- 8 



pruning or breaking of small twigs or suckers. Large cuts, however, are 

 important since they take longer to heal, and therefore, remain suscep- 

 tible longer than small ones. Of even greater significance is the fact 

 that the infection at these large cuts may even spread to remaining 

 large branches and greatly reduce the bearing surface of the tree. 



Most canker -causing fungi usually attack trees low in vigor. Quite 

 frequently, however, very small cankers can be found around wounds on 

 young, vigorous trees, but the spread of the fungus is prevented by a 

 layer of callus tissue produced by the vigorous, healthy cells surround- 

 ing the fungus. In this situation, the fungus may starve after a few 

 years -- unless the tree is suddenly weakened; then the fungus can over- 

 run the plant defenses and cause a larger infection. It is also pos- 

 sible, although there is no direct experimental proof concerning these 

 fungi, that the greater the number of infections occurring in an area, 

 and the longer the time they remain unchecked on normally resistant, 

 vigorous trees, the greater the chance for the appearance of new, more 

 virulent races of the fungus. Such new races can then attack vigorously 

 growing trees that would have been unaffected by the older, less viru- 

 lent races of the fungus. 



The presence of fungus cankers in well sprayed orchards indicates 

 that the fungicides used for the control of apple scab, rust, powdery 

 mildew and summer rots are either not effective against canker -causing 

 fungi or that the timing of these sprays is wrong for the prevention of 

 infection. Furthermore, the rapidity at which the fungi spread indicates 

 that these fungicides have no effect on the canker once infection has 

 taken place. This leaves growers defenseless against the apple canker 

 fungi . 



During the past year, growers found it necessary to remove an in- 

 creased number of branches and trees killed by the canker fungi. Some 

 growers estimate that this removal reduced the bearing surface in some 

 blocks by 10 per cent -- causing a proportional 10 per cent reduction in 

 yield. 



Eradication of all cankers by pruning or surgery is an extremely 

 difficult, and often an impossible task; therefore, usually enough can- 

 kers remain as a source of inoculum for renewed spreading of the disease. 



The systemic fungicide required to prevent or control infection by 

 the canker fungi has yet to be discovered. Until it is discovered, keep- 

 ing apple trees vigorous, removing cankers as soon as they appear by 

 frequent and proper pruning, and covering large wounds with a tree wax 

 or paint seems to be the most effective means of avoiding losses from 

 canker-causing fungi. 



Much more information is needed about the fungi and their habits. 

 A conscientious grower no longer fears apple scab, powdery mildew or 

 rust fungi, because he knows how to control them. But, it took many 

 years of work by scientists to obtain information about these diseases 

 and to develop fungicides for their control. Information available on 



