8 DISEASES OF CROP-PLANTS 



surface tissue. Habits or powers of this nature in parasites are 

 usually specific. The germinating spore typically produces a 

 preliminary hypha, the germ-tube, by or from which the entry 

 is made. 



Penetration being accomplished and the parasite brought 

 into relation with the living cells, the second line of defence, the 

 resistance of the protoplasm, is met with. Very little is under- 

 stood of the nature of this. The more or less mechanical process 

 of penetration may take place on a variety of plants, but only 

 when the host comes within the range of adaptation of the fungus, 

 which is often very restricted, can parasitism be established. 

 The subject is discussed further in the section relating to the host. 



When the resistance of the protoplasm is overcome the 

 fungus proceeds to occupy the tissues. The mycelium may grow 

 into and through the cells of the infected tissue (intracellular 

 mycelium), or ramify between and about them (intercellular 

 mycelium) . In the latter event the cells are either killed and their 

 walls thus rendered permeable, or absorbing organs (haustoria) 

 are introduced. Certain fungi, e.g., the Fusariums causing wilt 

 disease, develop principally in the vessels (vascular parasites). 

 In a few cases, as in the powdery mildews (Erysiphacese) , the 

 mycelium remains external, sending only haustoria into the 

 superficial cells. The black blight or sooty mould fungi (Cap- 

 nodise) develop similarly on the surface of leaves or other organs, 

 but they have no powers of parasitism and do not live at tb 

 expense of the plant but of insect or flower secretions of tl 

 nature of '■ honey dew." 



The Host : Resistance and Susceptibility, 



In every case of parasitism there are two organisms to be 

 considered, the host and the parasite, and the reactions of the 

 plant attacked are at least as important as the nature of the 

 attack delivered. The growing recognition of this truth is re- 

 flected in the increasing fund of information regarding sus- 

 ceptibility, resistance, and immunity. 



Effect of Agricultural Conditions on Resistance. 



Some general power of resistance is conferred by the healthy 

 condition induced in a plant by growth in a congenial situation. 

 Each of the important crop plants of the West Indies is subject 

 to one or more fungoid affections which occur or are serious only 

 when temporary or permanent disabilities such as drought, 

 poverty of soil, and exposure have reduced the resistance normal 

 to the healthy plant. By avoiding unsuitable locations and by 

 the practice of good agriculture it is possible to maintain this 

 resistance at a high level, and by this means not only to escape 

 debility diseases but frequently largely to reduce the prevalence 

 of more definite parasites. To this effect is to be added the greater 

 ability of a well-grown plant to endure, to outstrip, or to recover 

 from the injuries inflicted. 



