Frequency of isolation, determination, and epidemiology of Alternaria species on potato in the Pacific Northwest
Tymon, Lydia Stephanie
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Many species of Alternaria have been isolated from potato, but only Alternaria solani and A. alternata have been described as pathogenic in the US. Isolations from early blight and brown spot type lesions on potato leaves collected in the Pacific Northwest were made between 2008 and 2011. Large-spored isolates were isolated less frequently than small-spored isolates. Pathogenicity of isolates was assessed on detached leaves of 'Russet Norkotah'. Phylogenetic species were distinguished with a restriction digest and 23 of 66 isolates were identified as A. alternata and 36 isolates as A. arborescens. No correlation was observed between pathogenicity and species (r= 0.12, P= 0.33). Seven isolates had no restriction site and were identified as A. oregonensis, A. arbusti, or A. metachromatica in section infectoria by sequencing. Aggressiveness of three Alternaria species was quantified on 'Russet Norkotah', four other potato varieties, and whole plants of `Russet Norkotah'. The effects of wounding were also evaluated. While wounding had a significant impact on mean infection frequencies (MIF) and area under the lesion expansion curve (AULEC) for A. alternata and A. arbusti, it did not affect MIF or AULEC of A. solani. AULEC of whole plants was positively and significantly correlated with AULEC on detached leaves. Umatilla Russet was the most susceptible and Russet Alturas and Russet Burbank were the least susceptible cultivars for all three pathogen species. Fungicide resistance to azoxystrobin and boscalid is expected due to the specific modes of action of each fungicide. Prescreening at a threshold concentration was done to estimate the range of fungicide resistance of A. solani and A. alternata in the region. Growth ratios (GR) and percent spore germination (PSG) were evaluated. Only an increase in the resistance to boscalid by A. solani was observed between 2010 and 2011. Resistance of five isolates was then fully characterized at three concentrations of both fungicides. The lowest concentrations of fungicides were ineffective at reducing PSG or GR. In general, PSG of A. solani and A. alternata were not significantly reduced by the highest concentrations of either azoxystrobin or boscalid and as fungicide concentrations increased, mycelial growth decreased.