FUNGOUS DISEASES. 57 



agus plant and the rust fungus is approximately this : In the East, 

 where summer dews are frequent, and where there is at night 

 moisture on the leaves practically sufficient to assure the germina- 

 tion of the spores, the fungus is commonly most severe on poor, 

 light soils in dry weather. It is believed that there the plant is 

 less resistant, that is, less hardy. On the other hand, in California, 

 there is generally no spread of rust during the summer in those 

 regions where there is no dew, except when the ground is constantly 

 moist and air drainage insufficient. The local condition in any 

 case is said to be a sufficiently moist soil to promote the most 

 vigorous development of the plant. This is not without some 

 surprising suggestions, for I shall show subsequently that any 

 condition affecting the vigor or full vegetative growth of certain 

 grasses tends to prevent the production of disease by rust. So 

 far as I am aware, no one has been able to demonstrate that rust in 

 wheat or oats is inversely proportionate to the vigor of the plant. 



We are now well aware that chrysanthemum rust in the green- 

 house may be largely controlled by sub-irrigation, and a complete 

 cessation of the use of a nozzle or spray. Under similar conditions 

 the carnation rust is certainly far less abundant. This, however, 

 seems to be a simple matter of the prevention of rust germination 

 through lack of moisture upon the leaves or in the air. It is quite 

 possible in these cases, as is held by many florists, that weak stock 

 predispose to these diseases, but we lack information on this point ; 

 for weak stock, as previously suggested, may mean simply stock 

 which rusts, and that which is subject to rust will in the course of 

 time appear weak. An examination of the conditions under which 

 epidemics occur in the case of such fungi as the leaf blight of celery, 

 of violets, or others of this tyi^e, leads to some interesting suggestions, 

 as already indicated. Numerous similar diseases of truck and 

 floricultural crops are commonly most important in what we would 

 consider to be insufficient moisture for vigorous crop production. 

 Atkinson found a leaf blight of cotton abundant on certain soils in 

 the South. These soils responded promptly to the application of 

 potassic fertilizers. It seems that here, too, the ultimate factor is 

 that of water, which the plant is far more able to get in the presence 

 of potassium salts in the soil. Reference has already been made, 

 on the other hand, to the increased abundance of the rot of stone 



