Tropical Fruits 



141 



called "gum diseases." Their causes are not definitely 

 known; by some they are attributed to mechanical injury 

 alone; by others to poor cultivation or water injury in ir- 

 rigation; by others to the presence of soil around the trunk; 

 by still others to fungi. 



. Blue-mold rot ^"^ (Penicillium italicum Wehni., antl Penicil- 

 lium digitatum (Fr.) Sacc). — This decay of citrus fruits in 

 transit from California causes a loss estimated in 1908 as 

 being between $500,000 to $1,500,000 annually. Careful in- 

 vestigations of Powell and his assistants have shown that the 



lui. 73. — Two forms of blue-mold: at left, Penicillium italicum; at 

 right, P. digitatum. After R. E. Smith. 



chief trouble results from wounds on fruit in handling, which 

 thus provide an entering point for the fungus. Without me- 

 chanical injury no decay from blue-mold can occur. Hence 

 the method of control is painstaking care in handling the 

 fruit to prevent bruising or puncturing the skin. 



Black-rot ^"'^ (Alternaria citri Pierce). — This disease was 

 described and its cause given by Pierce in 1902. It attacks 

 only the navel orange, causing from 2 to 5 per cent loss 

 of the whole crop. It is easily recognized by the prema- 

 ture ripening which it induces, and by the deep red color 

 and extra large size of the fruit. The diseased fruits are 

 very conspicuous on the trees before the main crop has col- 



