Abstract
A total of 88 cucurbit powdery mildew (CPM) isolates, 52 of Golovinomyces cichoracearum (Gc) and 36 of Podosphaera xanthii (Px), collected in the period 2005-2007 in the Czech Republic (CR), were screened for tolerance and/or resistance to the three freauentlv used fungicides (fenarimol, dinocap, thiophanate-methyl). Benomvl served as ineffective control. Bv employing benomyl, which used to be the most frequently used fungicide for CPM protection in the CR, we could monitor and compare the development and changes in variation of tolerance/resistance among Czech CPM populations. A total of 36 (21 Gc, 14 Px) CPM isolates from 2007 were also tested for azoxystrobin tolerance and/or resistance. Fungicide sensitivity was determined by a modified leaf-disc bioassay with five concentrations. Significant differences among fungicides and even between the years 2005 and 2006-2007 were observed. Occurrence of resistant and/or tolerant isolates of both powdery mildew species in different locations was observed. Fenarimol (Rubigan 12 EC) and dinocap (Karathane LC) expressed decreasing efficacy in the year 2005, however in 2006-2007 these two fungicides showed a high level of effectiveness and all isolates of both CPMs were controlled by the recommended concentration (36 μg a.i./ml for fenarimol, 105 μg a.i./ml for dinocap). Benomyl (Fundazol 50 WP) and thiophanate-methyl (Topsin M 70 WP) were found to be ineffective, because the majority of screened isolates (96%) belonged to the highly resistant strains, with a resistant reaction at the recommended concentration (250 μg a.i./ml for benomyl, 525 μg a.i./ml for thiophanate-methyl) and tolerance or resistance at a higher concentration than recommended (500 μg a.i./mi for benomyl and 1050 μg a.i./ml for thiophanate-methyl). Some 89% of CPM isolates (except the year 2005 only 22% Px isolates tested on thiophanate-methyl) showed profuse or limited sporulation at a concentration 2× higher than recommended (1000 μg a.i./ml for benomyl and 2100 μg a.i./ml for thiophanate-methyl). Azoxystrobin (Ortiva) expressed decreasing efficacy, with 37% of isolates (Gc and Px) tolerant or resistant to the recommended concentration (500 μg a.i./ml) and also to higher concentrations (1000 μg a.i./ml and 2000 μg a.i./ml).
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