62 Bulletin 140. 



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 litde or n® 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 

 generally 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 aldtude. In such places the fungus 

 may develop regularly every year, and the severity of the attack will 

 be modified chiefly by abnormal atmospheric conditions. 



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 

 passes through the outer layer, enters an intercellular space, and then 

 rapidly extends its thread-like growth between neighboring cells, 

 drawing its nourishment by means of minute suckers which pene- 

 trate the cell walls." * The entire destruction of the leaf may be 

 accomplished. A stoma or breathing pore may also serve as a point 

 of entrance. 



The rapidity with which the fungus advances within the leaf tissues 

 depends very largely upon external conditions, and the appearance 

 of the affected parts is also modified to a very considerable extent. 

 Unfavorable conditions frequently render the identification of the 

 parasite a difficult matter without the aid of a glass, but under such 

 circumstances the disease may be fairly widespread, and still cause 

 little injury. In serious attacks, however, many characteristic symp- 

 toms may be easily recognized. 



The figure represents a leaf which has been entered in several 

 places by the fungus, causing late blight or potato rot. The growth 

 of the parasite has been rapid, and the illustration may be con- 

 sidered as a typical example in which the normal development of the 

 disease has taken place. The following points should be noted : 



The diseased areas are of considerable extent, and may be started 



B. M. Diiggar. 



