392 PHYSIOLOGY OF THE FUNGI 



related or unrelated to those which serve as the natural host of the fungus. 

 In such cases the failure to cause disease may be due to unfavoraVjle 

 nutritional relations. The theory of a toxin-antitoxin, or toxin-counter- 

 toxin, between parasite and host has been suggested by a number of 

 investigators (Ward, 1905; Marryat, 1907; Stakman, 1914; Allen, 1923; 

 Walker, 1924) as a possible explanation for resistance to the rusts. 



Cytological studies of Puccinia graminis tritici infections of both 

 susceptible and resistant varieties of wheat were made by Allen (1923), 

 who concluded that secretions from the fungus stimulate the metabolic 

 activities of the susceptible host to produce more food, while in the 

 resistant host the same secretions cause disintegration and death of the 

 host cells near the infection. More distant cells may be stimulated. 

 The haustoria usually die soon after the host cells are killed. Leach 

 (1919) believes resistance to P. graminis tritici and P. graminis tritici- 

 compacti can best be explained on the basis of specific food requirements 

 of the parasite and specific food production by the host. It was sug- 

 gested that the injury to the host cells might be due to an excess in amount 

 of enzymes stimulated by a limited supply of food in resistant hosts. 

 Similarly, Wellensiek (1927) believes that this theory best explains the 

 resistance of corn to strains of P. sorghi. He suggests that the difference 

 between susceptibility and resistance is of a quantitative nature and that 

 the amount of the specific nutrient determines resistance or susceptibility. 



Walker (1924) points out that resistance may be due to the action of a 

 number of factors and that a clear understanding of resistance must be 

 based upon a thorough understanding of parasitism. Walker's excellent 

 discussion of the nature of disease resistance gives many references to the 

 literature on this subject. 



Host nutrition and its effect on the development and severity of disease 

 is a relatively new phase of study, and much more investigation is neces- 

 sary before general conclusions can be drawn. The fungi vary widely 

 in their reactions to differences in host nutrition, the type of parasitism 

 apparently being a determining factor. The action on the pathogen is 

 believed to be principally indirectly through the effects of nutrition on 

 the host, although it is possible that some of the vascular parasites may 

 be directly affected by the nutrients which pass through the xylem. An 

 increase in the salt concentration of the nutrient solution increased the 

 development of clubroot, w^hile it decreased the severity of cabbage 

 yellows (Walker, 1946). The development of Fusarium wilt of tomato 

 was affected in a way similar to cabbage yellows. More recently, Gallegly 

 (1949) reported that the development of Verticillium wilt of tomato was 

 reduced with a reduction in salt concentration of the balanced solution 

 used to grow the tomato plants. Stakman (1914) and Ward (1902) came 

 to the conclusion that deficiencies in nitrogen and phosphorus salts avail- 



