In this study control of spread by insect vectors of non-persistent Lily symptomless virus and Lily mottle virus in lily, Tulip breaking virus in tulip, Iris mild mosaic virus, Narcissus latent virus and Iris severe mosaic virus in bulbous iris, and semi-persistent Dahlia mosaic virus and persistent Tomato spotted wilt virus in dahlia has been evaluated with weekly sprays of mineral oil, beta-pinene emulsion, polydimethylsiloxane emulsions and pyrethroid insecticide. In lily, beta-pinene in 'Wilt Pruf' gave 40duction of virus spread. In 1995-97 deltamethrin in 'Decis' gave 22-58duction. Deltamethrin added to sprays of mineral oil 'Luxan oil H' and polydimethylsiloxane(PDMS), e.g. in 'Dow Corning 36', efficiently improved control efficacy. The latter was also observed in tulip and dahlia. Mineral oil and deltamethrin gave best control by 81-97duction of virus spread at standard spray volumes (6.25 litre ha-1 0.4 litre ha-1). 'Luxan oil H' at 3.125 litre ha-1 with deltamethrin gave 69-91ontrol. Efficacy of coby polydimethylsiloxane in 'Dow Corning 36' was superior to 'Luxan anti-foam'. 'Dow Corning 36' with deltamethrin (7 0.4 litre ha-1) gave satisfactory control (68-87 In tulip, the control by 'Dow Corning 36'/deltamethrin sprays proved satisfactory compared with 'Luxan oil H'/'Decis'-sprays. In bulbous iris the efficacy of tested PDMS-brands was clearly different in favour of 'Dow Corning 36'. In dahlia mineral-oil and PDMS-sprays gave some control of semi-persistent DaMV (16-24 This ranged at higher level(65-80 when deltamethrin was added to the spray mixture. Similar trends were observed in the control of persistent TSWV. The effect of polydimethylsiloxane emulsions in the spectrum of virus-control agents is described for the first time. The effect of PDMS compared with that of mineral oils and synthetic pyrethroid insecticides is discussed with respect to efficacy, mode of action to prevent virus transmission and possible reduction of bulb weights in vegetatively propagated bulb crops
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