920 



Rural StiiooL Lkaflet 



blight rot, is observable. These spots are sometimes sunken, forming 

 little pits. Later, and often during storage, the lesion extends over the 

 surface of the tuber or two or more lesions unite to form a large, sunken, 

 discolored area. Bacteria sometimes enter the tuber through these 

 lesions and produce a soft, slimy rot with a bad odor, which is quite 

 different from the typical blight rot. 



The cause of the downy mildew. — The downy mildew of potatoes is 

 caused by one of the lower fungi, known to botanists as Phytophthora 



infestans. This fungus lives 

 over winter in the form of 

 very minute vegetative 

 root -like strands, called a 

 mycelium, within the tissues 

 of the diseased tubers. If 

 the storage room is moder- 

 ately warm and moist the 

 mycelium continues to 

 grow, obtaining food from 

 the cells of the potato and 

 extending the rotten spots 

 as it grows. 



When a diseased potato 

 is planted, the mycelium 

 produces the fruiting stalks 

 of the fungus on the cut 

 surface. These are small 

 branched stalks that extend 

 out into the spaces between 

 the soil particles and bear at their extremities a small pear-shaped spore 

 corresponding to the seed in higher plants. When the spore is fully formed 

 it is pushed to one side by another branch extending out from the end of 

 the first one and producing at its extremity another spore. In this way 

 a single stalk may produce as many as fifty spores; and ?s several hun- 

 dred stalks may be found on the cut surface of a single diseased potato, it 

 is easily seen that thousands of spores may be produced under fairly 

 warm, moist conditions. 



The spore germinates in the soil by the production of several swarm 

 spores — animal-like bodies destitute of cell walls. These move about 

 in the soil water and when the soil is saturated probably make their way 

 to the surface, where they come in contact with the lower leaves of the 

 potato plant that are often found resting on the surface of the soil after 

 a rainstorm.. The swarm spore soon germinates by sending out a slender 

 tube, called the germ tube. This tube secretes an enzyme that dissolves 



Section through a part of a diseased potato leaf show- 

 ing the spore stalks, with spores attached, extending 

 down through the breathing pores of the leaf 



