SUSCEPTIBILITY OF GRAFTED MALUS TAXA AND HYBRIDS TO APPLE PROLIFERATION DISEASE

Seedlings of a great number of Malus species, subspecies, cultivars and hybrids were graft-inoculated with the apple proliferation MLO. The scion cultivars were M. pumila cv. 'Cox's Orange Pippin' and 'Golden Delicious'. The grafted trees responded very differently to infection. According to recovery rate, witches' broom formation, mortality, and development of the MLO population, the tested material could be divided into 5 groups. Group I corresponds to the domestic apple M. pumila and is characterized by a low recovery rate, a low mortality, a high frequency of witches' broom formation, and a high MLO titer. Group II differs from group I mainly by a higher mortality. In group III, mortality is like in group II but recovery is higher while witches' broom formation and MLO titer are significantly lower. Group IV is characterized by a mortality of more than 50 %. Both witches' broom formation and the numbers of MLOs in the phloem are usually low. In group V, most of the trees recovered or showed never symptoms while mortality was low. After inoculation the MLO population was low or appeared degenerate. During recovery the number of MLO-positive samples decrease so that by the end of the observation period the organisms could not longer be detected in most cases. Group V consists of apomictic rootstock selections deriving from crosses of M. sieboldii and M. sargentii with M. pumila. Due to the combination of low mortality with the apparent elimination of the MLOs within a few years this group fulfills the requirement of resistant rootstocks suitable for controlling apple proliferation.