Vol.12_No.2 - Pesticide Alternatives Lab - Michigan State University

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Spring 2003 Resistant Pest Management Newsletter Vol. 12, No.2integrated disease management program that includesnon-chemical practices, such as use of resistantcultivars (McGrath, 2001). Nova should be used at themanufacturer's highest label rate (full rate) and shortestapplication interval. One suggested change to improveresistance management is to apply a contact fungicidewith strobilurins as well as DMIs. Sulfur (MicrothiolDisperss®) and mineral oil (JMS Stylet-oil®) arerecommended for resistance management because theyare more effective than Bravo and other contactfungicides for powdery mildew on the lower leafsurface (McGrath 2002a). Quadris applied inalternation with Nova and Microthiol Disperss wasmore effective than Quadris alternated with Nova andBravo in 2002 (Table 1). Sulfur is very inexpensive,but can be phytotoxic to melon (McGrath 2002a).This is the first report of resistance inPodosphaera xanthii to this group of fungicides in theUS. Resistance has already developed in Didymellabryoniae, which causes gummy stem blight, in the US(Stevenson et. al. 2002).REFERENCESAlexander, S. A., and Waldenmaier, C. M. 2002. Evaluation of fungicidesfor control of powdery and downy mildew in pumpkin, 2001.Fungicide and Nematicide Tests 57:V077.Drennan, J. L., and 2000. Comparing fungicides for powdery mildew andgummy stem blight control in butternut squash, 1999. Fungicideand Nematicide Tests 55:259.Everts, K. L., and Armentrout, D. K. 2002. Evaluation of fungicides forcontrol of powdery mildew and Plectosporium blight of pumpkin,2001. Fungicide and Nematicide Tests 57:V080.Hausbeck, M. K., Wendling, N.J., and Linderman, S.D. 2002. Evaluationof fungicides for managing powdery mildew of pumpkin, 2001.Fungicide and Nematicide Tests 57:V081.Ishii, H., Fraaije, B. A., Sugiyama, T., Noguchi, K., Nishimura, K.,Takeda, T., Amano, T., and Hollomon, D. W. 2001. Occurrence andmolecular characterization of strobilurin resistance in cucumberpowdery mildew and downy mildew. Phytopathology 91:1166-1171.Langston, D. B., and Kelley, W. T. 2002. Evaluation of fungicides andbiological control materials for control of powdery mildew intransgenic yellow crookneck squash, 2001. Fungicide andNematicide Tests 57:V095.Matheron, M. E., and Porchas, M. 2003. Evaluation of foliar fungicidesfor control of powdery mildew on muskmelon in Yuma, AZ, 2002.Fungicide and Nematicide Tests 58: (accepted for publication).McGrath, M. T. 1996a. Increased resistance to triadimefon and tobenomyl in Sphaerotheca fuliginea populations following fungicideusage over one season. Plant Disease 80:633-639.McGrath, M. T. 1996b. Successful management of powdery mildew inpumpkin with disease threshold-based fungicide programs. PlantDisease 80:910-916.McGrath, M.T. 2000. Evaluation of fungicide programs for managingpowdery mildew of pumpkin, 1999. Fungicide and NematicideTests 55:253-254.McGrath, M. T. 2001. Fungicide resistance in cucurbit powdery mildew:Experiences and challenges. Plant Disease 85(3):236-245.McGrath, M. T. 2002a. Alternatives to the protectant fungicidechlorothalonil evaluated for managing powdery mildew ofcucurbits. Pages 213-221 in: Proceedings of Cucurbitaceae '02:Evaluation and enhancement of cucurbit germplasm, D. Hopkins,ed.McGrath, M. T. 2002b. Evaluation of fungicide programs for managingpowdery mildew of pumpkin, 2001. Fungicide and NematicideTests 57:V86.McGrath, M.T., and Shishkoff, N. 1999. Evaluation of fungicideprograms for managing powdery mildew of pumpkin, 1998.Fungicide and Nematicide Tests 54:230-231.McGrath, M.T., and Shishkoff, N. 2000. Evaluation of fungicideprograms for managing powdery mildew of muskmelon, 1999.Fungicide and Nematicide Tests 55:176-177.McGrath, M.T., and Shishkoff, N. 2001. Evaluation of fungicideprograms for managing powdery mildew of pumpkin, 2000.Fungicide and Nematicide Tests 2001:V76.McGrath, M. T., and Shishkoff, N. 2003. Evaluation of fungicideprograms for managing powdery mildew of pumpkin, 2002.Fungicide and Nematicide Tests 58: (accepted for publication).McGrath, M. T, Staniszewska, H., Shishkoff, N., and Casella, G. 1996.Fungicide sensitivity of Sphaerotheca fuliginea populations in theUnited States. Plant Disease 80:697-703.Olaya, G., Moreno, M., Lum, B., and Heaney, S. Azoxystrobin resistancemonitoring study for Sphaerotheca fuliginea populations collectedin the United States and Mexico. Phytopathology 90:S57, 2000.Stevenson, K. L., Langston, D. B., and Seebold, K. 2002. Resistance toazoxystrobin in the gummy stem blight pathogen in Georgia.Resistant Pest Management 12(1).Turini, T.A. 2002 Comparison of fungicides for control of powderymildew on muskmelon, 2001. Fungicide and Nematicide Tests57:V050.Turini, T.A. 2003 Comparison of fungicides for control of powderymildew on muskmelon, 2002. Fungicide and Nematicide Tests 58:(accepted for publication).Zitter, T.A., and Drennan, J. L. 2001. Comparing fungicides for powderymildew and gummy stem blight control in butternut squash, 2000.Fungicide and Nematicide Tests 56:V91.Margaret T. McGrath and Nina ShishkoffDepartment of Plant PathologyCornell University Long Island Horticultural Research and ExtensionCenter3059 Sound Avenue, Riverhead, NY 11901-1098United StatesBaseline Sensitivity of Cucurbit Powdery Mildew (Podosphaera xanthii) to the Fungicide Azoxystrobin in theUnited StatesINTRODUCTION Azoxystrobin is the first syntheticfungicidal compound derived from naturally occurringstrobilurins (Ypema and Gold 1999). It is in fungicidegroup 11, the quinone outside inhibitor (QoI) activitygroup. The purpose of this study was to examinebaseline sensitivity of Podosphaera xanthii toazoxystrobin in the United States.MATERIALS and METHODS Isolates of Podosphaeraxanthii were obtained from six states in 1996 (Table 1).A reference isolate collected in 1990 was also included.A leaf-disk bioassay was used to determinesensitivity to azoxystrobin (McGrath et al 1996). Twoweekold squash seedlings ('Seneca Prolific') weresprayed with active ingredient (azoxystrobin 96%)dissolved in methanol: acetone: water (1:1:2 v:v:v) at0, 0.25, 0.5, 1.0, 2.5, and 5 µg/ml. Test solutions were72
Spring 2003 Resistant Pest Management Newsletter Vol. 12, No.2sprayed onto plants using aDeVilbiss bottle attached to acompressed air source (20 psi).Treated plants were allowed to dry,then disks were cut from thecotyledons using a #9 cork borer (9mm diameter). Five disks of eachtreatment were placed on water agarin divided petri plates (threetreatments per plate). Disks wereinoculated by transferring 5-10conidia to the center of each disk. Ineach trial, duplicate sets of treateddisks were inoculated with eachmildew isolate, then plates wereincubated for approximately 2 weeksat 24 C, at which time the controltreatment showed good growth, withsporulating mildew covering anaverage of 40-60% of leaf disk area.For each fungicide concentration,growth of the mildew was considered to have occurredif sporulation was observed on 3 out of 5 disks (or 2out of 4, if a disk died during incubation). The percentleaf disk area colonized by sporulating mildew wasrecorded for each disk and averaged for each treatment.RESULTS and DISCUSSION Seventy-two powdery mildewisolates from six states showed little variation insensitivity to azoxystrobin. All were able to grow at0.25 µg/ml, 60% could grow at 0.5-1 µg/ml but nohigher, 35% grew slightly (0.4-11% disk areacolonized) on leaf disks treated at 2.5 µg/ml, and 6% (4isolates) grew slightly (0.2-2.8%) at 5 µg/ml. Some ofthe variation in fungus growth may have been due tovariation in the actual concentration of test solutionsused to spray test plants; the fungicidal activeingredient was not readily soluble in water at 5 µg/mland took some time to dissolve in anacetone:methanol:water (1:1:2) solution. It wasnecessary to dissolve the fungicide in acetone:methanol(1:1) and then add water. There was no evidentcorrelation between sensitivity and geographic locationor sensitivity and race. An isolate with resistance totriadimefon and benomyl (isolate 4at) was unable togrow on disks treated with >1 µg/ml azoxystrobin,indicating that resistance to triazoles andbenzimidazoles was independent of resistance toazoxystrobin.REFERENCESMcGrath, M.T., H. Staniszewska, and N. Shishkoff, 1996. Fungicidesensitivity of Sphaerotheca fuliginea populations in the UnitedStates. Plant Disease 80:697-703.Ypema, H. L. and Gold, R. E. 1999. Kresoxim-methyl: Modification of anaturally occurring compound to produce a new fungicide. PlantDisease 83:4-19.Nina Shishkoff and Margaret T. McGrathDepartment of Plant PathologyCornell University Long Island Horticultural Research and ExtensionCenter3059 Sound AvenueRiverhead, New York 11901-1098United StatesManaging Phenylamide Resistance in Potato Late Blight in Northern IrelandINTRODUCTION Formulations containing phenylamides+ mancozeb were approved for the control of potatolate blight in the UK in 1978 and rapidly becamewidely used. In summer 1980, in the Republic ofIreland metalaxyl alone failed to control the diseaseand isolates of Phytophthora infestans from the foliagewere found to be phenylamide-resistant (Dowley andO'Sullivan, 1981). In Northern Ireland, phenylamideresistantisolates of P. infestans were obtained fromblighted tubers from the 1980 crop and annual surveysof the incidence of phenylamide resistance wereinitiated starting in 1981 (Cooke, 1981). In the early1980s, in Northern Ireland the percentage of isolatescontaining phenylamide-resistant strains was generally10-20% (except in 1984), but in the late 1980s therewas a dramatic increase to c. 90% in 1987-89. This wasattributed to the selection pressure resulting fromwidespread and season-long use of formulationscontaining phenylamides + mancozeb (mainlymetalaxyl + half-rate mancozeb) and a succession ofvery wet summers which favoured late blight. Asimilar build-up of phenylamide-resistant strains73
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