196 



THE NURSERY-MANUAL 



gall infections can be reduced by using a root and cion of ap- 

 proximately the same size and by wrapping the grafts carefully. 



APPLE- AND PEAR-SCAB. The scab diseases exist apparently 

 in every country where apples and pears are grown commercially. 



In the nursery, the pear- 

 scab is usually of impor- 

 tance only on the Flemish 

 Beauty buds. The apple- 

 scab is most destructive 

 in the nursery on the vari- 

 eties Mclntosh, Tran- 

 scendent and Martha. 



Description. The leaves 

 and the fruit and some- 

 times the twigs are affected. 

 The disease appears on 

 the leaves as olivaceous 

 to dark brown or nearly 

 black spots (Fig. 212). 

 There is a tendency for 

 the lesions to extend along 

 the veins of the leaf, mak- 

 ing them irregular in ap- 

 pearance. 



Cause. - - The scab 



lesions are caused by the fungi Venturia incequalis and Venturia 

 pyrina, which produce a large number of spores in the spots. 

 The spores are disseminated by wind and rain and are respon- 

 sible for the new infections which occur throughout the summer. 

 On the diseased leaves that fall to the ground in autumn, 

 special fruiting bodies (perithecia) of the fungi are produced 

 and by this means the organism lives over winter. In spring 

 the perithecia produce spores which attack the new foliage. 



FIG. 212. Scab on apple leaves. 



