FUNGOUS DISEASES. 63 



tions into the host. In spite of one or two alleged successful ex- 

 periments it is not possible at the present time to hold that any 

 value whatever can be expected from this line of work. It is thor- 

 oughly well known that a great majority of the fungi are far more 

 resistant to toxic or poisonous substances than the cultivated agri- 

 cultural plants. If such solutions, therefore, enter the living roots 

 or are injected and pass into living tissues, it must be assumed, 

 and indeed is known, that they would kill the host plant long before 

 they had accumulated sufficiently in the li\'ing cells of leaves and 

 stems to prevent infection. It is not difficult to try an experiment 

 of this nature. An auger hole may be made in a peach tree, and by 

 means of rubber tubing and a reservoir of copper sulphate solution 

 one may force this toxic agent into the tree. During a hot period, 

 when evaporation is rapid, the tree will promptly respond in the 

 course of a day or two by shedding its leaves, and great injury will 

 result to the trunk in a short time. I am well aware that in Florida 

 it is now the practice with some growers to insert crystals of copper 

 sulfate beneath the bark of orange trees affected with dieback. 

 This dieback seems to be a case of malnutrition brought about by 

 some unknown en^-ironmental factor. It is not a fungous disease. 

 The amount of copper used is small, and furthermore we lack as 

 yet any well-controlled experimental proof of its efficacy. It is 

 true that the germination of certain rust fungi may be readily in- 

 hibited or prevented by very small amounts of ammonium com- 

 pounds, but it is doubtful if this can be made use of practically, 

 for the obvious reasons that they may not as ammonia compounds 

 reach the cells they are designed to protect. 



Temperature. 



The relations of temperature to disease may not be made so 

 evident by illustration, but it is unquestionably important. 



Lack of aeration of the roots is shown to predispose certain 

 plants to disease. One of the most conspicuous examples cited is 

 that of the Texas root disease of cotton. On the other hand, 

 aeration favors the growth of the nodule-forming clover bacteria, 

 which, while not in the strictest sense disease producing, are never- 

 theless parasitic organisms. 



