Response to treatment also varies by cultivar. S-201 showed the greatest response when compared with M-201 and L-202 at three different locations.

Kernel Smut Declines During 3-Year Study

Three years of extensive sampling show that kernel smut is widely  distributed in California rice areas and that it is most prevalent in Glenn, Colusa, Butte, Sacramento, and Placer counties.

The good news is that there were fewer smut contaminated fields in 1985 and 1986 than there were in 1984. This suggests that the incidence of smut is highly dependent on environmental conditions and is probably not significantly spread through infested seed lots.

Long-grain rices are more susceptible to kernel smut and short-grain rices are the least susceptible. These results are consistent with other studies. Wider florets and longer anthesis in the long-grain cultivars may be making these cultivars more susceptible to smut. Greenhouse innoculation studies produced no differences in smut susceptibility among cultivars.

The incidence of kernel smut also is higher on the early maturing short-grain, S-201, and the early long-grains, L-202 and California Belle.

Researchers involved with this three-year study conclude that kernel smut will not become a major California rice disease because the environment does not appear to enhance severity of kernel smut. However, they caution that new long-grain cultivars should be carefully evaluated for the disease before they are released. They add that since the disease is already widely distributed throughout most of the state's production areas, quarantine of infected seed lots is not warranted.

Too Much Nitrogen Enhances Stem Rot

Trials to determine the effect of nitrogen on stem rot show that too  much nitrogen can increase severity of the disease. Eight levels of nitrogen, ranging from none to 210 pounds per acre were tested using the cultivar M-401.

Stem rot severity increased as nitrogen increased. Maximum yields of 71.8 cwt/acre were produced when the nitrogen rate was 150 pounds per acre. Stem rot severity increased significantly when nitrogen was increased from 150 to 210 pounds per acre.

Yields decreased to 66.8 cwt/acre at the higher nitrogen rate. Similar results were obtained in an identical trial using the cultivars M-201 and M-202.

Aggregate sheath spot was most severe when nitrogen was applied at a rate of 60 pounds per acre.

The researchers conclude that excess nitrogen fertilization increases stem rot damage on all rice cultivars but that aggregate sheath spot and boardered sheath spot are more severe when nitrogen is inadequate to obtain maximum yield.

 Potassium has Little Beneficial Effect on Disease

Adding 100 pounds of potassium per acre does little to increase yields or reduce disease. In fact, adding the potassium increased stem rot severity in all three locations where the effects of potassium were studied. However, the addition of potassium did reduce aggregate sheath spot severity in one of the fields.

Certain Residue Organisms Enhance Biodegradation

Several fungi found in rice residue after harvest were isolated and tested for their ability to inhibit S. oryzae sclerotial formation. Three of these - two Penicillium species and one Trichoderma specie were highly inhibitory. Similar attempts to isolate and identify bacteria found in rice residue are not yet complete.

Fungicide Controls Prove Questionable

The fungicides Tilt, Benlate, and Rovral were tested for control of stem rot and aggregate sheath spot. None of the chemicals satisfactorily controlled both diseases. Tilt gave fair control of stem rot. Benlate worked best against aggregate sheath spot. Rovral showed promise for control of aggregate sheath spot but not stem rot. None of the treatments increased yields.
 

Non-Chemical Controls Attack Diseases California producers have an inherent edge in combating rice diseases. "The nature of the diseases and the nature of our climate offer the best hope," says project leader Robert K. Webster. "We could control them with fungicides, but the future of chemicals is in doubt. The next best approach is cultural or biological control." What makes UC Davis plant pathologist Robert K. Webster tick is the never-ending search for the causes and cures of California rice diseases. For instance, kernel smut - which infects the rice panicle's florets and turns seeds into a black powdery pulp - has strict environmental requirements. Because of that, Webster believes it won't become a "devastating thing" to the industry as a whole, even though yield losses can range up to 50 percent for individual producers. "We're trying to find the organisms that will lend some biocontrol for kernel smut," Webster noted, "And we've found some good sources of resistance in wild species. but that's long range." In the short run, growers must refine their cultural practices. "All approaches deal with minimizing inoculum levels," he said. "That's why we've been fairly successful with burning." Webster, who has studied rice for more than 20 years and is chairman of the Plant Pathology Department at UC Davis, noted the project has also yielded valuable new data on controlling stem rot. "Too much nitrogen enhances disease and inoculum carry-over levels," he said. "Some diseases just flourish better on plants with excess nitrogen." These findings underscore the need to delve into the internal mechanisms of rice diseases, Webster says.