OBSTRUCTIONS TO DEVELOPMENT 113 



absent from the potato crops, although in some years 

 it is not so destructive as in others. 



The disease appears during damp, muggy weather in 

 August and September. It is often noticed as small 

 brownish spots on the lower leaves, which rapidly en- 

 large. In moist weather the edges of these spots, 

 on the under surface of the leaf, appear to be covered 

 with a white downy mildew. In dry weather this 

 may be difficult to detect. Later the leaves appear 

 as though burnt, and finally the whole plant, and 

 in some cases the whole field, will become a putrid, 

 offensive mass of decaying stems and leaves. The 

 tubers may be attacked also, and rot in the field or 

 in storage. Sometimes the disease runs a very rapid 

 course, and a field will wilt down in twenty-four 

 to forty-eight hours. 



Cause. The cause is a parasitic fungus which com- 

 pletes its life history in four or five days or less. The 

 whitish mold is made up of stalks bearing branches 

 (Fig. 34). These bear spore cases (Figs. 34 and 35), 

 which break up to form spores (Fig. 35). These 

 spores send out small tubes (Figs. 35, 36, 37), which 

 enter the potato leaf through a stomata, or breathing 

 pore (Fig. 37), or penetrate the cell wall (Figs. 36, 37). 

 The tubes spread in the walls of the leaf cells (Fig. 

 34) like mushroom spawn in a mushroom bed, utiliz- 

 ing the plant-food which should go to form tubers. 

 At intervals they send out spore-bearing branches 

 through the stomata (Fig. 34), which perpetuate the 

 trouble. Unless the tubers are well covered with soil, 

 the spores may fall on the ground, and, reaching the 

 tubers, transmit the disease to them. 



