For plant pathogenic fungi, such as powdery mildews, that survive only on a limited number of host plant species, it is a matter of vital importance that their spores sense that they landed on the right spot to initiate germination as quickly as possible. We investigated a barley (Hordeum vulgare) mutant with reduced epicuticular leaf waxes on which spores of adapted and nonadapted powdery mildew fungi showed reduced germination. The barley gene responsible for the mutant wax phenotype was cloned in a forward genetic screen and identified to encode a 3-KETOACYL-CoA SYNTHASE (HvKCS6), a protein participating in fatty acid elongation and required for synthesis of epicuticular waxes. Gas chromatography-mass spectrometry analysis revealed that the mutant has significantly fewer aliphatic wax constituents with a chain length above C-24. Complementation of the mutant restored wild-type wax and overcame germination penalty, indicating that wax constituents less present on the mutant are a crucial clue for spore germination. Investigation of Arabidopsis (Arabidopsis thaliana) transgenic plants with sense silencing of Arabidopsis REQUIRED FOR CUTICULAR WAX PRODUCTION1, the HvKCS6 ortholog, revealed the same germination phenotype against adapted and nonadapted powdery mildew fungi. Our findings hint to an evolutionary conserved mechanism for sensing of plant surfaces among distantly related powdery mildews that is based on KCS6-derived wax components. Perception of such a signal must have been evolved before the monocot-dicot split took place approximately 150 million years ago.

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Small deposits result in high AI-adjuvant association. With larger deposits, annulus structures allow for enhanced AI-adjuvant association (5-20 times greater). The formation of annuli appears to be important in enhancing the biodelivery of particulate AIs in adjuvant-containing suspoemulsion formulations at intermediate spray volumes.

Carpropamid {(JRS, 3SR)-2, 2-dichloro-N-[1-(4-chlorophenyl)ethyl]-1-ethyl-3-methylcyclopro-panecarboxamide} strongly inhibited pigmentation in shaking culture as well as in agar plate culture of Pyricularia oryzae and Verticillium dahliae. White crystals were isolated from the extract of the cultures of wild type strains of P, oryzae and V. dahliae treated with carpropamid. Analytical data of the crystals obtained from a spot at Rf 0.36 on the TLC plates coincided with those of scytalone. A large amount of scytalone and a small amount of vermelone accumulated in the culture treated with l0ug/ml of carpropamid, while 2-hydroxyjuglone was not detected in these analyses. When scytalone was added to the shaking culture of albino mutants of P. oryzae (P2-alb), it promptly disappeared from the culture and the culture was pigmented to light brown. But scytalone remained in the culture if treated with carpropamid. Scytalone and vermelone administered beside the colony of P2-alb on the agar plates were converted to black pigments but the pigmentation was inhibited by carpropamid. These results suggest that carpropamid inhibits dehydration of scytalone and vermelone in the fungal melanin biosynthesis.

With three plant pathogens, Botrytis cinerea, Venturia inaequalis and Puecinia graminis f. sp. tritici, the time course of sterol biosynthesis during spore germination was examined by labeling experiments along with the question whether this pathway could be inhibited by triazole fungicides. Conidia of B. cinerea and V. inaequalis are able to synthesize sterols immediately after the beginning of the germination process when the germ tubes have not yet emerged. On the contrary uredospores of P. graminis start sterol biosynthesis after 6 to 8 h germination time almost at the end of the germ tube phase, indicating that sterol reserves of the spores are likely to be used for the germ tube growth.The sterol C-14 demethylation appeared to be the rate limiting step within the sterol biosynthetic pathway: the half life of 24-methylenedihydrolanosterol was less than 1 h for B. cinerea. It was more than 1 h for V. inaequalis and 3 h for P graminis. Independent of these differences in the time course of sterol biosynthesis and in the C-14 demethylation rate, the synthesis of sterols in germinating spores was strongly inhibited by triazole fungicides in all three pathogens examined. In contrast to P. graminis, this inhibition could be demonstrated with B. cinerea and V. inaequalis even in ungerminated conidia, indicating that the fungicides were rapidly taken up and reached their target within 1 or 2 h. These results are discussed along with the question whether spore germination can be used as a bioassay for the estimation of sensitivities of triazole fungicides.