Breeding for tolerance to beet yellows virus and beet mild yellowing virus in sugar beet: II. The response of breeding material to infection with different virus strains

Abstract

SUMMARYIsolates of beet yellows virus (BYV) and beet mild yellowing virus (BMYV) were collected from different parts of eastern England. Individual strains of BYV were isolated, from field plants infected with mixtures of strains, using a single lesion technique; all the strains thus isolated appeared to be closely related and there was complete cross‐protection between them. Individual strains of BYV produced symptoms of differing severity on sugar beet and other test plants, and caused different losses of sugar yield in sugar beet in the glasshouse and in the field.Considerable differences in sodium contents of the roots, and in serological titres of leaf sap, were observed between plants infected with different BW strains. Simultaneous inoculation of two BYV strains, using Myzus persicae Sulz., usually resulted in a disease which was intermediate in severity between the diseases caused by the strains separately. However, in some experiments, simultaneous inoculation of BYV strains caused a more severe disease than was expected and in others a milder disease.Differences between BMYV isolates were apparently very small in the field and under glass. Field experiments showed that inbred lines and experimental varieties, derived from plants selected for tolerance to BYV and BMYV, were more tolerant than the control varieties when subjected to many different infection treatments. Infection with BYV and BMYV reduced sugar yields and sugar contents and increased sodium and potassium contents of the root. Isolates of BMYV reduced the sugar contents, and increased the sodium contents, more than any strain of BYV. Simultaneous inoculation of BYV and BMYV produced more severe effects on sugar beet than either virus alone. The virus‐tolerant breeding material usually showed smaller reductions in root weight, sugar content and sugar yield than the control varieties, and smaller increases in sodium and potassium contents of the root. The results of experiments indicated that tolerance to one strain of BYV was associated with tolerance to a wide range of BYV strains. Much less is known about BMYV strains but the breeding material yielded well after infection with all the BMYV isolates against which they were tested. It seems unlikely that a rapid breakdown of tolerance would occur if virus‐tolerant sugar beet were widely grown in the field.