PLANT DISEASES 



445 



bruising of the crown that permits the gall-forming bacteria 



to enter. 



Peach leaf curl. It is a frequent trouble in peach orchards 



that the leaves become curled up and twisted into various 



shapes, the surface looking 



wrinkled and blistered. This 



interferes with the work of 



the leaves so much that 



there may be an extensive 



failure of the crop. This 



curling and twisting is due 



to the fact that the presence 



of the parasite causes the 

 leaf cells to grow very un- 

 evenly. 



Black knot. This is an 



exceedingly common disease, 

 and among cultivated plants 

 it is most commonly seen on 

 plum (Fig. 95, a) and cherry 

 trees, but it is common also 

 on shrubs, as currants (Fig. 

 95, 6) and gooseberries. It 

 appears as small hard knots 

 or " warts " that break 

 through the bark and finally 

 become dark brown or black. 

 The knot is made up of a 

 mixed mass of cells devel- 

 oped by the host plant because of the presence of the fun- 

 gus, and interlaced in the tissues of the knot is the thready 

 body of the parasite. The twigs of the plant may not be 

 killed, but when they are girdled by knots they are destroyed. 

 143. Diseases of the third group. In this group of dis- 

 eases the parasite invades the water-conducting vessels and 



FIG. 94. Crown gall on daisy. After 

 ERWIN SMITH. 



