345 



"What is the economic importance of that specialization of 

 parasitism now proved to exist in many fun^^i ? 



"What degree of importance, from the economic point of view, 

 is to be attributed to the saprophytic stage in the life-history of 

 any fungus causing a plant disease ? 



"What are the conditions under which some saprophytic species 

 of fungi become parasites? 



"What are the conditions under which a parasitic fungus attacks 

 a new host species ?"' 



Some few words of explanation are necessary to elucidate the 

 subject involved in the first problem. It has been shown by inocu- 

 lations, notably among members of the rust family (Uredineae) 

 and of the family of powdery mildews (Erysiphaceae), that of a 

 fungus species occurring on a large number of host species one 

 form on a host species a cannot attack a host species h, and vice 

 versa. Thus although the two forms of fungus cannot be distin- 

 guished from one another by their morphological characters, that 

 is by those characters that together may be said to make up their 

 general appearance, yet biologically they are different in that their 

 powers of parasitism are very strictly limited. The economic 

 aspect of this is that if the host species a and b are growing to- 

 gether and only the biological form of fungus parasitic on a is 

 present, then the species h will appear immune. But if the fungus 

 strain parasitic on h is also present or is introduced, b also will be 

 attacked or its immunity will appear to break down. There is, 

 moreover, another means whereby b might becoiue attacked from 

 a. It has been found in some cases that the fungus parasitic on a 

 can attack a third species .r and that when it has grown on .r for 

 one or more generations, spores from .r can infect b. Thus .r 

 serves as a bridging species to carry the fungus from a to b, and 

 the introduction of x into a cultivation where a is attacked and b 

 is immune would naturally result in the breaking down of &'s 

 immunity. Finally, it has also been shown tliat the immunity of 

 the species b to the fungus strain on a may be partly broken down, 

 if the parts of b liable to attack are damaged by adverse condi- 

 tions, wounds, or the depredations of insects. 



Very little, if anything, is known of the existence of biological 

 species in the tropics-; yet the matter is clearly worthy of attention, 

 particularly in relation to the production or introduction of im- 

 mune varieties of host plants and in considering legislation re- 

 stricting the introduction of plants from one country into another. 

 These applications are so evident that they do not call for further 

 elaboration here. In conclusion it may be added that the related 

 genera Colletotrichum and Gloeosporium, to mention only two, 

 might well repay investigation from this point of view. 



The next point raised by Professor Salmon is that of the de- 

 gree of importance, from the economic point of view, of the 

 saprophytic stage of a fungus causing a plant disease. _ It has 

 been found that the mycelium producing conidial fructifications 



