|Gachango, Esther - Michigan State University|
|Kirk, William - Michigan State University|
|Rojas, Alejandro - Michigan State University|
|Tumbalam, Pavani - Michigan State University|
|Shetty, K - Syngenta Crop Protection|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/17/2010
Publication Date: 1/20/2011
Citation: Gachango, E., Kirk, W.W., Hanson, L.E., Rojas, A., Tumbalam, P., Shetty, K. 2011. First report of in-vitro fludioxonil-resistant isolates of Fusarium spp. causing potato dry rot in Michigan. Plant Disease. 95:228.
Interpretive Summary: Fusarium dry rot of potato is an important postharvest disease caused by several different species of Fusarium. Measures for controlling dry rot in storage are limited, and primarily involve reducing wounding of tubers during harvest and storage. However, fungicides may be used to protect seed tubers in some cases. Samples were taken from tubers with dry rot symptoms from Michigan seed lots in 2009 and 2010. Tubers had not been given any fungicide treatments. From these tubers, 170 Fusarium were isolated. These included 54 F. oxysporum and 23 F. sambucinum isolates. Both species caused typical dry rot symptoms when inoculated onto potato tubers. When screened for the response to three fungicides, 15 of the 23 F. sambucinum isolates and 34 of the 54 F. oxysporum isolates grew at levels of the fungicide fludioxonil at levels considered to indicate resistance to this fungicide. To our knowledge, this is the first report of resistance to fludioxonil in isolates of these two species in Michigan.
Technical Abstract: Fusarium dry rot of potato (Solanum tuberosum) is a postharvest disease caused by several Fusarium species and is of worldwide importance. Measures for controlling dry rot in storage are limited. Dry rot has been managed primarily by reducing tuber bruising, providing conditions for rapid wound healing, and by applying fungicides. Dry rot symptomatic tubers were collected from Michigan seed lots in 2009 and 2010. Samples were taken from the edge of lesions on untreated tubers and Fusarium was purified from these samples. 170 Fusarium isolates were obtained and used for further studies. Among them 54 were identified as F. oxysporum and 23 as F. sambucinum. Identification was based on colony and conidial morphology on potato dextrose agar (PDA) and carnation leaf agar respectively. The identity was confirmed through DNA extraction followed by amplification and sequencing of the translation elongation factor gene region. Pathogenicity testing was done on disease-free potato tubers, cv. FL 1879. Three tubers per isolates were injected with 20µl of a conidial suspension (106 conidia/ml) made from cultures grown on PDA for 7 days. Control tubers were injected with 20µl of sterile distilled water. All tubers inoculated with F. sambucinum and F. oxysporum developed typical potato dry rot symptoms consisting of a brown and dry decay lesions. F. sambucinum and F. oxysporum were re-isolated from all the symptomatic tubers. An effective concentration for 50% reduction in growth (EC50) was determined for each F. sambucinum and F. oxysporum isolate for thiabendazole (TBZ), fludioxonil and difenoconazole using the spiral gradient endpoint (SGE) method. Both sensitive and resistant F. sambucinum and F. oxysporum isolates were reported. Fifteen isolates of F. sambucinum and 34 of F. oxysporum were resistant to fludioxonil with and EC50 greater than 130 mg/L. The rest of the isolates were sensitive to fludioxonil with EC50 ranging from 0.8 to 4.9 mg/L. To our knowledge, this is the first report of resistance to fludioxonil in isolates of F. sambucinum and F. oxysporum in Michigan.