Quinone outside inhibiting (QoI) fungicides represent one of the most widely used groups of fungicides used to control agriculturally important fungal pathogens. They inhibit the cytochrome bc 1 complex of mitochondrial respiration. Soon after their introduction onto the market in 1996, QoI fungicide-resistant isolates were detected in field plant pathogen populations of a large range of species. However, there is still little understanding of the processes driving the development of QoI fungicide resistance in plant pathogens. In particular, it is unknown whether fungicide resistance occurs independently in isolated populations or if it appears once and then spreads globally by migration. Here, we provide the first case study of the evolutionary processes that lead to the emergence of QoI fungicide resistance in the plant pathogen Plasmopara viticola. Sequence analysis of the complete cytochrome b gene showed that all resistant isolates carried a mutation resulting in the replacement of glycine by alanine at codon 143 (G143A). Phylogenetic analysis of a large mitochondrial DNA fragment including the cytochrome b gene (2,281 bp) across a wide range of European P. viticola isolates allowed the detection of four major haplotypes belonging to two distinct clades, each of which contains a different QoI fungicide resistance allele. This is the first demonstration that a selected substitution conferring resistance to a fungicide has occurred several times in a plant-pathogen system. Finally, a high population structure was found when the frequency of QoI fungicide resistance haplotypes was assessed in 17 French vineyards, indicating that pathogen populations might be under strong directional selection for local adaptation to fungicide pressure.Preventing and delaying resistance adaptation to fungicides in pathogen populations is a major goal in sustainable plant pathogen management. The development of effective strategies to control pathogen populations requires precise understanding of the conditions under which fungicide resistance alleles appear, spread, and are maintained in natural populations. Among the factors that influence the evolution of resistance, the rate of mutation in fungicide resistance-associated genes plays an important role. It is still uncertain whether fungicide resistance-associated mutations commonly arise independently or whether pest adaptation occurs by convergent evolution through recurrent mutations at selected sites in the genes coding for the fungicide target. In other words, do alleles conferring fungicide resistance arise once before spreading across populations, or do they arise independently in each locality, and are they favored by large local population sizes of the pathogen population?Numerous experimental and phylogenetic studies in extensive geographical contexts have already examined the pattern of the evolution of genes coding for antimicrobial drugs or pesticide targets in animals, plants, and bacteria. For instance, the question of parallel genetic adaptation to drugs and p...

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SUMMARYThe sequence of the 7356 nucleotide [excluding the 3' poly(A)] RNA-1 of tomato black ring virus (TBRV) was determined from overlapping cDNA clones. A putative initiation codon at nucleotide 26l was considered to be the start of an open reading frame which terminates at a UAG codon at position 7053. The predicted translation product is a protein of 2264 amino acids with an Mr of 253 680 (254K). Comparison of the amino acid sequence of this 254K protein with other viral proteins revealed three regions, each having about 60~ homology with a region of the three cowpea mosaic virus (CPMV) proteins (58K, 24K, 87K) which are thought to be involved in replication of the CPMV RNAs. The same regions show similarities, although less extended, with proteins 2C, 3C and 3D of picornaviruses. From these results we suggest that the genome organization of TBRV is similar to that of comoviruses and picornaviruses.

Following inoculation of host and nonhost plants with Plasmopara viticola, the grapevine downy mildew, a histological survey was undertaken to identify the stage where its development is contained in nonhosts and in resistant host plants. Three herbaceous nonhost species, Beta vulgaris, Lactuca sativa, and Capsicum annuum, and three grapevine species displaying different level of resistance (Vitis vinifera [susceptible], Vitis riparia [partially resistant] and Muscadinia rotundifolia [totally resistant]) where inoculated by P. viticola using a controlled leaf disk inoculation bioassay. During the early steps of infection, defined as encystment of zoospores on stomata, penetration of the germ tube, and production of the vesicle with the primary hypha, there was no evidence of a clear-cut preference to grapevine tissues that could attest to host specificity. The main difference between host grapevine species and nonhosts was observed during the haustorium formation stage. In nonhost tissues, the infection was stopped by cell wall-associated defense responses before any mature haustorium could appear. Defense responses in resistant grapevines were triggered when haustoria were fully visible and corresponded to hypersensitive responses. These observations illustrate that, for P. viticola, haustorium formation is not only a key stage for the establishment of biotrophy but also for the host specificity and the recognition by grapevine resistance factors.