14 DISEASES OF CROP-PLANTS 



fruits are rapidly infested and destroyed. The parasitism of 

 the smut fungi, which also destroy flowers, is of a special type 

 in which infection proceeds from within, and is described in 

 another section. 



Fruits while yet unripe may be very quickly destroyed by the 

 Phytophthoras (soft-rot of cotton bolls, cacao pod-rot), may 

 be spotted or cankered by the anthracnose fungi and similar 

 parasites, may be scabbed or russeted by surface infections, may 

 develop cracks in consequence of the inability of infested spots 

 to keep pace in growth, and may be internally infected through 

 bug punctures while preserving an uninjured appearance. 



As fruits ripen they lose, like senescent leaves, much of the 

 resistance which accompanies the condition of living activity. 

 A ripe fruit is a fruit that is finished with so far as the plant 

 is concerned, and if not protected soon becomes the natural 

 prey of various saprophytic fungi (see Chap. XIII). Some of the 

 ripe rots may commence prematurely before the fruit is picked ; 

 usually this happens in consequence of a wound or bruise. 



Further examples of fruit affections will be found described 

 under diseases of Citrus spp. 



The shedding of young fruits, of which cotton affords the 

 pre-eminent type, may be due to infection or insect injury, but 

 is frequently a result of the disturbance of physiological balance 

 by some change in external circumstances. It may also be an 

 adjustment of numbers to the amount of fruit which can be 

 brought to maturity, as seems to some extent to be the case 

 with coconuts. A similar adjustment in cacao occurs not by 

 shedding, but by the drying up of young fruits, and as these 

 invariably become infested with fungi the loss is often attributed 

 to fungus disease. That the inherent bearing capacities of 

 individual coconut and cacao trees vary widely does not affect 

 the fact of the adjustment, which is demonstrated by the pro- 

 portionate increase in the amount of fruit which matures when 

 cultivation and manuring are improved. 



Stem Diseases. 



Tip-wither, the dying of young green shoots, is commonly 

 due to fungi which also attack the young leaves, and may arise 

 from terminal infections or from the ringing of the young stem 

 by infections in the cortex. It may, of course, also be produced 

 by insect injuries or physical causes. 



Dieback of twigs often follows on successive defoliations by 

 fungi or insects. It is frequently associated with infestation by 

 weak parasites such as Diplodia spp. or Colletotrichum glceo- 

 sporioides on trees rendered susceptible by exposure, drought, 

 or^ poor nutrition. These conditions are themselves capable of 

 originating dieback, and it is usually very difficult to form an 

 opinion as to how far the failure is due to or is aggravated by the 

 presence of the fungus. 



