748 ANNUAL. REPORT OF THE Off. Doc. 



days, with occasional showers, and a close damp air are especially 

 favorable to its growth; aud if such periods occur during August 

 and September, the disease may appear at any time. But, on the con- 

 trary, if the season is dry and hot the fungus is unable to develop, 

 and little or no injury of this nature cau 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 altitude. In such places 

 the fungus may develop regularly every year, and the severity of the 

 attack will be modified chiefly by abnormal atmospheric condi- 

 tions. 



"The manner in which the germ tube of a spore penetrates the tis- 

 sues is interesting. It is now generally believed tlhat 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 rap- 

 idly- extends its thread-like growtih between neighboring cells, draw- 

 ing its nourishment by means of minute suckers which penetrate 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 tis- 

 sues depends very largely upon external conditions, and the appear- 

 ances of the affected parts is also modified to a very considerable ex- 

 tent. Unfavorable conditions frequently render the identification 

 of the parasite a different 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 charac- 

 tertistic symptoms may be easily recognized. 



"The following points should be noted: The diseased areas are of 

 considerable extent, and may be started in any part of the leaf, but 

 the edges appear to suffer more from new infection than the more 

 central portions of the leaflets. This is probably due to the fact that 

 in the case of rains these portions remain moist for a longer period 

 than the center, since the water drains to the lower parts of the leaf- 

 lets, and collect there in the form of drops of greater or less size. 

 It is to be expected that under such conditions a fungus could gain 

 an entrance more easily than in drier places. The decayed por- 

 tions are inclined to droop; this is especially true in cases of rapid in- 

 vasions, for at such times the parts do not dry so fast as the parasite 

 adivances. The rapid decay also prevents the edges of the leaflets 

 from curling, although this takes place when the air becomes warm 

 and dry. 



"The distribution of colors over the affected leaf is very suggestive. 



