ENVIRONMENTAL FACTORS 67 



black rot of the grape, apple scab, and late blight of the potato 

 with humidity is a matter of almost annual record. 



Moisture generally augments the production of spores, and it is, 

 of course, essential to the germination of these. It may at times, 

 however, have another effect, that of promoting the suscepti- 

 bility of the host to attack. The potato appears to be more readily 

 affected by the Phytophthora when it has at least sufficient water 

 for vigorous growth. Kiihn observed that there are two stages of 

 growth when the plant is most susceptible, first, when the plant is 

 young and tender, and second, when tuber formation begins more 

 vigorously. Ward thinks these two stages correspond to periods 

 of rapid movement of water and soluble food materials. 



He has also cited certain conditions under which Botrytis cin- 

 erea is parasitic, and the suffusion of the host with water is a 

 prominent feature of this case. 



It is commonly stated by grape growers that not only is the 

 black rot of this fruit most abundant in humid weather, but that, 

 further, it is more abundant upon vines which have made a vigor- 

 ous " sappy " growth. This would indicate that moisture acts here 

 also indirectly to render the host more sensitive. 



Pseudomonas campestris, producing the black rot of cabbage, 

 gains entrance to its host by reason of beads of water over the 

 marginal water pores of the leaf. These droplets are, when there 

 is sufficient soil moisture, a normal occurrence on cool mornings 

 succeeding warm days. It signifies a state of "guttation," and, 

 practically speaking, means a water way between the external and 

 internal environment. 



Smith and Stone (see asparagus rust) have demonstrated an 

 interesting water relation as affecting the prevalence of the rust of 

 asparagus. The fact that chrysanthemum rust may be largely 

 controlled by subirrigation, and carnation rust greatly reduced by 

 the same treatment, is perhaps to be explained simply by the 

 prevention of germination. 



An examination of the conditions under which epidemics occur 

 in the case of such fungi as the leaf blight of celery, leaf spot of 

 strawberry, and many others lead to some interesting suggestions. 

 These diseases may disappear during a moist summer, which 

 affords a relatively succulent growth of the hosts. In fact, such 



