Objectives

• Control seedling diseases of drilled rice.
• Control sclerotial fungi diseases.
• Determine effects of cultural practices on disease severity.
• Determine distribution and severity of kernel smut.
• Chemical control of rice diseases.

The UC Department of Plant Pathology has conducted research on the diseases of rice and their control with Rice Research Board support since 1969. Although California is relatively free of some of the serious rice diseases occurring in some other rice growing areas, there are several that can and sometimes do significantly reduce yields. These include diseases that destroy rice seedlings; damage or kill stems, leaves and leaf sheaths; and kernel smut.

Aggregate sheath spot, although a minor disease, has been increasing in recent years.

Drill seeding at 1 1/2- and 3-inch depths resulted in no differences in the presence of fungi or the number of tillers per plant. More plants were established at the 1 1/2- depth, and the yields per acre were greater, but these differences were not statistically significant. These studies were conducted at the Rice Research Facility at Davis with the variety M-101 drilled at the rate of 90 pounds per acre. Plots were flooded for 24 hours, then drained, reflooded, and drained again after 14 days.

There are four species of Rhizoctonia-causing rice diseases that exhibit somewhat similar symptoms. Primary infection of tillers comes from sclerotic of the fungi floating on the surface of the water. First symptoms usually are not observed in the field until rice plants have reached maximum tillering stage. Sheath blight is the most aggressive and injurious of the Rhizoctonia-causing diseases. Aggregate sheath spot and bordered sheath spot, although present in California, are weak pathogens in rice. A sheath spot disease caused by Rhizoctonia zeae is a minor pathogen. Each of these diseases responds differently to different fungicides, complicating attempts to use fungicidal control.

The incidence and severity of aggregate sheath spot have increased in Cali= fornia in recent years, possibly a result of shifting to more susceptible semi-dwarf varieties. For the past two years, sheath spot severity has been compared among varieties and at different rates of nitrogen fertilization. Varietal susceptibility accounts for most of the differences. The tall varieties S-6 and Earlirose were the most disease resistant. S-201 was the most resistant among the semi-dwarf varieties; M-201 and L-202 were the most susceptible. The varieties M9, Cal Pearl and California Belle were intermediate in susceptibility.

Intermediate levels of nitrogen fertilizer decreased the severity of stem rot arid aggregate sheath spot. In general, disease severity was greatest with no applied nitrogen fertilizer and increased again at very high rates. Aggregate sheath spot was not affected by seeding rates or by various water management treatments.

Kernel smut severity varies from year to year because of seasonal differences in conditions required for sporidial production, survival and infection. A survey in 1984 showed that 17.6 percent of 1,100 fields had kernel smut. The following year only 5.1 percent of fields surveyed had kernel smut. Long-grain varieties had more kernel smut than short- or mediumgrain varieties. The reason for this difference, may be that florets of long-grain varieties open wider and remain open longer than those of short- or mediumgrain varieties.

Among several chemicals used to control aggregate sheath spot, only Benlate significantly reduced disease severity without reducing grain yield. In previous studies, Benlate did not control stem rot or kernel smut, possibly because their levels were very low when the chemical was applied.

In summary, the 1986 studies revealed varietal differences in susceptibility to aggregate sheath spot and kernel smut. Cultural practices had either limited or no effect on disease severity, and the fungicide Benlate provided some protection from aggregate sheath spot disease. Kernel smut, which appeared threatening in 1984, was a minor problem in 1985, indicating environmental conditions at flowering time may influence its severity.