Articles
THE VALUE OF TOPLEAF INOCULATION TO DEMONSTRATE GENETIC RESISTANCE IN POMOIDEAE SPECIES TO ERWINIA AMYLOVORA (BURR.) WINSLOW ET AL.
Results of the experiments demonstrated that, when resistance was absent or when a very high level of resistance was present in the original material, all 10 cuttings of one seedling reacted similarly.
If only a certain level of resistance was present the type of symptom expression of all cuttings of one seedling and the degree of symptom development varied considerably, due to the physiological variation of the individual cutting.
These results indicate that by using a topleaf inoculation test on a progeny of a breeding program no information can be obtained about the presence of resistance factors if shoots do wilt only partially or not at all, but that only the very susceptible seedlings can be sorted out.
In our paper presented to the Fireblight Symposium in Wageningen in 1977, the conclusion was drawn that in Pyracantha "the results indicate the presence of, at least, two independent resistance factors, one in the leaf, one in the stem" (1). Moreover "analogous, but less distinct, findings were obtained by screening other Pomoideae".
Continued screening in the past three years of ornamental Malus graftings, Crataegus- and Cotoneaster seedlings of various origins, and Pirus- and Pyracantha seedlings resulting from breeding programs again confronted us with the fact that after topleaf inoculation of actively growing plants the classification of the observed symptoms seemed to indicate different levels of susceptibility for the pathogen Erwinia amylovora (Burr.) Winslow et al.
A fair number of pear seedlings was inoculated by cutting off the top of the unfolding leaves during the submersion of the shoot in a bacterial suspension of app. 106 bact./ml.
The majority of the treated plants showed wilting symptoms after one week and collapsed totally within a month.
In a considerable number, however, though showing wilting of the inoculated leaves and the apical bud, there was no symptom development into the stem.
It seemed as if, to a certain extent these stems were resistant to the bacterial infection.
Moreover, quite a number of inoculated leaves showed a delayed wilting, as if the spread of the bacteria in the leaf was inhibited.
When, at last, the symptoms reached the stems, either the stem symptom progressed very quickly or stemcankers of various sizes developed around the leaf scar of the inoculated leaves.
A last group of symptoms could be described