Vol.12_No.2 - Pesticide Alternatives Lab - Michigan State University
Vol.12_No.2 - Pesticide Alternatives Lab - Michigan State University
Vol.12_No.2 - Pesticide Alternatives Lab - Michigan State University
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Spring 2003 Resistant Pest Management Newsletter Vol. 12, <strong>No.2</strong>A.ButturiniServizio FitosanitarioRegione Emilia-Romagna, BolognaItalyR.CornaleCentro Agricoltura AmbienteCrevalcore (Bologna)ItalyS.VergnaniCentro Ricerche Produzioni VegetaliCesenaItalyFungicide ResistanceResistance to QoI Fungicides in Podosphaera xanthii Associated with Reduced Control of Cucurbit PowderyMildew in Research Fields in the Eastern United <strong>State</strong>sINTRODUCTION Application of fungicides continues tobe the principal practice for managing powderymildew, the most common disease of cucurbit cropsthroughout the world. Powdery mildew, which iscaused by Podosphaera xanthii, needs to be controlledon both leaf surfaces to avoid premature death ofleaves. It is especially important to control powderymildew on the underside of leaves where conditions aremore favorable for disease development than on uppersurfaces. Using systemic or translaminar fungicides isthe best approach. Unfortunately, most of thesefungicides are at risk for resistance developmentbecause they have single-site modes of action. Thecucurbit powdery mildew fungus has demonstrated ahigh potential for developing resistance. Each chemicalclass active for powdery mildew that is at risk forresistance somewhere in the world following repeateduse has developed resistance. Presence of resistantstrains has been associated with control failure. Thusmanaging fungicide resistance is an important aspect ofeffectively managing powdery mildew (McGrath2001).The fungicide program that has beenrecommended recently in the United <strong>State</strong>s is astrobilurin fungicide (azoxystrobin formulated asQuadris® or trifloxystrobin formulated as Flint®)applied in alternation with the DMI fungicidemyclobutanil (formulated as Nova® or Rally®) tankmixedwith a protectant fungicide. This program usestwo strategies for managing resistance:1. alternation among systemic fungicides in at leasttwo chemical classes, and2. inclusion of protectant fungicides which are not atrisk for resistance development because they havemulti-site modes of action.Strobilurins are in fungicide group 11, the quinoneoutside inhibitor (QoI) activity group. Myclobutanil isa triazole fungicide in the DMI activity group, which isfungicide group 3. Quadris and Nova have beenavailable for commercial use for powdery mildew oncucurbits in the US beginning in 1998 when theyreceived Section 18 registration in some states. USfederal (Section 3) registration was granted for Quadrisin March 1999 and Nova in May 2000. Flint wasregistered in September 1999.Resistance is a major concern with QoI fungicidesused for cucurbit powdery mildew due to past historywith this pathogen developing resistance. Therefore itis important to monitor fungicide efficacy and, if poorperformance occurs, to determine if it is due toresistance. These were the goals of this study. Efficacywas monitored in NY. Pathogen isolates were alsoassayed for resistance from other areas where poorcontrol was reported.MATERIALS and METHODS Quadris applied in alternationwith Nova tank-mixed with chlorothalonil (formulatedas Bravo Ultrex®) was included in fungicide efficacyexperiments conducted with pumpkin from 1998 to2002 in Riverhead, NY. Quadris was used alone on aweekly schedule for another treatment in 2002 becauseother fungicides used with Quadris in a programdesigned for managing resistance, as recommended forproduction fields, might provide enough control ofpowdery mildew to mask the presence of strobilurinresistant strains, especially if they were at a lowfrequency. Fungicides were applied weekly with atractor-mounted boom sprayer. Applications wereinitiated after the IPM threshold of one leaf withsymptoms of 50 old leaves examined was reached in all(or almost all) plots (McGrath, 1996b). A randomizedcomplete block design with four replications was used.Upper and lower surfaces of 5 to 50 leaves, dependingon incidence, in each plot were examined weekly forpowdery mildew. Average severity for the entirecanopy was calculated from the individual leafassessments. Area under disease progress curve(AUDPC) values were calculated as a summationmeasurement of powdery mildew severity over thetreatment period. Cultural practices used and otherfungicide treatments tested are described in previousreports (McGrath 2000, 2002b; McGrath and Shishkoff1999, 2001, 2003). One other treatment was Flintapplied alone weekly in 1998 to obtain manufacturerrequestedefficacy data.An additional opportunity to evaluate Quadrisapplied alone or with Nova plus Bravo was provided69