Agrochemicals suitable for downy mildew control in New Zealand boysenberry production

Abstract

The effectiveness of different agrochemical treatments for downy mildew control in boysenberry was evaluated in situ during the 2000/01 and 2001/02 growing seasons in New Zealand. Agrochemicals tested were azoxystrobin, dichlofluanid, phosphorus acid, metalaxyl-M (metalaxyl-M+dithianon and/or metalaxyl-M+mancozeb). Agrochemical treatments consisted of single or multiple applications integrated into a commercial spray programme. Downy mildew was assessed for primocane, leaf and/or berry infections (‘dryberry’). Disease pressure was very different in the two seasons, with growers experiencing small losses (<10%) during 2000/01 and high losses (20–100%) during the 2001/02 season. At low disease pressure the 2 metalaxyl-M, 3 phosphorous acid and 3 azoxystrobin plus 1 dichlofluanid applications significantly reduced downy mildew fruit losses compared with the unsprayed control treatment. At high disease pressure, only the phosphorous acid treatment (3 applications) gave acceptable downy mildew control (less than 20% fruit loss). The 3 azoxystrobin applications resulted in approximately 50% fruit loss, whereas all metalaxyl-M treatments resulted in 100% downy mildew infections. During the 2001/02 season, neither leaf nor primocane infections were indicative of fruit infections. Downy mildew incidence increased extremely rapidly within 6 days: fruit assessment increased from less than 5% downy mildew infection in the control treatment (14 December 2001) to over 75% (20 December 2001). The research not only identified agrochemicals suitable for downy mildew control during low and high disease pressure, but also opened questions with regard to our current understanding of the disease cycle and pathways of infection.