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A Strain of Guignardia citricarpa, the Citrus Black Spot Pathogen, Resistant to Benomyl in South Africa. J. A. Herbert, Citrus and Subtropical Fruit Research Institute, Nelspruit, South Africa. N. M. Grech, Citrus and Subtropical Fruit Research Institute, Nelspruit, South Africa. Plant Dis. 69:1007. Accepted for publication 19 July 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-1007d.

Citrus black spot (CBS) caused by Guignardia citricarpa Kiely has been endemic in South Africa for many years. Since 1971, effective control has been achieved by a single benomyl application. In 1982, a loss of efficacy of benomyl was reported from a site in Eastern Transvaal. In a large orchard of Valencia oranges (Citrus sinensis Osbeck), over 50% of fruit showed three or more CBS lesions that rendered them unacceptable for export. All isolates of the pathogen from lesions on fruit from this orchard showed strong growth on benomyl-amended potato-dextrose agar (PDA) at concentrations up to 500 ppm. Some isolates from nearby orchards were resistant and some were sensitive, the proportion varying among the orchards. Sensitive isolates showed no growth at 10 ppm of benomyl. On nonamended PDA, resistant isolates grew at approximately half the rate of sensitive ones. Laboratory tests showed the resistant fungus to be cross-resistant to other benzimidazoles but sensitive to mancozeb. A field trial showed that after exclusion of benomyl and use of mancozeb only for 2 yr, the proportion of resistant isolates was reduced from over 90% to 30%. This reduction was attributed to the stronger competitive ability of the susceptible fungus in the absence of benomyl. The presence of the resistant strain was confirmed at two additional sites during 1984.