azoxystrobin and famoxadone

Phytophthora capsici is a highly destructive plant pathogen that has spread worldwide. To date, the quinone outside inhibitor (QoI) azoxystrobin has been the choice of farmers for managing this oomycete. In this study, the sensitivity of 90 P. capsici isolates collected from Yunnan, Fujian, Jiangxi, Zhejiang, and Guangdong in southern China to azoxystrobin was assessed based on mycelial growth, sporangia formation, and zoospore discharge. Furthermore, the mitochondrial cytochrome b (cytb) gene from azoxystrobin-sensitive and -resistant P. capsici isolates was compared to investigate the mechanism of QoI resistance. The high values for effective concentration to inhibit 50% of mycelial growth and large variation factor obtained provide strong support for the existence of azoxystrobin-resistant subpopulations in wild populations. The resistance frequency of P. capsici to azoxystrobin was greater than 40%. Sensitive P. capsici isolates were strongly suppressed on V8 medium plates containing azoxystrobin supplemented with salicylhydroxamic acid at 50 µg ml

Topics: Amino Acid Sequence; Amino Acid Substitution; Capsicum; Cytochromes b; Fungicides, Industrial; Phytophthora; Plant Diseases; Pyrimidines; Sequence Alignment; Strobilurins

Resistance to QoI fungicides (strobilurins, famoxadone and fenamidone) in populations of Plasmopara viticola (Berk & Curt) Berlese & de Toni developed soon after their introduction in France and Italy. Current resistance management strategies include limitation of the number of applications, use of mixtures and alternation of fungicides with different modes of action. The selection pressure resulting from QoI fungicides applied alone or in mixtures with non-QoI fungicides was investigated in whole plant experiments under controlled conditions. QoI-resistant populations of P. viticola gradually reverted to full sensitivity following consecutive transfers to untreated plants, suggesting that resistant phenotypes were less competitive than sensitive ones. When cycled on QoI-treated plants, reduction in sensitivity was greater for the QoI fungicide which had greater intrinsic activity on P. viticola. Sensitivity decreased at each subsequent cycle, resulting in almost full resistance after four generations. Mixture experiments indicated that selection pressure was affected most by the dose of the QoI fungicide and the nature of the partner fungicide. Folpet delayed selection pressure most effectively when it was associated with famoxadone or azoxystrobin. Mancozeb was least effective at reducing the rate of selection compared with the QoI alone, and fosetyl-aluminium was intermediate. Higher rates of selection were recorded when the dose of the QoI fungicide, solo or in a mixture, was increased from 1 to 4 microg ml(-1). Increasing the dose of the non-QoI partner fungicide in the mixture from 10 to 30 microg ml(-1) resulted in reduced selection pressure. These results suggest that the choice of the fungicide partner and its dosage in the mixture can significantly affect the success of QoI resistance management strategies under practical conditions.

Topics: Drug Interactions; Drug Resistance, Fungal; Fungicides, Industrial; Imidazolines; Maneb; Methacrylates; Oomycetes; Organophosphorus Compounds; Oxazoles; Phthalimides; Plant Diseases; Pyrimidines; Selection, Genetic; Strobilurins; Vitis; Zineb

Famoxadone is a new cytochrome bc(1) Q(o) site inhibitor that immobilizes the iron-sulfur protein (ISP) in the b conformation. The effects of famoxadone on electron transfer between the iron-sulfur center (2Fe-2S) and cyt c(1) were studied using a ruthenium dimer to photoinitiate the reaction. The rate constant for electron transfer in the forward direction from 2Fe-2S to cyt c(1) was found to be 16,000 s(-1) in bovine cyt bc(1). Binding famoxadone decreased this rate constant to 1,480 s(-1), consistent with a decrease in mobility of the ISP. Reverse electron transfer from cyt c(1) to 2Fe-2S was found to be biphasic in bovine cyt bc(1) with rate constants of 90,000 and 7,300 s(-1). In the presence of famoxadone, reverse electron transfer was monophasic with a rate constant of 1,420 s(-1). It appears that the rate constants for the release of the oxidized and reduced ISP from the b conformation are the same in the presence of famoxadone. The effects of famoxadone binding on electron transfer were also studied in a series of Rhodobacter sphaeroides cyt bc(1) mutants involving residues at the interface between the Rieske protein and cyt c(1) and/or cyt b.

Topics: Acrylates; Animals; Cattle; Crystallography, X-Ray; Cytochromes c1; Electron Transport; Electron Transport Complex III; Enzyme Inhibitors; Iron-Sulfur Proteins; Kinetics; Methacrylates; Models, Molecular; Mutagenesis, Site-Directed; Oxazoles; Photochemistry; Protein Conformation; Pyrimidines; Strobilurins