38G PHYSIOLOGY OF THE FUNGI 



In the case of the dermatophytes, Nickerson has suggested that growth 

 and sporulation in vivo may be affected by a chemical supplied to the 

 hair and scales of the skin by diffusion from the adjacent resistant tissues. 

 There is some evidence that resistance of skin to fungus infection may be 

 influenced by the nutrition of the individual. 



For more complete discussions of the fungus diseases of man and 

 animals, the student is referred to Nickerson (1947), Wolf and Wolf 

 (1947), Conant et al. (1944), Emmons (1940), and Dodge (1935). The 

 pioneering work of Sabouraud (1910) should also be consulted. 



Cultural characteristics and pathogenicity. Studies of numerous 

 isolates of a given species or genus have indicated a possible correlation 

 between pathogenicity and some particular cultural characteristic. The 

 recognition of such relations and definite knowledge regarding them would 

 be of great value to plant pathologists. One such study was made by 

 Houston (1945) on 52 isolates of Corticium solani from various hosts. 

 These isolates fell into three culture types based upon the characteristics 

 of the mycelium and sclerotia. There was a certain degree of correlation 

 between culture type and pathogenicity and symptoms on certain hosts. 

 He concluded that the culture type of C. solani is more important in 

 predicting the pathogenicity of an isolate than the host from which it was 

 isolated. 



During a study of the physiological characteristics and pathogenicity 

 of 143 isolates of Actinomyces, mostly from scabbed potato tubers, it was 

 found (Taylor and Decker, 1947) that certain isolates produced a dark 

 ring of growth at the surface of separated milk. This characteristic was 

 correlated perfectly with the ability to produce typical scab lesions on 

 potato tubers. No attempt was made to explain the basic relation of 

 these two apparently unrelated physiological characteristics. 



RESISTANCE 



Resistance is the ability of a host to prevent or oppose the entrance or 

 subsequent growth and development of a parasite. It may be effective 

 either before or after penetration of the host. A host which cannot 

 successfully prevent such actions of the parasite is susceptible. Studies 

 in the nature of resistance have been only partially enlightening, and in 

 many cases the nature of disease resistance is still obscure. Some of the 

 present theories are based on what might be termed "circumstantial 

 evidence," such as a general correlation between resistance and some 

 characteristic of the host. There is sufficient evidence, however, that it 

 is dangerous to generalize about the nature of resistance. It seems likely 

 that in many cases the cause of resistance may be specific, being common 

 perhaps to but one or only a few host-parasite combinations. 



The types of resistance may be placed for convenience into three 



