A primary research goal has been to obtain thorough knowledge of the economic importance of rice diseases and to apply this knowledge in seeking control. Since stem rot is the most serious disease of rice in California, it has received primary emphasis.

In 1969, at the beginning of our research program, little was known of the role that disease played in California rice production. Preliminary study at UCD had confirmed the importance of stand establishment, although it was not known whether seed rot and seedling disease were important factors and could be controlled. Growers compensated by an ever-increasing rate of seeding and, in severe cases, by reseeding. Stem rot was first observed in 1933, but in the following years rice research workers and farmers considered it normal and unimportant. As research progressed, other diseases of rice were noted: sheath blight, Pythium crown blight, and Helminthosporium leaf blight.

Water-sown rice is often attacked by seed rot and seedling disease, reducing plant stands.

These infections often appear within a few days of seeding water-sown rice and, though prevalent throughout the rice-producing areas of California, are generally more severe when temperatures are cool and unfavorable for rice growth. The fungi Achlya klebsiana and Pythium species were demonstrated to be the major causes of seedling disease.

Difolatan, captan, and Kocide SD - all now registered for use - have been consistently effective in improving total stand establishment.

Over 50 chemical formulations have been tested on the organisms causing seedling disease. Treatment by either slurry or spray mist with flowable formulations, and a drying period on the seed before soaking proved more effective than applications after soaking, or adding the chemicals to the soak water. Seed treated before soaking showed no harm from the chemicals after storage for up to four months. This is the first year of use of Kocide SD, which has a potential in California, particularly where difolatan poses problems. The other chemicals have already found wide commercial use in California rice production because they provide good crop insurance, especially in cold spring weather.

Stand density increased 12-36% when seed was treated.

Trials were conducted over a three-year period to determine the feasibility of reducing seeding rates (commonly 150-200 lb/A) by using fungicide-treated seed. The results confirm findings of small trials evaluating seed treatment and clearly show that fungicide seed treatment reduces seed rot and seedling disease in water-sown rice under commercial conditions. Combined, the results lead to the following conclusions:

1. The increased stand from seed treatment justifies considerable reductions in seeding rates, in many areas to around 135 lb/A.
2. This reduction more than pays back the cost of seed treatment.
3. Seed treatment provides more uniform stands and reduces the need of reseeding.

Losses in commercial rice yields attributable to stem rot have run as high as 50%, with 10-12% losses occurring commonly.

Three sclerotial fungi found to cause symptoms similar to stem rot have been isolated and identified as Sclerotium oryzae-sativae Sawada (not previously known in the U.S.), Sclerotium hydrophilum Sacc. (reported in California for the first time), and Sclerotium oryzae. The first two were shown to be of minor importance as pathogens under California conditions on California varieties, while S. oryzae was found to be causing considerable economic loss to growers when conditions favored disease development. S. oryzae pervades the northern California rice-growing region, and occurs to a lesser extent in other rice areas of the state. It was found in over 35% of 209 fields examined.

Investigations have shown that sclerotia of S. oryzae overwinter from one season to the next either free in the soil or in association with plant residues, with the latter giving better survival. In water-sown rice, sclerotia float to the water surface and provide the primary inoculum infecting young rice plants at the water line. When infection occurs early in the season, tillers are either killed or fail to produce panicles. Additional losses often result from increased lodging of infected plants. All California varieties are susceptible, with Colusa affected least.

Since a high proportion of sclerotia as recovered from nature frequently fail to germinate, the cause of loss of viability was studied for possible use in control.

Results indicate that inhibition of sclerotial germination is associated with cellular organisms, fungi, bacteria, and actinomycetes isolated from rice soils. Of 56 organisms tested, only Trichoderma aureoviride and a mixed culture of two bacteria completely inhibited sclerotial germination. Two species of Penicillium, three isolates of actinomycetes, and two isolates of bacteria inhibited germination mildly. Hopes for using this information in control of stem rot will rest on our ability to manipulate populations of microorganisms that are fungistatic to S. oryzae. This will receive major research attention over the next three years.

Disease severity and yield loss are related directly to inoculum level. That correlation justifies the extensive studies on residue management carried out under this project. It is also the basis for recommending management decisions regarding method and time of residue disposal and potential for use of fungicides for stem rot control.

Open-field burning of residue is clearly the most effective method tested for minimizing losses to stem rot disease.

Sclerotia of S. oryzae form abundantly in infected stem tissues near the time of rice maturity, and remain and continue to form in crop debris as long as temperature and moisture permit. California rice growers have traditionally burned their rice residue, mainly because the large volume of straw and stubble interferes with preparation of fields for continuous rice culture. At the outset of the studies it was not known whether burning, or the likely alternative method of soil incorporation, would have any appreciable effects in reducing inoculum levels of S. oryzae. For this reason, long-term trials were established to compare the effects.

The effectiveness of moldboard plowing and stubble disk plowing as initial tillage practices, in both fall and spring, was compared with field burning for 4 consecutive years at one site and 5 years at another. In both locations, inoculum levels in seed beds increased significantly with each year of residue incorporation in all treatments except burning, where the inoculum remained at the original level.

Thus, burning rice residue not only is the most efficient and economical approach to continuous rice culture but also effectively minimized stem rot disease in areas where it is well established. Further, burning prevents build-up of the disease where it is not currently a problem.

Control measures in conjunction with burning are included in the research program.

Open-field burning of rice residue may even reduce inoculum levels in many cases. The next-most effective means of minimizing inoculum is deep plowing, providing the Straw and stubble are completely buried.

Plant breeders and pathologists are making every effort to ensure that the new varieties are evaluated for stem rot resistance, and extensive efforts have been made to obtain and identify effective sources of resistance.

Researchers have consistently observed that 10-12 plants per sq ft produce as high yields as stands ranging up to 34 plants per sq ft. On the other hand, stem rot incidence and severity increase as stand densities are increased. Thus, stand densities greater than needed for maximum yield should be avoided so as to reduce inoculum levels, particularly with continuous rice cropping.

Similarly, since excess nitrogen fertilization increases stem rot inoculum for future rice crops, plants should be grown at minimum adequate nitrogen levels.

Since Sclerotium oryzae is a wound-invading parasite, any practice that injures or stresses rice plants might be expected to enhance infection and disease development. Since phenoxyherbicides such as MCPA are used extensively for broadleaf weed control and also can injure rice under some conditions, researchers have compared rates and time of application of such herbicides with stem rot severity and rice yields. With MCPA, rate of application had less effect on disease severity than did time of application. Disease was more severe and yields were reduced significantly with all rates applied 55 days after planting. It is concluded that the late applications of MCPA predispose the rice to stem rot.

Du-Ter, the most promising fungicide recently evaluated, shows good prospects against stem rot. Its use will require government approval and registration.

Du_Ter's effectiveness as a fungicidal control of stem rot disease has been evaluated in field tests over the past 3 years. Single applications (from ground or air) at 1 and 2 INA at the mid-tillering stage significantly reduced disease severity at harvest. The fungicide reduced the number of early-season infections, thus delaying disease progress and minimizing final disease severity. Yield increases due to fungicidal disease control represent gains above those achievable by open-field burning, since all of the fields had been burned in the previous fall.

The EPA has not yet registered Du-Ter for use on rice. With emphasis now on research toward obtaining registration, the primary objective will be to obtain data on Du-Ter effects on nontarget organisms. Consequently, trials are being conducted to determine its effects on algae, weeds, and alternate crops that may receive water from treated fields.

Three minor rice plant diseases are being monitored but appear to pose no serious threat in California.

In southern rice-producing states, Rhizoctonia sheath blight is a serious disease often limiting production by 50% or more. From experiments during 1974-76, UCD plant pathologists have concluded that a similar Rhizoctonia sp. causing sheath blight in California is not identical to the southern states variety. The high occurrence of Rhizoctonia in field trials, however, suggests that it should be monitored as new varieties are released.

Pythium crown rot has been observed only sporadically in the last few years, and usually associated with nutritional disorders or extreme MCPA damage. It appears not to be a threat, warranting no further study.

Helminthosporium leaf spot has been observed only immediately after unseasonal rains in July or August. Since its occurrence depends on the availability of free moisture on the rice leaf surface, it is not considered a threat to California rice production.