The control of Alternaria species on leaves of sugar beet infected with yellowing viruses

SUMMARY Spraying the foliage with copper or captan increased the sugar yield of beet plants inoculated with two yellowing viruses but did not increase the yield of uninoculated plants. The severity of leaf‐necrosis symptoms, resulting from virus‐inoculation, was decreased by spraying with fungicides. These results could be attributed to a partial control of Alternaria spp. which invariably attack the older leaves of beet plants infected with beet yellows virus (BYV) and beet mild yellowing virus (BMYV). Alternaria infection also apparently affected the processing quality of the roots by lowering the sugar content and increasing the sodium content. It seems, therefore, that Alternaria spp. may have caused much of the damage previously attributed directly to virus infection. Spraying virus‐infected plants with copper or captan caused smaller yield increases in an experimental, virus‐tolerant, variety than in a commercial variety. The virus‐tolerant variety showed a useful degree of inherited resistance to Alternaria and this gave yield increases comparable with those obtained by fungicide treatment of the commercial variety. These preliminary experiments suggest that Alternaria‐induced necrosis is of considerable economic importance in beet crops infected with yellowing viruses. The losses due to the disease can be reduced by growing resistant varieties and by spraying with fungicides, although the need for further experimentation, using different strains of BYV and BMYV and also different fungicides, is emphasized.