Abstract
A shift towards declining azoxystrobin sensitivity has been previously documented in a collection of Brazilian Monilinia fructicola isolates, but information on the stability of this trait after in vitro sub-culturing is not available. In this study, one highly-sensitive isolate and two isolates with reduced sensitivity to azoxystrobin, not exhibiting point mutations at codons 129, 137 and 143 in the target gene of cytochrome b were used. Two independent experiments consisted of 10 weekly transfers of each strain in potato dextrose agar (PDA). Conidial production and germination were quantified in the initial culture and, again, after the third, sixth and tenth transfer. Measures of mycelial growth were obtained in colonies growing on PDA amended with azoxystrobin at 1 µg mL−1 and salicylhydroxamic acid at 100 µg mL−1. Data were normalized to mycelial growth rate after each of the 10 transfers. The colony diameter in the less sensitive isolates was stable across the transfers in the PDA amended with the fungicide. The conidial production and mycelial growth rate did not decrease in non-amended media either. Our results suggest that the trait of reduced sensitivity to azoxystrobin is stable under no selection pressure for azoxystrobin.
Highlights
Monilinia fructicola is the main causal agent of brown rot of peach in Brazil (Martins et al, 2005; May De Mio et al, 2014)
Measures of mycelial growth were obtained in colonies growing on potato dextrose agar (PDA) amended with azoxystrobin at 1 μg mL−1 and salicylhydroxamic acid at 100 μg mL−1
The colony diameter in the less sensitive isolates was stable across the transfers in the PDA amended with the fungicide
Summary
Monilinia fructicola is the main causal agent of brown rot of peach in Brazil (Martins et al, 2005; May De Mio et al, 2014). The intensive use of these fungicides selects for resistance to triazoles and reduced-sensitivity to azoxystrobin in M. fructicola isolates (May De Mio et al, 2011). Resistance instability has been reported for M. fructicola isolates with reduced sensitivity to propiconazole without a corresponding point mutation in the cytochrome P450 sterol 14α-demythylase gene (CYP51) (Cox et al, 2007). The levels of sensitivity to propiconazole increased in M. fructicola isolates after successive transfers in vitro and under different storage procedures, such as glycerol, filter paper, mineral oil and silica gel at low or ultralow temperatures (Cox et al, 2007; Zhu et al, 2012). Two azoxystrobin-resistant M. fructicola isolates, without point mutation in the target gene cytochrome b (cyt b), were detected in China and exhibited similar behavior with increased sensitivity to the fungicide after five transfers. The EC50 values of the resistant isolates significantly decreased from 19.6 and 9.1 μg mL−1, respectively, to 0.56 μg mL−1 for both isolates (Chen et al, 2014)
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