Some effects of inoculation with yellowing viruses on the susceptibility of sugar beet to fungal pathogens: II. Susceptibility to Erysiphe species

Abstract

Large differences in susceptibility to Erysiphe betae (Vañha) Weltzien (E. polygoni DC ex Merat) were observed, in field and glasshouse experiments, between individual virus-tolerant sugar beet lines and varieties. The severity of powdery mildew symptoms on plants of virus-tolerant varieties and of two commercial varieties was increased by infection with beet mild yellowing virus (BMYV) and decreased by infection with beet yellows virus (BYV). Simultaneous infection with both viruses did not apparently alter the susceptibility to powdery mildew, presumably because of the opposite effects of BYV and BMYV infection. It is suggested that leaves of plants infected with BMYV provide a more suitable substrate for the fungus than leaves of virus-free plants. The effect of BYV infection is obscure, but may be due to a reduction in the concentration of substances essential for the rapid development of the fungus, in BYV-infected leaves. The results of these experiments show that, although virus infection appeared to have a similar effect on the powdery mildew symptoms of all varieties, the efficiency of selection tests for resistance to E. betae will be seriously reduced if plants are not all of the same virus infection status.