ABSTRACTCHARACTERIZATION OF SETOPHOMA TERRESTRIS CAUSING PINK ROOT IN ONION, DISEASE MANAGEMENT, AND AGE-RELATED RESISTANCEByPrissana WiriyajitsomboonSetophoma terrestris infects onion roots causing a characteristic discoloration called ‘pink root’. This understudied pathogen negatively impacts onions worldwide as a result of compromised root systems that result in reduced yields. Variability of S. terrestris isolates was determined based on genetic, morphology, and virulence. A collection of 98 isolates of S. terrestris was identified based on partial DNA sequences encoding the large-subunit ribosomal DNA. High identity (>97%) was revealed when compared to the nucleotide sequences of S. terrestris on the NCBI database. Seven inter-simple sequence repeats (ISSR) markers were used to determine population structure of S. terrestris across nine sites in Michigan. There was no population structure detected when the isolates were grouped by geographical origins. Variability in morphological characteristics and disease virulence were observed among isolates within the populations rather than among the populations.Thirty onion cultivars were evaluated for pink root susceptibility in commercial fields with a history of pathogen infestation. Roots were assessed 64, 76, 96, and 110 days after seeding for disease incidence in 2011 and 2012; disease severity was assessed at 110 days in 2013. The cultivars Frontier, Highlander, and Scout were highly susceptible to pink root, while the cultivars Hendrix, Redwing, and Sedona were the least susceptible. Fungicide treatments were evaluated for their efficacy against pink root in a greenhouse trial after they were drenched either 0 day post inoculation (dpi) or 0 and 14 dpi. The fungicide penthiopyrad resulted in the greatest plant height, fresh weight, bulb circumference, and root density, and was significantly different from other fungicide treatments and the inoculated untreated control. Age-related resistance to pink root was examined in ‘Hendrix’ and ‘Highlander’ onions. Plants were inoculated when 3, 5, 7, and 9 weeks old using millet seeds infested with S. terrestris and incubated in the greenhouse for 42 days. Microscopic observation revealed that pathogen colonization could be detected prior to root discoloration and that onion roots of all ages could become infected. The incidence of root colonization increased when older plants were inoculated compared with younger plants. Root density compared among plant ages of the two cultivars was not statistically different following inoculation. Reduction (%) of plant growth parameters (plant height, fresh weight, number of total leaves per plant, and bulb circumference) compared to the uninoculated onions was generally greater when young plants were inoculated compared to older plants regardless of cultivar. The growth of onions when inoculated at 3 weeks was significantly less for ‘Highlander’ than ‘Hendrix’. The variability among the S. terrestris isolates in Michigan may be useful for developing onion cultivars that are less susceptible to pink root. Identification of effective fungicide treatments and efficient application timing may reduce crop losses.
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