Cause and Control of Rice Diseases, 2000

The major goals of this project are to gain an improved understanding of the biology of the diseases that affect rice in California and to develop methods to minimize their damage to the crop. Emphasis is on stem rot, aggregate sheath spot, blast, and the newest fungal disease, Bakanae. Control methods under study include cultural practices that affect occurrence and severity of diseases and the potential use of fungicides. Progress on these fronts is discussed below.

Blast monitoring

Plant pathologists continued to monitor occurrence and spread of blast. For the most part, blast disease was of little consequence in last year’s rice crop. Until last year, the blast disease was known to occur only in Colusa, Glenn, Sutter and Butte counties. The disease did appear in a few fields in Yuba County last year. The continuing spread of blast in the California rice producing areas indicates the continuing need for caution in using seed sources free of the pathogen and also to avoid the possibility of spread through transport of and use of equipment that may be contaminated with residue or dust containing the pathogen.

Collection of blast infected plants in 1999 and DNA analyses of the pathogen isolates obtained from them revealed only one known race (IG-1) of Pyricularia oryzae exists in California. Analysis of samples collected again in 2000 are under way. This information is essential for plant breeders attempting to develop blast resistant cultivars for California.

Fungicide Trials

Results of trials to determine optimum time and rate of application of the fungicide Quadris for control of aggregate sheath spot (AGSS) were consistent with previous results. (See table on opposite page.) A decrease in AGSS severity and increases in paddy yield and percent head rice were observed on M-202 at a rate of 0.2 pounds active ingredient per acre 70 days from planting.

Decreases in AGSS severity and an increase in paddy yield and percentage total and percentage head rice were observed on M-204 with a single application at 70 days from planting. Reductions in stem rot severity were not observed at treatment times and rates tested.

No sites were found in 2000 where the blast disease occurred with enough infection to carry out meaningful fungicide trials for blast control.

Effect of Application Time and Concentration of Quadris on Aggregate Sheath Spot Severity, Plant Height, Yield and Quality (Cultivar M-202)
Treatment Days/Planting Rate
AGSS severity Height (cm) Yield 14% lbs/acre % total % head
1 – 70 0.2 1.73 102 9021a 70.82 52.02
2 – 95 0.2 1.65 103 8514ab 70.37 53.42
3 – 70+95 0.2 + 0.2 1.62 101 8633ab 70.80 56.55
4 – 70 0.3 1.71 101 8485ab 69.60 51.57
5 – 95 0.3 1.69 98 8416ab 70.65 55.37
6 – 70+95 0.3 + 0.3 1.48 99 8463ab 70.85 54.27
7 – Control 0 2.19 98 8032b 70.4 48.07
LSD P<0.05 0.33 ns ns 4.58
1 – 70 0.2 1.80 89.5 10401 72.22 59.22
2 – 95 0.2 2.04 91.0 9964 70.20 59.40
3 – 70+95 0.2 + 0.2 1.65 90.9 10471 69.92 58.87
4 – 70 0.3 1.91 91.0 10337 70.17 54.55
5 – 95 0.3 1.69 87.0 10006 69.52 60.02
6 – 70+95 0.3 + 0.3 1.67 90.5 9957 70.07 58.90
7 – Control 0 2.22 91.0 9846 69.77 52.95
LSD P<0.05 0.44 ns ns 1.34 3.19
AGSS severity determined by progression of disease up the tiller (2=lesions on lower leaves, 3=lesions on leaf below the flag leaf, 4=lesions on flag leaf, 5=lesions above flag leaf collar). Quadris treatments were applied at 0.2 and 0.3 lbs/ai/acre, sprayed at 20 gals/acre, for each application time at 70 and 95 days after planting.

Bakanae Disease

Bakanae disease was first found in California in 1999. Last year plant pathologists set out to determine the distribution and impact of Bakanae disease and the cultural practices that foster its development. The disease was observed in fields in Colusa, Glenn, Butte, Sutter and Yuba counties.

All available information indicates that the disease is primarily seed born and that the pathogen is not particularly well suited to overwintering and survival in the soil. The pathogen is abundant in residue of freshly killed plants at harvest time.

Preliminary studies have shown that the pathogen is primarily a surface contaminant of the seed and that it may be easily removed by treatments with sodium hypoclorite. Additional study is needed to determine concentrations that are effective in removing the pathogen and not detrimental to rice seed and seedlings.

Safe Harbor Sampling technique

Another part of this project is development of a sampling technique to detect incidence of stem rot and aggregate sheath spot. This technique would be used to determine whether burning permits should be issued under the “safe harbor” clause of the Agriculture Burning Reduction Act.

Studies on burning alternatives provided the basis for predicting the risk of disease in following years. UC scientists are cooperating with the Air Resources Board and county Agricultural Commissioners to develop a procedure for sampling and disease identification to justify the “safe harbor” burning permits for fields at risk.

Project Leader and Principal Investigators

Robert K. Webster, professor, Department of Plant Pathology, University of California, Davis