Plant Pathology Conference
December/y, 1949
J* R* Shay: RESISTANCE TO APPLE SCAB IN MALPS
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I. Introduction* The high resistance to apple scab of certain Russian appleshas
been known for at least 60 (probably more) years (2) • In this country, no
concerted effort was made to incroporate this resistance with the high quality
of susceptible American apple varieties. German workers beginning about 1930
(5) h&ve used a limited number of species of Malus and forms of the Russian
variety Antonovka as resistant parents* At the present, greatest emphasis in
the German program is placed on improving Antonovka hybrids. In 1946, a
program aimed toward further discovering and evaluation of scab resistance in
species and hybrids of Malus and the use of this resistance in a breeding
program was begun cooperatively by the Illinois and Purdue Experiment Stations*
More recently the Virginia and New Jersey Stations have entered into formal
cooperative agreements while unofficial but active cooperation has been
established with Minnesota and with the Central Dominion Experimental Farm at
Ottawa*
II• Search for and Evaluation of resistance* Surveys in three different years
of the Malus growing at Illinois and at Morton Arboretum, Lisle* Illinois (6)
yielded 19 resistant species all with email fruits (\ to 1 inch) and several
selections with intermediate-si2ed fruits (1 to 2 inches)* In visits at the
Minnesota, South Dakota, Iowa, and Morden, Manitoba Experiment Stations.
Additional items have been collected, particularly scab-resistant hardy forms
stemming from Malup baccata* Evaluation of these selections has been attempted
ty controlled inoculations during three greenhouse seasons, and by growing
certain of them outdoors in various apple growing regions during the past 3
seasons* In the outdoor regional tests none of the selections has shown any
evidence of natural scab infection. In the controlled tests, symptoms ranged
from none through pin-point or larger non-sporulating lesions to restricted,
sporulating lesions which did not result in leaf abscission.
HI. Progress in breeding. Thirteen of the Malus species have been crossed with
Wolf River or Tv/en-ty Ounce Pippin. Some 1350 seedlings from these creases are
being grown for fruiting tests* Three resistant selections with intermediate-
sized fruits have been crossed with Jonathan and other varieties prepotent for
quality. Some 6000 seedlings from these crosses are being grown for fruiting
tests. The occurrence of commercially desirable selections is theoretically
possible in this latter group. Crosses have been made annually since 1946 so
that the seedlings have just finished their first, second or third season of
growth in the orchard. Successful crosses of 1949 which will be grown and
subjected to inoculations during the 1950 greenhouse season include: 1) Kimball
tetraploid Mcintosh x (Rome x M. noi»ibunda) sib, and R#L2740-7A-1000 seeds
obtained from crosses made by Ifcughj and, Z)  Geneva x commercial varieties -
2000 seeds from crosses made by Spangelo, Ottawa. Many resistant x resistant
and resistant x self crosses were attempted at Illinois and in Minnesota but
seed set was very low*
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