Cytochrome b gene sequence and structure of Pyrenophora teres and P. tritici-repentis and implications for QoI resistance

Resistance to QoI fungicides in Pyrenophora teres (Dreschsler) and P. tritici-repentis (Died.) Dreschsler was detected in 2003 in France and in Sweden and Denmark respectively. Molecular analysis revealed the presence of the F129L mutation in resistant isolates of both pathogens. In 2004, the frequency of the F129L mutation in populations of both pathogens further increased. The G143A mutation was also detected in a few isolates of P. tritici-repentis from Denmark and Germany. In 2005, the F129L mutation in P. teres increased in frequency and geographical distribution in France and the UK but remained below 2% in Germany, Switzerland, Belgium and Ireland. In P. tritici-repentis, both mutations were found in a significant proportion of the isolates from Sweden, Denmark and Germany. The G143A mutation conferred a significantly higher level of resistance (higher EC50 values) to Qo inhibitors (QoIs) than did the F129L mutation. In greenhouse trials, resistant isolates with G143A were not well controlled on plants sprayed with recommended field rates, whereas satisfactory control of isolates with F129L was achieved. For the F129L mutation, three different single nucleotide polymorphisms (SNPs), TTA, TTG and CTC, can code for L (leucine) in P. teres, whereas only the CTC codon was detected in P. tritici-repentis isolates. In two out of 250 isolates of P. tritici-repentis from 2005, a mutation at position 137 (G137R) was detected at very low frequency. This mutation conferred similar resistance levels to F129L. The structure of the cytochrome b gene of P. tritici-repentis is significantly different from that of P. teres: an intron directly after amino acid position 143 was detected in P. teres which is not present in P. tritici-repentis. This gene structure suggests that resistance based on the G143A mutation may not occur in P. teres because it is lethal. No G143A isolates were found in any P. teres populations. Although different mutations may evolve in P. tritici-repentis, the G143A mutation will have the strongest impact on field performance of QoI fungicides.     

Field resistance to QoI fungicides in Podosphaera fusca is not supported by typical mutations in the mitochondrial cytochrome b gene

BACKGROUND: A single nucleotide polymorphism in the mitochondrial cytochrome b gene confers resistance to strobilurin (QoI) fungicides in phytopathogenic fungi. Recent studies have revealed worrying levels of resistance to strobilurins in Podosphaera fusca (Fr.) U Braun & N Shishkoff comb. nov. [ = Sphaerothecafusca (Fr.) S Blumer], the main causal agent of cucurbit powdery mildew in Spain. In the present study the underlying resistance mechanism to QoI fungicides in the Spanish populations of P. fusca was investigated. RESULTS: Analysis of the Q(o) domains of cytochrome b in a collection of isolates revealed that none of the typical mutations conferring resistance to QoI, including the G143A and F129L substitutions, was present in the QoI-resistant isolates. Moreover, although different amino acid polymorphisms were observed in the two regions spanning the Q(o) site, none of them consistently distinguished QoI-resistant from QoI-sensitive strains. Exposure to salicylhydroxamic acid (SHAM), a specific inhibitor of alternative oxidase, in the presence of trifloxystrobin did not have any effect on QoI resistance, ruling out alternative respiration as the mechanism of resistance. Sensitivity tests to a battery of respiration inhibitors revealed high levels of cross-resistance to all Qo-inhibitors tested but not to Qi-inhibitors, these features resembling those of a target-site-based resistance. CONCLUSIONS: The results indicate that the mechanism responsible for QoI resistance in P. fusca is not linked to typical mutations in cytochrome b gene and that the absence of the G143A substitution cannot be explained by an intron following codon 143. These are important observations, especially in relation to the possible molecular diagnosis of resistance.     

Characterization of the cytochrome b (cyt b) gene from Monilinia species causing brown rot of stone and pome fruit and its significance in the development of QoI resistance

BACKGROUND: Quinone outside inhibitor (QoI) resistance as a consequence of point mutations in the cytochrome b (cyt b) gene has been reported in numerous plant pathogenic fungi. To examine the potential for QoI resistance development in those Monilinia species causing brown rot of stone and pome fruits [Monilinia fructicola (G Winter) Honey, M. laxa (Aderhold & Ruhland) Honey and M. fructigena (Aderhold & Ruhland) Honey], an examination was made of the sequence and exon/intron structure of their cyt b genes for the presence of any point mutations and/or introns commonly associated with resistance to QoIs in fungal plant pathogens. RESULTS: None of the point mutations typically linked to QoI resistance was present in any of the Monilinia isolates examined. Furthermore, the cyt b genes from M. fructicola and M. laxa, but not M. fructigena, possessed a group‐I‐like intron directly after codon 143. Based on the results obtained, a simple PCR assay using a single primer pair was developed, allowing discrimination between the three Monilinia species without the need for culturing. CONCLUSIONS: Results suggest that resistance to QoI fungicides based on the G143A mutation is not likely to occur in M. fructicola or M. laxa. Conversely, M. fructigena may be at higher risk for developing QoI resistance owing to the absence of a G143‐associated intron. Copyright © 2010 Society of Chemical Industry     

Genetic relationship between the adult plant resistance gene Lr12 and the complementary gene Lr31 for seedling resistance to leaf rust in common wheat

A resistant phenotype similar to that conferred in wheat by the complementary genes Lr27  +  Lr31 was produced in the progeny of intercrosses of cultivars carrying Lr27 and a line possessing Lr12 . This confirms that Lr12 is either completely linked with Lr31 or is the same gene. On the basis of these findings and that Lr31 is located on chromosome 4BS, it is concluded that Lr12 must also be located on 4BS. Adult-plant genetic tests confirm that the Australian wheat cultivar Timgalen carries Lr12 , and stocks with Lr12 alone were established from this cultivar.