THE EFFECT OF DIFFERENT STRAINS OF APPLE PROLIFERATION ON THE GROWTH AND CROP OF INFECTED TREES

One-year old apple trees were infected by root grafting with different sources of apple proliferation. One experiment was initiated with three sources of the disease and the variety 'Golden Delicious' in 1968 (experiment A). In 1970 a second experiment was carried out with three other sources of the disease and five varieties (experiment B). In the following years the growth of the infected trees was remarkably smaller than that of the healthy check plants. The effect of the infection, however, differed significantly in relation to the source used for inoculation.

In experiment A in 1974 the average stem girth of the trees infected by the sources a, b and c were 10,2 cm, 12,1 cm and 15,6 cm, respectively, while the healthy plants had a stem girth of 24,6 cm. Significant differences in the fruit sizes were as well noted between trees infected by different sources, nevertheless almost none of the fruits produced by the diseased trees were big enough to be sold.

In experiment B all trees infected with source d grew very poorly. The average reduction in shoot growth of trees infected with source e and f (1972) were 40,6% resp. 19,8% and in the stem girth (1975) 22,1% resp. 14,9% in relation to the check plants. These reductions occured in trees which developed only mild symptoms or remained symptomless.

The differences in the effect of the inoculum sources indicate the existence of strains with different virulence for the causal agent of apple proliferation. In experiment B the differences in the effect of strains were stronger than differences in the sensitivity of the varieties.