16 BULLETIN 71, HAWAII EXPERIMENT STATION 



mites are so small they are practically invisible to the naked 

 eye. Both young and adults feed on the essential oils contained 

 in the rind. The surface of the affected orange becomes scurfy 

 and of a russet color, and that of the affected lemon silvery in 

 appearance. 



Control — Finely-powdered sulphur should be dusted on the 

 infested parts with a blow-gun. Lime-sulphur spray is also rec- 

 ommended as a control measure. Spraying should be thoroughly 

 done when the fruit is about half grown. 



Other Insect Pests 



The purple mite (Tetranychus citri) and red spiders (T. sex- 

 maculatus; Tennuipalpus irritans) may be controlled by the 

 methods recommended for the orange rust mite. 



DISEASES 



Citrus trees in Hawaii are attacked by a few serious diseases, 

 caused mainly by fungi and occasionally by physiological dis- 

 turbances. Many of the diseases are difficult to identify and 

 some of them may exist for years before being observed. 



Gummosis 



Higgins (16, p. 22), in 1905, noted that gummosis or foot rot 

 also commonly known in some other countries as mal di gomma 

 (Phytophthora parasitica; P. terrestris) , was "perhaps the most 

 widespread of any citrus disease." Rhoads and De Busk (23, 

 p. 71 ) , who studied citrus diseases in Florida, state that gummo- 

 sis "may be induced by various fungi, by chemical, mechanical, 

 and insect injuries, and by certain physiological stimulations." 

 The term "gummosis" is commonly applied to a number of dis- 

 eases that cause gumming. The amount of gum exuded in each 

 particular case is approximately proportional to the severity 

 and extent of the disease. Pathological study has revealed cer- 

 tain distinguishing characters that identify this particular spe- 

 cies from others. Thus, mal di gomma manifests itself when 

 the citrus trees are of bearing age by the presence of drops of 

 gum on the trunk near the ground. At first the inner bark and 

 sapwood die, then larger areas become involved, the disease 

 spreading upward to the basal portion of the main limbs and 

 downward to the main crown roots. The dead bark gradually 

 breaks away, leaving the dead sapwood exposed. Mal di gomma 

 is often associated with Pythiacystis gummosis, which "derives 

 its name from the causal agent, the citrus brown rot fungus 

 (Pythiacystis citrophthora) " (11, p. 125, 146). Both diseases 

 may be controlled by the same method. 



Control — Gummosis can best be controlled by selecting a good 

 orchard site having rich, well-drained soil, and ample water sup- 

 ply ; using strong, clean, thrifty trees of standard varieties 

 grafted on disease-resistant stock; and by properly setting the 

 trees and maintaining them in vigorous condition. Particular 

 attention is called to mound planting as an important method 

 of preventing gummosis (15, p. 2U, 28, 33). The young trees are 



