Flutianil was discovered as a novel fungicide chemically characterized as a cyano-methylene thiazolidine. This chemical showed particularly high fungicidal activities against powdery mildew on various crops. Optimization of the fungicidal performance of the series of synthesized chemicals finally led to the identification of flutianil, which could control powdery mildew diseases. In this report, details of the structure-activity relationships from the lead compound to flutianil are described.

Flutianil, chemically characterized as a cyano-methylene thiazolidine, showed antifungal activity against powdery mildew on various crops but not any other plant pathogens tested. Flutianil showed high residual and translaminar activities and rainfastness against Podosphaera xanthii. It also exhibited curative activity against P. xanthii on cucumber at an extremely low concentration of 10 mg/L. There was no cross-resistance between flutianil and other existing fungicides. Morphological studies revealed that flutianil did not inhibit the early infection behavior of Blumeria graminis f. sp. hordei i.e., conidium bursting, primary and appressorial germinations, appressorium development and hook formations, but it did inhibit haustorium formation and further fungal development. Nutrient absorption by haustoria and subsequent secondary hyphal elongation was inhibited by flutianil but not by the fungicide that showed a similar inhibitory pattern up to the haustorium formation stage of the infection process. These findings suggest that flutianil might have a novel mode of action.

The genes required for the pathogenicity of plant pathogens can be divided into two groups: those for eliciting virulence and those for establishing host-parasite interactions. Basically, the former genes are required for efficient production of the virulence factor(s) leading to the development of symptoms, while the latter genes are needed for well-balanced production of the factors required at each step of the parasitism. To understand the overall picture of plant pathogenicity, we need to understand not only the list of both genes but also how their expressions are regulated. We chose the major virulence genes of soft rot causing Enterobacteriaceae (represented by Pectobacterium carotovorum subsp. carotovorum and Dickeya dadantii) as a model system for dynamic study of the former genes. For the dynamic study of the latter genes, we chose the hrp genes, many of which are responsible for the Type III secretion system (T3SS) and the genes for the T3SS-dependent effectors of xanthomonads. Particularly since the major effector of xanthomonads, the AvrBs3 group or TAL (transcription activator-like) effectors contain NLS (nuclear localization signal) and an acidic activation domain, their studies are also expected to lead us to dynamic analyses of plant genes too.

S Supplementary materialFlutianil, a fungicide effective only on powdery mildew, was previously reported to affect the host cell's haustorial formation and nutrient absorption. Studies were conducted to investigate flutianil's primary site of action on Blumeria graminis morphology using transmission electron microscope (TEM) observation and RNA sequencing (RAN-seq) techniques. TEM observation revealed that flutianil caused the extra-haustorial matrix and fungal cell wall to be obscured, without remarkable changes of other fungal organelles. RNA-seq analysis indicated that, unlike other powdery-mildew fungicides, flutianil did not significantly affect the constantly expressed genes for the survival of B. graminis. Genes whose expression is up-or downregulated by flutianil were found; these are the three sugar transporter genes and various effector genes, mainly expressed in haustoria. These findings indicate that the primary site of action of flutianil might be in the haustoria.