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.
Fernández-Ortuño
Dolores
D
Estación Experimental La Mayora (CSIC), Algarrobo-Costa, 29750 Málaga, Spain.
Torés
Juan A
JA
de Vicente
Antonio
A
Pérez-García
Alejandro
A
D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D001203:Ascomycota / Q000145:classification / Q000187:drug effects / Q000235:genetics* / Q000378:metabolism; D001483:Base Sequence; D045303:Cytochromes b / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D025141:Drug Resistance, Fungal; D005656:Fungal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D005659:Fungicides, Industrial / Q000494:pharmacology*; D024101:Mitochondrial Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D008969:Molecular Sequence Data; D009154:Mutation; D010802:Phylogeny; D010935:Plant Diseases / Q000382:microbiology; D020641:Polymorphism, Single Nucleotide; D016415:Sequence Alignment; D013030:Spain
D005656:Fungal Proteins; D005659:Fungicides, Industrial; D024101:Mitochondrial Proteins; D045303:Cytochromes b