DISEASES CAUSED BY FUNGI 17 



root fungi (Rosellinia, Sphaerostilbe, and others) infest dead wood 

 or other vegetable material and either spread along the roots of 

 living trees or infest them at the collar from the soil. The 

 parasitic Fusariums obtain access by the roots to the vascular 

 system of the stem. These are all largely or entirely independent 

 of the vigour of the host, and their prevalence depends closely on 

 physical conditions. 



The root fungi of sugar-cane (Marasmius, Odontia) live 

 normally on the withered leaves and other dead material of this 

 and other grasses, but by invading the roots contribute, to an 

 extent which is very variable and in any given case difficult to 

 define, to the results of adverse conditions in general. Under 

 certain special conditions Marasmius seems able to become a 

 more active parasite and completely infest the bases of the 

 shoots. 



Notable Groups concerned in Fungus Diseases. 

 The Rusts {Uredinales). 



The family Uredinales is made up of some two thousand 

 known species of obligate parasites on trees, shrubs, and her- 

 baceous plants (including ferns). Wherever vegetation of these 

 types exists, from the polar regions to the equator, some repre- 

 sentatives of the family are found. Conditions in the West 

 Indies are not so favourable to the development of rust-fungi 

 as are those of temperate regions. While recent research has 

 disproved the idea that the species are extremely few, it remains 

 true that with few exceptions they are scanty in their distribution 

 and inconspicuous in their development. A few species occur 

 on minor crop plants, but the damage done is usually quite 

 insignificant and affords a great contrast to the immense losses 

 frequently caused in temperate crops, especially in continental 

 areas. As regards tropical countries generally, there is an out- 

 standing exception to this position in the coffee leaf rust 

 (Hemileia), which is regarded as responsible for the ruin of the 

 coffee industry in Ceylon. That fungus is not known to occur 

 in the Western Hemisphere. 



The rust fungi are strictly confined to growth on living 

 tissue, and the species are limited to individual or to related 

 species of host plants — the general tendency being to narrow 

 limitation in this respect — except that in many species the 

 character of heteroecism is exhibited In these cases the fungus 

 occurs in certain stages on one host, and is then capable of transfer 

 by spores to a second and sometimes a third host — commonly of 

 widely different botanical af&nities — and there goes through 

 other stages in an entirely different form. Only by infection 

 trials can the relationship of the forms on separate hosts be 

 determined. 



Five distinct kinds of spores occur in the Uredinales, described 



c 



