148 EXPERIMENT STATION. [Jan. 



various spores which are likely to infect crops. It is Avell known 

 that the tops of trees are less likely to become infected by fungi, 

 owing to the smaller amount of moisture there than about the 

 branches nearer the ground ; and asparagus plants when grown 

 under trees or covers which protect them from the dew seldom 

 show any indications of rust. 



Too ffreat a deg-ree of heat and moisture in the soil mves rise 

 to serious troubles, as may be seen in the case of Qi^dema of 

 tomatoes ; and when seedlings are grown in soil that is kept 

 too moist and at too high a temperature, they are likely to 

 " damp off." The presence of water in a plant in excess of 

 certain amounts is favorable to disease, as is shown in the car- 

 nation's susceptibility to rust ; for example, those carnation 

 plants possessing the greatest amount of water in their tissues 

 appear to be the most susceptible to rust. The stinmlating 

 effects of electricity, fertilizer and sterilized soil often prove 

 injurious by developing too high a water content in the tissues, 

 thus rendering them more susceptible to disease. Tillage, 

 manuring, irrigation, mulching, etc., are important factors in 

 securing vigorous plants, and go a long way towards rendering 

 them immune to certain diseases. An excessive amount of 

 moisture in the soil stimulates growth and often renders plants 

 more susceptible to fungous diseases, and a lack of water has 

 the same effect ; in fact, stimulation of various sorts may result 

 in weakening a plant and rendering it less immune to disease. 



The life history of an organism presents different stages of 

 susceptibility or immunity to disease, corresponding to different 

 stages of development ; for example, young plants may be more 

 susceptible to certain diseases than older ones. Very young 

 seedlings often fall a prey to the "damping off" fungus, but 

 when they have reached a certain stage of development they 

 become immune to fungi, and the younger and less-developed 

 parts of mature plants are more susceptible than the older 

 parts. Vegetative rest and overmaturity are also favorable to 

 disease, while the conditions associated with isolation are un- 

 favorable for infection. Weakened plants are more susceptible 

 to disease than strong ones, and in most cases, if not all, vital 

 depressions are the real causes of disease. Vital depressions 

 are brought about by the abnormal conditions which modify and 



