Articles
SEVERITY OF ERWINIA AMYLOVORA (FIRE BLIGHT) INFECTION ON SHOOTS OF APPLE CULTIVARS: CUMULATIVE RESULTS
To provide objective data on the severity of fire blight on different apple genotypes, the tips of vegetative shoots of single-shoot plants were injected with a suspension of 1010 cells/ml of a virulent strain of E r w i n i a amylovora, and the length of the resultant lesion was determined as a proportion of the length of current season’s shoots (%LL).
Over 300 named cultivars, numbered selections and Malus species have been tested in large replicated groups in this way.
Differences between cultivars within each test have been determined.
Since the plant condition and environment are unavoidably different for each test, direct comparisons between tests cannot be made.
Several standard cultivars have been included in all tests.
The severity of infection of a test cultivar may be expressed in the form
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This has been done for all the clones in seven large tests using Delicious, McIntosh, and Rome Beauty as standard cultivars.
Severity of infection of cultivars in different tests then may be compared and a unified list compiled.
Additional clones may be examined in future tests and included in the list as long as the standard cultivars are also present in the tests.
Among the clones that have been tested already, large differences in susceptibilty have been observed.
It is now possible to recommend which cultivars should not be planned in regions where fire blight is endemic now or likely to become so in the near future.
If available, breeders’ selections are tested before introduction so that fire blight susceptibility can be considered as an important criterion for release.
Knowledge of the susceptibility of cultivars in existing plantings can be used in forecasting disease and the advisability of protectant sprays.
Highly resistant genotypes are being utilized in programs for breeding new resistant cultivars and rootstock clones.
Complete results of this work will be published elsewhere.