First Report of Penicillium expansum Isolates With Low Levels of Resistance to Fludioxonil From Commercial Apple Packinghouses in Washington State

Abstract

HomePlant DiseaseVol. 101, No. 5First Report of Penicillium expansum Isolates With Low Levels of Resistance to Fludioxonil From Commercial Apple Packinghouses in Washington State PreviousNext DISEASE NOTES OPENOpen Access licenseFirst Report of Penicillium expansum Isolates With Low Levels of Resistance to Fludioxonil From Commercial Apple Packinghouses in Washington StateA. Amiri, K. A. Mulvaney, and L. K. PanditA. AmiriSearch for more papers by this author, K. A. MulvaneySearch for more papers by this author, and L. K. PanditSearch for more papers by this authorAffiliationsAuthors and Affiliations A. Amiri K. A. Mulvaney L. K. Pandit , Washington State University, Tree Fruit Research and Extension Center, Wenatchee 98801. Published Online:1 Mar 2017https://doi.org/10.1094/PDIS-09-16-1353-PDNAboutSections ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat Fludioxonil has been used since 2004 in the state of Washington to control several postharvest pathogens of pome fruit, including Penicillium expansum, the causal agent of blue mold. In a recent survey conducted between February and May of 2016, decayed apples were collected from multiple packinghouses (n = 95) across Washington State. Blue-mold symptomatic fruit were cut in half with a sterile scalpel and a small plug was taken from the inside of the lesion and transferred to Petri plates containing potato dextrose agar (PDA) acidified to pH 3.5 using lactic acid. Plates were incubated for 5 days at 22°C and a mycelial plug was cut from the growing colony and transferred to PDA plates and incubated until sporulation was observed. Isolates were single-spored, stored in 30% glycerol at –80°C, and identified to species level as described by Pitt (2002). A total of 1,200 isolates were collected and tested for sensitivity to fludioxonil using a germ tube elongation inhibition assay on PDA amended with 0.5 and 1.0 µg/ml fludioxonil (Scholar SC, Syngenta Crop Protection, Inc.). The 0.5 µg/ml dose discriminated lab-mutants with low resistance to fludioxonil (Li and Xiao 2008) whereas the 1.0 µg/ml dose was used in this study to detect highly resistant isolates. Overall, germ tube elongation on 0.5 µg/ml-amended PDA was observed in 128 (10.6%) P. expansum isolates, whereas none germinated at 1.0 µg/ml. The ability of fludioxonil to control P. expansum isolates with different levels of sensitivity was evaluated on detached apples. Four sensitive isolates and six isolates with different germ tube length in vitro were tested. Organically grown Red Delicious apples were surface sterilized in 0.8% sodium hypochlorite for 2 min, rinsed twice with tap water, and allowed to dry. Fruit were wounded twice (4 mm wide × 3 mm deep) near the stem-end area and immersed for 30 s in a Scholar SC suspension at 0.78 and 1.25 ml/liter, which correspond to the low and high label rates, respectively. Nontreated fruit were immersed in water and served as controls. Fruit were inoculated 4 h post treatment with 25 µl of spore suspension at 105 spores/ml and incubated for 6 days at 20°C in sealed boxes containing 200 ml of sterile water. Eight fruit were inoculated for each isolate and the assay was repeated twice. All isolates developed blue mold on control fruit, whereas only six isolates characterized with low resistance in vitro developed blue mold lesions on fruit treated with Scholar at 0.78 ml/liter. Disease severity in low-resistant isolates ranged from 51.1 to 71.4% relative to nontreated control. All isolates were fully controlled on fruit treated with the higher labeled rate regardless of their in vitro sensitivity. This is the first report of low resistance to fludioxonil in P. expansum from commercial apple packinghouses in Washington State. The emergence of such a population is likely due to frequent fludioxonil usage in recent years because of the widespread resistance to thiabendazole and the emergence of resistance to pyrimethanil in Washington and Pennsylvania (Caiazzo et al. 2014; Yan et al. 2014), the two other postharvest fungicides registered on pome fruit. These findings warrant continuous resistance monitoring and the implementation of adequate management strategies to avoid selecting for highly resistant pathogen populations.