Resistance of Helminthosporium solani strains to selected fungicides applied for tuber treatment

Helminthosporium solani strains were isolated from potato tubers collected in Russia or taken from imported German and Dutch seed tubers. Sequences of the nuclear ribosomal genes and internal transcribed spacers (ITS) for all 24 tested strains were identical and had 100% similarity to the sequences from GenBank identified as Helminthosporium solani. The obtained molecular data confirmed the morphological identification based on the width and length of conidia, the shape of conidiophores and the colony morphology. Screening for resistance to the fungicides Score 250 SC (active ingredient difenoconazole 250 g/l), Quadris (azoxystrobin 250 g/l), Tecto 500 SC (thiabendazole 500g/l), Zeroxxe [colloidal silver particles (3 g/l) stabilized with amphoteric surfactant] was done. Agar blocks with pure cultures of the fungal strains were placed in the centre of Petri dishes containing malt agar amended with fungicide concentrations of 0.1, 1, 10, 100 and 1000 mg/l (accounted for the concentration of the active ingredient). Malt agar free of fungicide was used as the control. Growth inhibition of 50% (EC50) compared to the control was detected based on the dose-response curves. Difenoconazole (EC50 < 0.12mg/l) and colloidal silver (EC50 < 76 mg/l) were the most effective fungicides. No strains resistant to the aforementioned fungicides were found. In most cases, azoxystrobin was effective against H. solani (EC50 < 7 mg/l), but there were several strains with high resistance to this fungicide (EC50 > 100 mg/l). Thiabendazole appears to be effective against the sensitive strains of H. solani (EC50 < 7.3 mg/l); however, six studied strains from Russia and the Netherlands were found to be extremely resistant to it (EC50 > 1000 mg/l). The sequence of their β-tubulin gene contained a SNP mutation in the 198 codon or 200 codon, translating to Gln (CAG) instead of Glu (GAG) or Tyr (TAC) instead of Phe (TTC), respectively. Thus, the resistance to thiabendazole of the Russian, European and American strains had the same genetic background and was conferred by the same mutations. © 2017, Edizioni ETS. All rights reserved.