298 Agricultural Experiment Station, Ithaca, N. Y. 



most rapid growth in a temperature of about 70° F. when much 

 moisture is present in the atmosphere. Cloudy days, with occa- 

 sional showers, and a close damp air are especially favorable to its 

 growth ; and if such periods occur during August and September, 

 the disease may appear at any time. But, on the contrary, if the 

 season is dry and hot the fungus is unable to develop, and little or 

 no injury of this nature can appear. It is for this reason that the 

 potato rot is not a regular visitor in most parts of the state, but is 

 more orenerallv confined to certain localities. These are found in 

 the more northern potato districts, in the regions near the sea coast, 

 and in some parts which have a high altitude. In such places the 

 fungus may develop regularly every year, and the severity of 

 the attack will be modified chiefly by abnormal atmospheric con- 

 ditions. 



The fungus causing the late blight of potatoes passes the winter 

 in two forms. The mycelium, or vegetative portion of the parasite, 

 may retain its vitality until the following spring, when growth may 

 again begin and further attacks of the fungus take place. These 

 are followed perhaps by less serious results than those which result 

 from the other form. This second method of surviving the winter 

 is effected by means of a small fruiting body known as an oospore. 

 It is surrounded by a comparatively hard covering and is able to 

 resist considerable extremes of temperature and moisture. The 

 spores are produced in the fall within the tissues of the potato 

 plant, and here they remain until the following spring or summer. 

 By the gradual decay of the surrounding tissues these spores become 

 liberated and when dry may easily be spread over wide areas by 

 means of winds and other natural agencies. Those which even- 

 tually rest upon potato foliage soon germinate under proper condi- 

 tions, and reproduce the fungus at the new point of infection. 

 Here the development of the parasite takes place so fast that in a 

 very short time such places become centers from which the disease 

 is rapidly disseminated. 



The manner in which the germ tube of a spore penetrates the 

 tissues is interesting. It is now generally believed that the ends 

 of the tube secrete a ferment which has the power of dissolving 

 the walls of the cells comprising the outer layer of leaf tissue. 

 When such an opening has been made, the small thread of the 

 parasite enters and it then rapidly extends to other cells, and soon 



