212 DISEASES OF CROP-PLANTS 



GUMMOSIS. 



The formation and exudation of gum in citrus trees is a general 

 reaction to injuries which may be caused by- mechanical means, 

 by chemicals, by physiological disturbances, or by infestations 

 with fungi. There are consequently various types of disease 

 of which it is a symptom. Two of these, foot-rot or mal-di- 

 goma and Diplodia disease are separately dealt with. 



Symptoms closely resembling those of foot-rot, but not 

 confined to the base of the stem, have been regarded as evidence 

 of a distinct disease. On lemon trees in California, according 

 to H. S. Fawcett, gummosis of this type is very common, and has 

 been shown to be due in many cases to the fungus Pythiacystis 

 citrophthora of Sm. and Sm. In another variety of gummosis on 

 lemons in California the fungus found is Botrytis cinerea. The 

 first-named fungus inhabits the soil, infects the bark about 

 soil-level, and spreads indefinitely upwards. It is closely related 

 to Phytophthora. The conditions favouring the disease are the 

 same as in the case of foot-rot, and the same methods of pre- 

 vention and treatment are successful. 



There are other forms of gummosis of stem and branches in 

 which, as in many cases of foot-rot, parasitic causation has not 

 been demonstrated. These affections begin with the degenera- 

 tion of a patch of the inner bark, resulting in the production of 

 gum or " sour sap " which oozes out if any crack in the bark is 

 available. The bark may die completely over this patch, or 

 the gum layer may be enclosed by new growth and form a charac- 

 teristic pocket. 



The external development varies from dead sunken or bare 

 patches to rough bark broken up into scales owing to the irregu- 

 larity of its production by the cambium. The latter form of 

 affection closely resembles one aspect of the Florida scaly bark 

 disease, which differs in further attacking the twigs and fruit. 



For practical purposes these types of bark affection on citrus 

 trees, including foot-rot, are best regarded as manifestations of 

 some unfavourable relation, which may be temporary or perman- 

 ent, between the plant and the physical conditions of its environ- 

 ment. Injuries due to temporary conditions may be met by 

 the treatment given for foot-rot. The subject of more general 

 incompatibility is discussed in the introduction of this section 

 on citrus diseases. 



Foot-Rot or Mal-di-goma. 



The type of disease to which these names are applied is 

 characterised by the decay of bark and wood on the crown and 

 collar of the tree, accompanied by the formation of gum and its 

 exudation from the affected parts. Disease of this general 

 type occurs in most if not all the citrus-growing countries of 



