Tohoku National Agricultural Experiment Station, Yotsuya, Omagari, Akita, 014-0102 Japan

In Japan, owing to conducive environmental conditions, rice diseases annually cause 1-7% yield losses in spite of the intensive use of fungicides. Yield losses due to blast (Pyricularia grisea) (0.5-5.6%) are the highest for all the rice diseases, while sheath blight (Rhizoctonia solani) (0.3-1.3%) follows blast in losses. In the past 13 years, depending on disease pressure, farmers have applied fungicides for blast and sheath blight to 74-87% and 56-68%, respectively, of the rice cultivated area [1]. Several methods are used to apply fungicides including nursery box and submerged applications, and dusting and foliar spraying by ground and aerial applications.

Fungicides currently in use are highly effective and cause few problems with regard to environmental pollution. However, less toxic and less expensive chemicals with fewer applications offer important advantages in the current state of heightened environmental and economic awareness. Furthermore, the majority of rice-growing farmers, being elderly or having side jobs, need more convenient applications of fungicides than those now available. Ten blasticides are currently registered in Japan: blasticidin S, kasugamycin, fthalide, edifenphos, IBP, isoprothiolane, probenazole, tricyclazole, pyroquilon, and ferimzone. In 1997 carpropamid was registered for blast control. In addition, five new blasticides are awaiting registration: azoxystrobin, metominostrobin (SSF-126), acibenzolar-S-methyl (CGA 245704), diclocymet (S2900), and NNF-9425.

Azoxystrobin as well as metominostrobin are strobilurin analogues which inhibit mitochondrial respiration in P. grisea. Carpropamid blocks the dehydration of scytalone in the melanin biosynthesis pathway and thus has a different site of action than the older melanin biosynthesis inhibitors such as fthalide, tricyclazole, and pyroquilon. Acibenzolar-S-methyl is reported to induce host plant resistance as does probenazole.

Carpropamid, acibenzolar-S-methyl, diclocymet, azoxystrobin, and a new formulation of probenazole have a long lasting effectiveness after granular applications to rice seedling boxes before mechanical transplanting. This effectiveness persists longer than that of currently used fungicides, but is not sufficient to control panicle blast under severe disease pressure. Metominostrobin, which inhibits hyphal growth in rice plants, suppresses blast as a submerged treatment. Foliar treatments with azoxystrobin, carpropamid, diclocymet, and NNF-9425 are also effective.

Five fungicides are currently used to control sheath blight: validamycin A, mepronil, flutolanil, pencycuron, and diclomezine. Two others, furametpyr and thifluzamide (MON-240), were recently registered. Azoxystrobin, which is in the registration process, also suppresses sheath blight. Nursery box granular applications of azoxystrobin, thifluzamide, and furametpyr are effective as well as submerged and foliar treatments [2]. Azoxystrobin also controls brown spot (Bipolaris oryzae) and false smut (Ustilaginoidea virens) [2,3].

Most new fungicides for rice disease control are systemic and effective for long periods due, in part, to their slow-release granular formulations. These fungicides, especially in nursery box applications, are expected to reduce applications of chemical disease control agents.

1. Ministry of Agriculture, Forestry and Fisheries, 1983-1995. Crop protection annual report. Tokyo, Japan.
2. Japan Plant Protection Association, 1997. Results of fungicide field trials. Tokyo, Japan.
3. Japan Plant Protection Association, 1996. Symposium on disease and insect control of paddy rice and dry field crop. 74 pp. Tokyo, Japan.