126 DISEASES OF TROPICAL PLANTS CH . 



in localities where the orchards and surroundings are 

 not well kept. However, they may cause trouble in 

 orchards under the very best cultural conditions. Most 

 of these organisms are primarily saprophy tic, growing on 

 dead twigs, old stumps and debris either in or near the 

 orchard. They gain entrance to the tree through 

 wounds, and may grow for long periods of time and the 

 tree may die before the organism appears on the surface 

 as sporophores, or fruiting bodies. Borg has reported 

 Polyporus obliquus, which he says is a true parasite, and 

 P. fumosus from Malta. They kill their hosts in from 

 three to five years. He also reports Agaricus citri and 

 A. hesperiduum as saprophy tic on rotten roots, and 

 expresses the opinion that they may have some con- 

 nection with the gummosis. 



Root Rots. Root rots, which are due to various 

 causes, are more or less common, and have been reported 

 in one form or another from practically all citrus-grow- 

 ing districts of the world. In most cases they can be 

 traced directly to poorly -drained soil, improper planting, 

 or poor cultivation. 



Any of these causes w r ill be accelerated by various 

 root fungi, which are especially severe on weak trees. 

 They usually cause a chlorosis or yellowing of the leaves, 

 which eventually fall ; and if the cause is not removed 

 the tree finally dies. Kirk has reported a serious root 

 rot of the lemon and citron in New Zealand. The 

 leaves show the usual symptoms, the growth is arrested, 

 flowers are produced in great abundance, and a heavy 

 crop may be produced for several years. The fruits 

 become smaller from year to year, and finally fail to 

 ripen. The small roots or rootlets decay, the epidermis 

 flakes from the larger roots, which finally decay. The 

 collar softens and rots, giving off a vile odour. The 

 decay may extend a foot above the collar and the bark 

 fall away in fragments. If the rot travels longitudinally 

 the tree may live for many years, but if the trunk is 

 girdled, death results very quickly. It is due to cold 

 soil, excessive moisture, and too deep planting. 



