Occurrence and Molecular Characterization of Strobilurin Resistance in Cucumber Powdery Mildew and Downy Mildew

Abstract

Between 1998 and 1999, control failure of powdery mildew (Podosphaera fusca) and downy mildew (Pseudoperonospora cubensis) by the strobilurin fungicides azoxystrobin and kresoxim-methyl was observed in cucumber-growing areas of Japan. Results from inoculation tests carried out on intact cucumber plants and leaf disks clearly showed the distribution of pathogen isolates highly resistant to azoxystrobin and kresoximmethyl. Fragments of the fungicide-targeted mitochondrial cytochrome b gene were polymerase chain reaction amplified from total pathogen DNA and their sequences analyzed to elucidate the molecular mechanism of resistance. A single point mutation (GGT to GCT) in the cytochrome b gene, resulting in substitution of glycine by alanine at position 143, was found in resistant isolates of downy mildew. This substitution in cytochrome b seemed to result in high resistance to strobilurins in this pathogen. The same mutation was found in some but not all resistant isolates of powdery mildew. This study suggests that a mutation at position 143 in the target-encoding gene, resulting in an amino acid substitution, was probably a major cause of the rapid development of high strobilurin resistance in these two pathogens.

Site-Specific Risk Factors for Ray Blight in Tasmanian Pyrethrum Fields

“…In the current study, disease intensity assessments were made during 12 to 16 20 flowering stems were systematically selected from each of three linear transects covering the entire width of each nontreated plot (60 stems per plot in total). Flowering stems were destructively sampled by cutting stems at the soil-line, and defoliation severity, defined as the length at which leaves were either completely necrotic or abscised divided by the total stem length (expressed as a percentage of total stem length) (32), was assessed on each stem in the laboratory within 48 h of collection.…”

“…Fungicides included in the current management recommendations are from the strobilurin, carboxamide, and demethylation inhibitor groups. Rapid directional selection toward fungicide resistance has been reported for strobilurin (5,12,16,19) and carboxamide fungicides in other pathosystems (1,20,21,42). These fungicides are considered medium-to-high risk for the selection of resistant strains by the Fungicide Resistance Action Committee.…”

Fungicide Resistance in Cucurbit Powdery Mildew Fungi

“…& M. A. Curtis) Rostovzev.) in Japan (Ishii et al, 2001) and the USA. Cross-resistance among QoI fungicides has been documented with E. graminis f. sp.…”

“…It is generally known now that these site-specific fungicides have a high risk of resistance development in a pathogen population (McGrath, 2001). The type of resistance to strobilurins is qualitative (same as for benomyl) which means that individuals of the CPM pathogen are either highly sensitive to strobilurins or highly resistant (Ishii et al, 2001;McGrath & Shishkoff, 2003). This fact does not correspond to the situation in Czech CPM population in relation to azoxystrobin in the year 2007.…”

“…In many parts of eastern Asia and the northern Mediterranean area, strobilurin resistance in CPM has developed, sometimes within the first season of use (Hollomon & Wheeler, 2002). Resistance to azoxystrobin in Px was recorded from Spain (Fernández-Ortuño et al, 2006), Japan (Ishii et al, 2001) and the USA (McGrath & Shishkoff, 2003;McGrath, 2005McGrath, , 2006. Reduced efficacy of azoxystrobin was also claimed in cucurbit downy mildew (Pseudoperonospora cubensis (Berk.…”