DISEASES CAUSED BY FUNGI 15 



The death of branches may be the result of direct infestation 

 with a parasite such as that of pink disease (see Chapter XVI) 

 or of an infection near the base by a fungus which has attacked 

 the bark or gained access from a broken branch or other wound. 

 The progressive dying of the topmost branches, Hke the dieback 

 of twigs, is less often due to local infections than to the ebbing 

 of the vitality of the tree as a result of drought, poor nutrition, 

 root disease, or old age. 



With regard to the last-mentioned cause it should be always 

 remembered that the age and size to which a tree attains before 

 showing the signs of waning vigour are widely variable according 

 to situation. Trees grown in thin or poor soils, or in a situation 

 drier or more exposed than suits the particular species, reach their 

 natural limits in these respects in a period which is much shorter 

 than the normal one, while highly favourable conditions have 

 the opposite effect. 



Bark rots are often of fungus origin, but may be bacterial, 

 or, as it appears in some cases, non-parasitic. Their effects are 

 frequently much increased by insects, as in the case of Citrus 

 trees which become infested with a beetle larva (Leptostylus). 

 Bark rot in the region of the collar is rather frequent, and is 

 often due to soil-inhabiting fungi or to others enabled to exist 

 in this situation by the moisture of the soil or the shelter of 

 weeds. The peculiar liability of Citrus trees to collar rot is 

 discussed elsewhere. 



Cankers are usually local infestations of the bark in which the 

 persistence of the fungus and the reactions of the cambium give 

 rise to raised margins or irregular lumpy growth about the wound 

 produced. The application of the term is not clearly dis- 

 tinguished from that of bark rot, but the above would seem its 

 most appropriate use. 



Gummosis is a S3^mptom of very variable origin, consisting 

 in the production of gum by the degeneration of cell walls, usually 

 in the neighbourhood of the cambium layer, i.e. just under the 

 bark as that term is commonly used. The gum may saturate the 

 overlying bark and solidify in a crust or in drops on its surface, 

 as in Diplodia disease of limes and collar rot of oranges. New 

 layers of wood or bark may enclose a site of gum formation in 

 the cambial zone, giving rise to gum pockets. Gummosis can 

 occur as a result of fungus infection, wounds, or chemical 

 stimulation. 



Wood-rots, which are separately discussed in Chapter XI, 

 are as a rule the result of bark injuries of various kinds, the 

 breaking of branches, or unskilful pruning or trimming, which 

 expose the wood to infestation by fungi otherwise unable to gain 

 access. Their effect is ultimately to destroy the mechanical 

 support of the part affected. 



Galls are abnormal outgrowths from leaves or from green or 

 woody^stems, and their tissues are soft or woody according to 



