DISEASES CAUSED BY FUNGI 5 



The Nature of Fungus Parasitism. 



There are numerous gradations between the strictly sapro- 

 phytic and the strictly parasitic habit in fungi. Many species 

 which subsist typically on the dead or dying organs of plants 

 can advance some way towards the invasion of living tissue 

 if its resistance is weakened by age, poor nutrition, or insect 

 or mechanical injury. The debility diseases already referred to 

 are caused by the more advanced examples of this class, of which 

 the Diplodias afford the best illustration in the West Indies. 

 Certain fungi of this genus are abundant on recently dead her- 

 baceous stems, woody branches, and fallen fruits. Further, 

 they readily attack sugar-cane, cotton bolls, cacao pods, or 

 citrus fruits when these are over-ripe ; while in the same class 

 of material even a trifling injury at an earlier stage may afford 

 a foothold and permit of infestation. In the case of cotton it 

 would seem that under favourable circumstances the rot can 

 commence unaided. On lime and cacao branches, to mention 

 instances typical of many other trees, the Diplodias get a foothold 

 on a broken branch or dead twig and are only checked when they 

 reach a part situated in a line of normally vigorous growth. 

 Where conditions are adverse to the maintenance of vigour in 

 the tree they can do considerable damage. 



From this class of weak parasites advance is made by in- 

 sensible steps to fungi which can establish themselves on plants 

 in normal or even supernormal vigour. It is somewhat vain to 

 attempt to classify the numerous grades of parasites ; they vary 

 in so many ways that each case must be taken on its merits ; 

 the variations will be best appreciated from the descriptions of 

 individual diseases given later. The most notable division is 

 that which marks off the extreme class of parasites. On the 

 one side are those fungi which, though capable of parasitism 

 and frequently typically parasitic, can also live on a greater or 

 lesser range of dead material, often of their own killing : on the 

 other side are the strict or obligate parasites, of which the most 

 advanced forms enter into a close relationship with the living 

 cells of the host and are dependent on the continuance of its life 

 processes for support. Of this nature are the rusts (Uredineae), 

 the smuts (Ustilagineae) and the powdery mildews (Erysiphaceae) . 

 Unlike nearly all fungi in the previous class, the obligate parasites 

 naturally cannot be grown in culture apart from their host. 



Another distinction made, with more or less of value, is that 

 which marks off as wound parasites those fungi which cannot 

 attack living organs direct, being unable to penetrate the cuticle 

 of leaves or the cork of stems as the case may be, and only gaining 

 access when the protective layer is in some way broken. 



Very important differences exist in regard to adaptation to 

 special hosts. There are at one end of the scale general parasites 

 like the Rosellinias later discussed, which under suitable condi- 

 tions can infest almost any plant which comes in their way. 



