SUSCEPTIBILITY AND IMMUNITY 253 



(e.g. Nectria, a species of which causes Coral-spot Disease of 

 various trees) belong to this category, though their saprophytic 

 phase is of short duration. The majority of Fungi, however, 

 are either strictly saprophytic or strictly parasitic. 



In many cases special conditions, such as excess of moisture 

 (e.g. Pythium), the general state of health of -the host, or acci- 

 dental injuries to the latter, may be instrumental in bringing 

 about the attacks of parasitic Fungi. Epidemics of such wide- 

 spread diseases as the Potato Blight and the Gooseberry Mildew 

 have, for instance, often been associated with particularly damp 

 warm seasons. Fungi proper are rarely the cause of disease in < 

 man, but it may be mentioned that various skin diseases (e.g. 

 Ringworm, Favus) are due to Fungi. 



Owing to the small size of the spores, Fungi often become 

 widely disseminated by the wind, but so far as can be gauged 

 by the careful study of the spread of plant diseases due to these 

 parasites, infection by wind-borne spores seldom occurs beyond 

 a few miles. Long-distance carriage is generally to be attributed 

 to transport in infected plants or plant-fragments, through human 

 or other agency, hence the value of careful inspection and control 

 of imported horticultural produce. It is owing to such intro- 

 duction that the parasitic Fungus-flora of Botanic Gardens is 

 so extraordinarily rich. 



In dealing with the Rust Fungi, mention was made of the 

 fact that a particular species or strain of these parasites may 

 be so specialised as to be able to attack only one particular kind 

 of host, and the same is true of the Mildews. There is thus 

 often a difference, with regard to susceptibility to a certain disease, 

 between the various races of a cultivated plant ; for instance, 

 some varieties of Potato and Wheat are immune to Blight and 

 Rust respectively, and would tend to be grown in regions in 

 which these Fungi were known to be prevalent. Much has also 

 been done by the production of immune hybrids (cf. p. 384) 

 between immune and non-immune races. The ravages of a 

 disease may decrease in intensity after it has been rampant for 

 some years, the host presumably becoming adapted to the 

 presence of the parasite ; thus the Hollyhock Rust (Puccinia 

 malvacearum) , when first introduced into Europe about 1870, 

 played great havoc with its host, but now, though Hollyhocks 



