THE DOWNY MILDEWS 



221 



the stalk to the forming tuber below. Wherever they go, 

 they cause the death of the cells of living tissue, breaking 

 them up, turning them brown, and causing a foul odor to 

 be given off. In the case of the stalks and tubers the 

 result appears as a sort of rot. 



The prosperous, healthy plant is thus stricken with a 

 sudden sickness that may well be called a blight. The 

 tubers are no longer able to increase 

 in size through the addition of starch 

 cells. A field thus blighted is a 

 sight that may well bring dismay to 

 the owner who had hoped for a 

 bountiful harvest. 



But the parasite is not yet done 

 with its stricken host. All this 

 growth of mycelium has been but 

 a preparation for the development 

 of the reproductive spores, just as 

 the growth of the mushroom myce- 

 lium beneath the soil surface is but a 

 preparation for the sending up of 

 the spore-bearing mushroom. The mycelium in the leaves 

 sends out to the surface vertical branches that bear upon 

 their tips the tiny summer spores. These mature quickly 

 and are readily carried away by the slightest breeze. Thus 

 the disease is able to spread rapidly by means of the bil- 

 lions of spores produced. This development of summer 

 spores begins soon after the mycelium gets well started, 

 often- before the whole leaf is affected. Many of these 

 spores are likely to be washed through the soil till they 

 reach the tubers, and they may develop there, causing a 

 characteristic brown discoloration. 



This fungus commonly passes the winter by means of 



SPORES ON LEAF. Magni- 

 fied : a, spore ; b, spore 

 germinating; c, spore 

 with germinating tube 

 entering breathing pore 



