Mixture B New Formulation adjuvant increases the rainfastness and hence effectiveness of glyphosate for rhododendron control

Abstract

Rhododendronponticum(L.)isahighlyinvasiveandcompetitiveweedonmanyforestsitesinBritainandcanactas a host for the pathogens Phytophthora ramorum(Werres) and P. kernoviae(Brasier). Control of rhododendron can be difficult due in part to itsthick waxy leafsurfaces, which discourage the absorption of foliar-applied herbicides. The adjuvant Mixture B has been used for many years to improve efficacy, absorption and rainfastness of glyphosateusedforrhododendroncontrol,buttheformulationhaschanged,andthenewproduct,‘MixtureBNF’,hasnot beenindependentlytested.Anexperimentwasthereforesetuptotesttheeffectofaddingoneofthreeadjuvants (MixtureB,MixtureBNForBreak-ThruS240),totwoexampleglyphosateproducts(RoundupProBiactiveandClinic Ace) and one triclopyr product (Timbrel) on the control of rhododendron plants. Treatments were applied as foliar spraysandthensubjectedtofourdifferentartificialrainfallregimestoexaminerainfastness.Rainfallhadnoeffect on the efficacyof Timbrel, which performed well with or without adjuvants. Rainfall afterapplication reduced the efficacyofbothRoundupProBiactiveandClinicAce,butitmadenodifferenceiftherainfalloccurred1,3or6 hafter application. There were no differences between the effectiveness of the different glyphosate formulations, but the rainfastness of both products were substantially increased by both Mixture B and Mixture B NF. The adjuvant Break-ThruS240gavelittlebenefit.Itisrecommendedthatwhenfoliarspraysofrhododendronbushesarerequired, iftriclopyrisnotavailable,anequivalentof2.88 kg a.i. ha 21 glyphosateshouldbeused(e.g.as8 l ha 21 RoundupPro Biactive or Clinic Ace) and Mixture B NFadjuvant added at a rate of 2 per cent of final spray volume.