Mutation breeding of apple and pear started in 1965 using dormant scions which, after irradiation (X-rays), were grafted on rootstocks . The most efficient dose for apple was around 3 krad, for pear between 4 .5 and 7 krad . Primarily shoots exhibiting compact traits were selected, defining such shoots as being thicker than normal for their length or shorter than normal for their diameter while having shorter internodes than normal shoots . The selection was carried out on one-season-old shoots of the same trees during three successive seasons, this involved a cut back at the end of the first and second season . The selected compact shoots yielded with apple on average four times more clones with a distinct compact habit than normal appearing shoots ; four out of every five clones were found to be stable in both apple and pear . Averaging: the results for apple, 7 % of the surviving trees produced shoots the clones of which showed compact growth, this was only 0 .5% for pear . In all several dozens of such clones were obtained of the apple varieties Golden Delicious, Cox's Orange Pippin, Belle de Boskoop and Tydeman's Early and a few distinct compact and some dwarf clones of Beurre Hardy and Doyenne du Cornice .