Chemical control of Alternaria brown spot on Minneola tangelo in South Africa

The effectiveness of several groups of fungicides was compared with that of copper and mancozeb for their ability to control Alternaria brown spot on Minneola tangelo. Variables used for comparing fungicides in order of impor tance were: performance index, the number of exportable fruits per tree, th e number of fruits per tree free of Alternaria lesions and total number of fruits per tree. Percentage exportable fruits was not a reliable variable f or comparing treatment efficiency. During the 1992/1993 evaluations iprodione, difenoconazole, and procymidone showed promising control of the disease. Other than for difenoconazole, me mbers of the triazole group were not effective, especially at dosages below 10 g a.i. 100 litre(-1) water. Moderate control of Alternaria brown spot w as obtained using copper and mancozeb. During the 1993/1994 season spray pr ogrammes with copper oxychloride, mancozeb, maneb plus zinc oxide, procymid one, iprodione, and tebuconazole were compared in field trials, for the con trol of Alternaria brown spot on Minneola tangelo. Mancozeb applied at 2-we ekly intervals was the most effective spray programme, followed in order of efficiency by programmes using procymidone, maneb plus zinc oxide, and man cozeb applied at 4- and at 3-weekly intervals. Iprodione and tebuconazole w ere not particularly effective in this evaluation, although they performed significantly better than the control treatment. Trees treated with copper oxychloride produced poorly, in spite of a high percentage exportable fruit s.