Overview on the Chemical Control of Rice Blast Disease

Abstract

Rice blast disease is the most serious and damaging of all the plant diseases in rice production in such temperate and humid climate like that of Japan, which resulted in the development of many excellent chemicals for this particular disease, e.g., blasticidin S, kasugamycin, iprobenphos (IBP), edifenphos (EDDP), isoprothiolane, ferimzone, and metominostrobin. These chemicals mostly act on the growth of M. oryzae by exhibiting specific fungicidal effects on the pathogen. The wide use of these site specific chemicals sometimes causes development of resistance in the pathogens. In fact, resistance against kasugamycin and organophosphorus thiolate fungicides were recognized in the field where they were too intensively used. Solutions to the problem were contrived for the effective use of the chemicals at hand, such as application of mixtures with different action mechanisms or their use in rotation. While all modern fungicides are developed through extensive safety evaluation, there is a deep public concern about their side effects on non-target organisms and on the environment.

Thus there is a great deal of interest in non-fungicidal disease controlling agents since they are supposedly specific to target organisms and are less likely to lead to resistance problems. Actually, two groups of non-fungicidal rice blast chemicals are currently on the market; melanin biosynthesis inhibitors (MBIs), e.g., fthalide, tricyclazole, pyroquilon, carpropamid, diclocymet and fenoxanil, and the so called priming effectors or plant defense activators such as probenazole, acibenzolar-S-methyl and tiadinil, which induce host resistance against the pathogen’s attack.

Figure 1 shows “sales of blast fungicides in Japan”, which indicates that non-fungicidal plant activator, probenazole, and melanin biosynthesis inhibitors like pyroquilon, carpropamid, tricyclazole and fthalide are main fungicides for the blast control.

The exploitation and development of new anti-blast fungicides have been becoming more and more difficult in recent years; cost effective studies on the toxicity of the chemicals and residue analyses in crops, animals and the environment are highly required to secure human health and to preserve the ecosystems. Studies on molecular action mechanism of new MBIs and plant activators are currently attracting researchers’ interest for the unique features of these chemicals. In this article, recent topics of the blast-controllers used in practice were overviewed mainly in terms of their action mechanisms, which may hopefully suggest the qualities that ideal anti-blast fungicides should possess.