A FORTNIGHTLY REVIEW 



OF THB 



IMPERIAL DEPARTMENT OF AGRICULTURE FOR THE WEST INDIES. 



Vol. XL No. 274. 



BARBADOS, OCTOBER 26. 1912. 



Priob Id. 



Problems of Economic Importance 

 Regarding Plant Diseases. 



'N his presidential address* to the British 

 Mycological Society delivered in 1911, Pro- 



(fessor Salmon states and illustrates, among 

 others, four practical problems connected with cert.-.in 

 aspects of the life-histories of economic fungi. 



* Trantadions of the British Mycological Society, 1911. 



Although these problems were all considered and illus- 

 trated with reference to economic mycology in England, 

 yet in themselves they are equally important in con- 

 nexion with plant pathology in the tropics. Stated 

 shortly they are as follows: — 



'What is the economic importance of that special- 

 ization of parasitism now proved to exist in many 

 fungi ' 



'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 sapro- 

 phytic 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 inoculations, 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 distinguished from one 

 another by their morphologicalcharacters.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 .-pecies 

 a and h are gi-owing together and only the biological form 

 of fungus parasitic on a is present, then the species 6 will 

 appear immune. But if the fungus strain parasitic on 

 h is also present or is introduced, b also will be attacked 

 or its immimity will appear to break down. There is, 

 moreover, another means whereby b might become 



