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

Spring 2003 Resistant Pest Management Newsletter Vol. 12, No.2occurred in the Republic of Ireland, GreatBritain, and the Netherlands. In 1988, themetalaxyl + mancozeb formulation waschanged to give a full-rate of mancozeb. Inthe early 1990s, an anti-resistance strategybased on the one developed in theRepublic of Ireland (Dowley et al., 1995)was adopted in Northern Ireland withgrowers advised to use no more than threeapplications of phenylamides at thebeginning of the spray programme only.MATERIALS and METHODS Samples ofinfected potato foliage were obtained(mainly from seed crops) by members ofthe Department of Agriculture and RuralDevelopment (DARD) Potato InspectionService (Cooke et al., 2000). Isolates werederived by bulking together the sporangiaobtained from all foliage samples within asingle crop and maintained on detachedglasshouse-grown potato leaflets thentested, using the floating leaf disctechnique (Cooke, 1986), on 100 and 2 mgmetalaxyl litre-1. Isolates were designatedresistant if they sporulated on 100 mgmetalaxyl litre-1-treated discs andsensitive if they sporulated on untreateddiscs, but not on any metalaxyl-treateddisc. Isolates that grew on discs floating on2 mg, but not on 100 mg metalaxyl litre-1,were designated intermediate. At the endof each season, inspectors providedestimates of fungicide usage for all seedpotato crops in their areas.RESULTS and DISCUSSION In the mid-1980s,a high proportion of Northern Irelandpotato crops were sprayed withphenylamide fungicides for most of thespray programme. However, during mostof the 1990s and up to 2002, PotatoInspectors' estimated that c. 40% of seedpotato crops were phenylamide-treated(Fig. 1) and the majority of growersfollowed DARD advice and used no morethan three phenylamide applications perseason (Table 1). The proportion ofisolates containing phenylamide-resistantstrains declined in the early 1990s andappeared to have stabilised around 50%(Fig. 2) up to 1999. However, in 2000 andparticularly in 2001, there was a markedincrease in the incidence of phenylamideresistance. The reason for this was notclear, since fungicide usage had notchanged. The weather in the summermonths of 1998-2000 was unusually wet74