Chapter VII. 



Fungi. Plant Diseases* 

 Jff 



Disease in plants. It is not always an easy matter to tell 

 whether or not a given plant is healthy or diseased. For in- 

 stance, a plant may be placed under very slightly unfavorable 

 conditions of moisture and sunlight. If it were to obtain 

 slightly more or less moisture or sunlight, as the case might be, 

 it would thrive or sicken. Still the unhealthiness of such a 

 plant would hardly be termed a disease. If, however, we were 

 to further change the unfavorable surroundings, we might bring 

 the plant to a condition where its life would be seriously threat- 

 ened and such a plant would unhesitatingly be called diseased. 

 We can therefore see that one may conceive of all sorts of 

 possible conditions between so-called good health and undoubt- 

 ed disease in plants, and that disease and health are only con- 

 ventional marks, as it were, on an artificial scale of the life con- 

 ditions of organisms. No plant ever enjoys all of the best con- 

 ditions possible for it can only approach such a condition. If 

 it could, it would touch the top mark of the life-scale ; the bot- 

 tom mark is the disease-death of the plant. We might say a 

 premature death instead of a disease-death because it must be 

 remembered that all plants as we know them today are destined 

 to die sooner or later. Some, as many of our common weeds, 

 live only a year, while, on the other hand, our great forest trees 

 live for centuries, but sooner or later their constructive powers 

 are no longer successful in repelling the attacks of unfavorable 

 conditions and they succumb. Such a "natural death" is not in 

 the nature of a disease, as we commonly understand that term. 

 Yet disease merely hastens this death, and again we might 

 trace all conceivable conditions between an imperceptible has- 

 tening of death to a violent death from a well-defined disease. 

 All of the efforts of agriculturists and horticulturists are 

 summed up in saying that the conditions of growth of selected 



