EXPERIMENT STATION BULLETINS. 259 



best when the temperature is about 70° F. and the air is full of mois- 

 ture. It is worse in wet than in dry seasons and the general opinion 

 is that it causes more rotting of the tubers in clayey than in sandy 

 soils. The fact that the disease flourishes best in wet seasons has 

 caused many farmers to believe that '^blight is caused by wet weather". 

 This is true in part, in that the wet weather furnishes the ideal con- 

 ditions for the rapid development of the fungous; but the real cause 

 is the presence of the disease upon the plant when the wet weather 

 comes along. We can have wet weather without having blight; and, 

 vice versa, we can have blight without having wet weather. The two 

 are not inseparable. 



The disease usually appears on the lower leaves of the plant in 

 the form of a yellowish-brown spot that rapidly turns brown and 

 dies. The trouble rapidly spreads upon the plant until soon nothing 

 green remains to be seen but the extreme top leaves. These too, soon 

 succumb and the whole top dies and rots. If we examine one of these 

 yellowish-brown spots on the leaves, early in the morning when the 

 dew in on, we shall probably find a white mildew-like substance on 

 the lower side of the leaf. If this is examined with a microscope it 

 will be found to consist of a large number of egg-like spore-bodies on 

 minute stalks projecting out of the leaf tissue. (See Fig. I.) These 

 spores which correspond to seeds, soon fall off the parent stalk and 

 either fall to the ground or are blown to a neighboring plant. If the 

 spore finds lodgment on a potato leaf, and if moisture is present, it 

 starts to germinate and sends out a slender, thread-like branch (my- 

 celium). It is believed that this mycelium has no power in itself to 

 penetrate the tissues of the leaf, so it grows on until it reaches some 

 opening into the leaf. This opening may be one of the numerous 

 breathing pores, (stomata), of the leaf, or it may be a hole made by 

 some insect. On reaching such an opening the little branch enters 

 and pushes its way between the cell walls of the leaf robbing them 

 of their nourishments. Once in the plant this mycelium grows very 

 rapidly, branching again and again and finally penetrating every part 

 of the plant tissues, robbing them of their juices and causing them 

 to decay. Occasionnlly a branch of the mycelium rnmes to the surface 

 and sends out many fruiting stalks that bear myriads of new spores. 

 These mature, fall off, and go on their mission of destruction. 



If the spore falls to the ground and rains follow, it may be washed 

 downward through the soil until it finds lodgment upon the tuber. 

 Here it finds warmth and moisture and begins to grow, sending its 

 mycelium into the tuber, robbing it of its starch, breaking down its 

 tissues and causing rot. Here its action may or may not be rapid; 

 the spore may sprout, enter the tuber, and make only a slow growth, 

 not manifesting itself until the potato is dug and stored, or perhaps 

 not until the tuber has been planted for the next crop. So the dis- 

 ease may be passed on from generation to generation of potatoes by 

 means of infected seed potatoes. 



We have seen that the disease passes a greater part of its life his- 

 tory within the plant tissues where it is safe from harm. To fight 

 it successfully it must be attacked at some time when it comes to the 

 surface, during the spore stage, or when the spore is germinating. So 

 we see that any remedy used must be a preventive one. 



