Biology of Scytalone Dehydratase Inhibitors

Several fungicides inhibit 1,3,6,8-tetrahydroxynaphthalene reductase (MBI-Rs) (Table 20.2). These inhibitors have been used since the 1970s, such as pentachlor-obenzyl alcohol (PCBA) (6), tricyclazole (7) (Eli Lilly, 1975), pyroquilon (8) (Ciba, Pflzer, 1985) and fthalide (9) (Kureha, 1971), without any resistance problems and retain considerable economical importance, mainly in Northeast Asia. They are not discussed in more detail here. 

Common properties of MBI-Rs and MBI-Ds are protective control activity against rice blast and inhibition of appressorium pigmentation of Pyricularia oryzae on agar plates or cellophane membrane [Hattori et al., 1994 Soma et al., 1999 Sie-verding et al., 1998]. Inhibition of spore liberation from leaf lesions is also commonly observed in all known MBI-Rs and MBI-Ds [Kitamura et al., 1976 Okuno 

Abbreviations used in Tables 20.1-20.3 P inhibition activity in vitro, Pyricularia C inhibition activity in vitro, Colletotrichum RE Blast control efficacy in rice, foliar spray RS Blast control efficacy in rice, systemic application MP commercial product in 2006 Res Field resistance authorized by FRAC. 

However, the overwhelming prevalence of the one-shot application with long-lasting MBI-Ds must have enhanced the relatively early outbreak of field resistance, which had not been expected at all for these secondary metabolism inhibitors, based on the long-term experience of using MBI-Rs without resistance problems. 

Another common property of the three commercial MBI-Ds that is not shared by MBI-Rs is specific systemic damage to Solanaceae plants. The relevance of this specific sensitivity and MBI-D activity has not been investigated. However, since MBI-s are exclusively used in an isolated environment for rice, this is not regarded as a serious problem in practice. 

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