60 [ DISEASES OF CITRUS TREES 



its stead, or the soil left fallow for two or three years. 

 When an orange-tree or a lemon-tree budded on Seville 

 orange stock is badly infected with gummosis and the 

 stock is healthy, it may be cut down or pollarded in 

 spring a little below the graft line, and the vigorous 

 shoots which are thrown up during summer, may be 

 budded in the following spring. 



Another disease which is often met with on clayey 

 low-lying soils is the root-rot, which attacks and destroys 

 the roots of Citrus trees. It is ascribed to a fungus, 

 Rhizoctomia violacea. The generative zone or cambium 

 k of the roots is killed by the branching mycelium of 

 the fungus, the bark becomes soft, of a dark purple 

 colour, and easily separable from the wood. The wood 

 deprived of its cambium dies in its turn, and on its 

 surface may be seen the dark purple filaments of the 

 old mycelium. The tree attacked by root-rot ceases to 

 grow, its leaves assume a yellowish colour as in chlorosis, 

 the usual undoubted sign of impaired health. Large 

 branches die one after another, and shortly afterwards 

 the tree succumbs. The disease usually appears sporadi- 

 cally, but at times it may assume the form of an epi- 

 demic, particularly in very wet years. When the disease 

 is surprised at its beginning, it may be possible to save 

 the tree by cutting off and removing those roots which 

 show signs of infection, painting the wounds with tar, 

 and replacing the earth about the roots with fresh earth 

 taken from a sunny part of the grove, at some distance 

 from the infected tree. At the same time, the tree 

 should be isolated by means of a circular ditch about 

 50 c. m. deep, and about 2 metres away from the trunk. 

 Drainage should be promoted and stagnant humidity 

 avoided. 



A large wound however well protected, if it fails 

 to close in a few years, is sure to give rise to canker. 

 The genesis of this common disease is very imperfectly 

 known. Several fungi were found in the rotten wood, 



