60 Predisposition. 



CHAPTER XVIII. 

 PREDISPOSITION. 



Wherever epidemic diseases caused by parasitic fungi occur, the ques- 

 tion is raised as to the relative susceptibility of different varieties or indi- 

 viduals. The fungus has the power of causing disease in the host plant 

 attacked, but the latter in its turn may either be favorably or unfavor- 

 ably disposed towards its development. This predisposition of the host 

 for the attacks of the parasite is very variable, and is influenced by vari- 

 ous factors. It is generally considered that a sickly plant is more liable 

 to rust than a sound one ; but, on the contrary, strong and sound individuals 

 are more easily and more virulently attacked, so that for artificial infection 

 strong-growing plants are selected. Ward 7 has shown that when the host- 

 plant is starved by withholding/ certain mineral salts, and thus stunted in 

 growth and generally enfeebled, it is not affected in its susceptibility or 

 otherwise. A starved plant certainly develops smaller pustules and fewer 

 spores on account of the diminished supplies of food available for the 

 mycelium, ibut the power of infection is just as great as in normal plants. 

 As far as brome rust is concerned and we have no reason to doubt that 

 it holds good for others as well predisposition and immunity on the part 

 of the host, and impotence and virulence on the part of the parasite are 

 alike independent of mere nutrition. But, as we shall see afterwards, 

 certain substances not of the nature of food-material, introduced into the 

 plant may affect its liability to disease. There are various factors, how- 

 ever, which may either dispose the plant towards disease or tend to render 

 it immune, and some of these may be given here. 



The age of the part attacked has an important influence on infection, 

 particularly where the sporidiola are concerned. Young leaves and shoots 

 are most easily infected, and when they get older little or no effect is pro- 

 duced. This is owing to the germ-tube of the sporidiolum penetrating the 

 epidermis direct, and it is well known that this laver becomes firmer and 

 tougher and less easily penetrable as it gets older. The uredospores 

 and aecidiospores, on the other hand, infect the older leaves as well ; as the 

 younger, and this is easily explained from the fact that their germ-tubes 

 enter through the stomata. 



Different parts of the same plant are also variously affected. Some- 

 times it is the leaves, sometimes leaf and stem, and it may be on one or 

 both sides of the leaf. The different species of rust on the same host- 

 plant are apt to choose different portions. Thus Puccinia triticina, from 

 its earlier attack, is found most commonly on the lower leaves, and extends 

 on to the sheath, mostly near its junction with the leaf, while P. graminis 

 5 worse on the upper leaves, and often particularly bad on sheath and 

 stem. 



^Different varieties or sorts of the same species vary considerablv in 



;ity, and, as will be shown later, it is by the selection 'and 



such rust-resistant sorts that solution of the rust-in-wheat ques- 



m Australia is being attempted. The same is the case with other 



have seen one kind of flax (Linum usitatissimum) badly 



l.y Mclampsora /*/, and another kind growing alongside quite 



Jennings 5 has recently made observations which tend to show that 



lants previously susceptible to the attacks of a parasitic fungus mav gradu- 



come immune when they are changed to rich ground where they are 



better nourished and more vigorous. 



