58 [ DISEASES OF CITRUS TREES 



to kill a Citrus tree. The tree dies slowly, but in early 

 autumn death may be sudden, the leaves and small fruits 

 dry and remain hanging on the tree, and but for the 

 presence of half-rotten roots or of a complete ring of 

 black shining bark around the trunk, the disease is easily 

 mistaken for brontesis. 



Not every case of gummosis is necessarily fatal. 

 When the disease attacks a bitter orange-tree or a bitter 

 orange stock, it is often checked by the robust growth 

 of the tree. In this case a new layer of healthy bark 

 grows out, and covers the dead wood in the same manner 

 as when it closes upon a wound. Of all the Citrus trees, 

 the lemon tree is the most subject to gummosis, and 

 suffers most severely from it. The citrons and the 

 sweet orange or China Orange are less liable than 

 th'e lemon, but more than the ordinary or common 

 orange and its varieties or forms. The mandarin orange 

 is occasionally liable to a dry form of gummosis in which 

 gum is rarely produced. The Seville or bitter orange 

 is the least subject to gummosis and is also the most 

 resistant to the disease, recovery taking place in about 

 70 per cent, of the total cases. Even when the disease 

 attacks the roots of the bitter orange, a form known 

 as mal detta cagna by Italian writers, new roots may 

 be developed, and the tree may continue to flourish 

 and to produce large crops for many years. 



In the lemon, the citron and the common orange, 

 gummosis may attack also the twigs of a year's growth. 

 In this case the twig dies, but the tree suffers no further 

 injury if not already infected lower down. As a measure 

 of precaution it is advisable to remove and burn the 

 diseased twigs, cutting them some distance lower down 

 the point of infection, upon the healthy wood. 



The predisposing causes of gummosis are want of 

 moisture in the soil and subsoil, an excess of un- 

 fermented manure, lesions or wounds of the trunk, the 

 branches and the roots, abrasions or punctures of the 



