16 PROTECTION OF PLANTS — 1923-24 



of them more speculations. All however, support the general fundamental 

 theory that such immunity is owing "to a definite antagonistic chemical 

 inter-action between host and parasite." 



Maryatt, and later Stakman, showed that resistance of some plants to rust 

 is due to the dying of the cells surrounding the ooint of infection, and thus 

 starving the obligate parasite. This is termed super-sensitivity (1). The 

 cause for the dying of the cells surrounding the point of infection in advance 

 of the invasion of the parasite has not been determined, bub it would seem from 

 the recentwork of Allen that it is bio-chemical. J. G. Leach states that in a 

 variety of bean highly resistant to Colletotrichum lindemuthianum seldom more 

 than one or two cells are attacked, and that all evidence seems to indicate 

 that resistance is due to the inability of the fungus to obtain nourishment 

 from the living pi-otoplasm. What is the bio-chemical property of the host 

 cells of resistant varieties of beans that makes it impossible for the invading 

 hyphae to obtain nourishment form them ? This question has not been answered 



Tannin is very frequently found in vegetable cells and is, as a rule, toxic 

 to fungi. Cook and Taubenhaus point out the fact that the beans are more 

 susceptible to Colletotrichum lindemuthaianum during those stages of their 

 growth when the enzyme which acts on gallic acid in the cells to form tannin 

 is least abundant. In apples pears and persimmons, and other fruits, this 

 enzyme is less abundant when the fruits are ripe than when they are green (3) . 

 Thismay explain the decreased resistance to rots observed in ripe fruits. Cook 

 found that tannin effects the various species of Endothia very differently. 

 E. radicalis and E. Gyrose are inhibited by it, while E. parasitica is at first 

 retarded and later is able to feed on the tannin (4). 



Acidity of the cell sap evidently plays some part in resistance to Guignardia 

 bidwellii, and varieties of grapes containing less tartaric acid in thier celle sap 

 have been found to show to show greater resistance to Guidnardia bidwellii 

 than thcjse containing higher quantities. Botrytis has been shown to be 

 repelled by certain acids found in plant cells. The resistance of grapes to 

 powdery mildew (Uncinula necator) has also been correlated with the acidity 

 of the cell sap, the resistant varieties containing more tartaric acid than the less 

 resistant kinds. Comes, an Italian investigator has drawn attention to the 

 fact that Rieti, a variety of wheat strongly resistant to rust in Italy, has a more 

 acid sap than that of any of the other less resistant kinds tested, and that loss 

 of resistance in this wheat when grown in warmer localities than its native 

 area is correlated with reduced acidity of the sap (5). Popp, in his investiga- 



1. Stakman, E. C. Phytopath, 4:400. D. 1914 (Abstract). 



2. Leach, I.e. 



3. Cook, M. F. & Fautenhans, J.J. Deleware Agr. Erp. Sta. Bui. 98, Jy. 1912. 



4. Cook, M. F. & Wilson, G. W. New Jersey, Agr. Exp. Sta. Bui. 291, 1916. 



5. Butler, I.e. 



