less than in conventional tillage systems.
• Differences were observed in severity of aggregate sheath spot among three different varieties in a minimum tillage experiment. M-201 exhibited greater disease symptoms than S-201.
• L-202 was intermediate.
• No varietal differences in susceptibility to stem rot were observed.
• Nitrogen fertilization rates did not affect disease development of either aggregate shear spot or stem rot.

Fungicides for Rice Disease Control

Six fungicides were tested at different rates and application times for control of aggregate sheath spot and stem rot on the variety S-201. There was essentially no stem rot disease present, so no disease ratings were made. Three of the six fungicides indicated some control of aggregate sheath spot, but these differences were not reflected in plot yields. No recommendations are possible at this time for use of fungicides to control these two diseases.

Water Management

Water management experimental plots on Gordon Wylie's ranch in Colusa County.

The primary innoculum for aggregate sheath spot and stem rot is carried to the infection site by flood water. Purpose of the water management studies was to determine the persistence and effectiveness of the primary innoculum at different water depths and under continuous and discontinuous flooding. Field studies were conducted in Colusa and Sutter counties.

The incidence of stem rot was very low and no significant differences were observed among different treatments.

Aggregate sheath spot was significantly less severe in plots that were drained after the initial holding period. There were no significant differences among the other water management treatments. In the Sutter County trials, stem rot incidence was greater in herbicide treated plots than in untreated plots.

Differences in disease among water management treatments may have been apparent if the disease pressure had been greater. Continuation of the study is necessary to determine long term effects of water management on inoculation levels of both stem rot and aggregate leaf spot.

Kernel Smut of Rice

Kernel smut of rice caused by the fungus Neovossia horrida has been detected in California rice. Studies were conducted to determine the extent of infestation, to eliminate smut spores from infested seed, determine the disease cycle, develop reliable innoculation methods for determining varietal susceptibility and to develop control measures.

Kernel smut was found in 11 percent of the 1,100 fields sampled in 1984. The disease appears to be concentrated in the Sacramento Valley. Although most varieties appear to be susceptible, the highest incidence of smut was in long grain varieties. Methods for reliable inoculation and screening of cultivars are now available. Sodium hypochlorite seed treatments eliminate kernel smut from seed.

Results thus far indicate that kernel smut is widely distributed and chat control methods ocher than clean seed or seed treatment are needed.