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

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