DISTRIBUTION OF BEET MILD YELLOWING VIRUS AND BEET YELLOWS VIRUS IN SUGAR-BEET IN THE COURSE OF THE GROWING-SEASON

All leaves and one root sample of five sugar beets in a field naturally infested with beet mild yellowing virus (BMYV) or beet yellows virus (BYV) were examined by means of DAS-ELISA at the end each of July, August, September and October. At all these dates, extinction values in leaves and roots were significantly lower for BMYV than for BYV. Regarding BMYV, the beet root values were always higher than the values recorded in the leaves. For BYV, these values reached about the same levels in roots and leaves. For both viruses, the share of positive samples and the level of mean extinction values in leaves and roots increased from July to August and September, and afterwards they declined again. For BMYV, in August and September the middle-aged leaves had significantly higher extinction values than older and younger ones. This was not the case, however, in July and October. As to BYV, in July and October the extinction values declined from the older to the younger leaves. For reasons of experimentation it was not possible in August and September to distinguish between the various leaf stages (extinction values higher than 2). Studies with BYV-infected field plants in the following year proved that same tendency for August, too.