PARASITISM AND RESISTANCE 393 



able to the host had no appreciable direct effect upon the resistance to 

 rusts. A summary of the work on the effect of soil nutrients and environ- 

 ment upon resistance to disease has been presented by Wingard (1941). 



The carbon metabolism of a plant likewise influences resistance to 

 certain rusts. Waters (1926) found that urediospores of Uroniyces fabae 

 developed on detached leaves floating on 5 per cent sucrose solution in the 

 dark, while none formed when leaves were floated on water. These 

 observations were confirmed by Yarwood (1934) for rust and powdery 

 mildew of clover. It follows that active carbon assimilation increases 

 susceptibility of the host to the obligate parasites. 



Although the environmental factors are of great importance in deter- 

 mining the resistance or susceptibility to a disease, their effects are usually 

 upon the host and only indirectly upon the parasite. Abundant refer- 

 ences on this subject can be found in the literature. The effect of tem- 

 perature upon the metabolism and resistance of certain hosts may be 

 illustrated by Gihherella zeae on wheat and corn (Dickson, 1923). Seed- 

 ling infection of wheat was found to occur at high temperatures and of 

 corn at low temperatures; i.e., the temperatures unfavorable to host 

 development. In the germination of w^heat at low soil temperatures the 

 starch of the endosperm is hydrolyzed more rapidly than the proteins, 

 which results in abundant sugar but little available nitrogen for seedling 

 growth. Thus, the cell walls are thickened and more resistant. At 

 higher temperatures both starch and proteins are rapidly hydrolyzed; 

 there is a greater supply of available nitrogen, and growth is more rapid. 

 The cell walls remain longer in the pectic condition and are more suscepti- 

 ble to attack. In corn the situation is reversed. At high temperatures, 

 which favor the corn, the cell w'alls develop more rapidly and are more 

 resistant. 



Sharvelle (1936) concludes that the resistance of flax to flax rust cannot 

 be attributed to any single factor but probably results from a number of 

 factors operating together. Doubtless, the same statement could be 

 applied to many other diseases to which the nature of resistance is not well 

 understood. 



SUMMARY 



Some of the different types of parasitism may be summarized as fol- 

 lows: (1) The parasite produces extracellular enzymes, particularly 

 pectinase, w'hich dissolves the middle lamellae of the host cells, allowing 

 the cells to separate (rotting). This may or may not be accompanied 

 by toxic substances but results in the death of the cells. The soluble 

 food materials are then free to be absorbed by the fungus. The insoluble 

 foods stored in the host cells may be digested by other extracellular 

 enzymes. This type is illustrated by the rots of fruits and vegetables. 



