VEGETABLE AND FIELD CROPS 



251 



ticularly destructive in New York, causing a loss of half 

 the yield, or more in some instances. 



The attack in the field occurs first 

 upon a few plants during warm, damp 

 weather. Examined while the dew is 

 still present, these plants, seen from 

 considerable distance, display a pecul- 

 iar violet tint especially easy to discern 

 through the dewy covering. Close 

 examination shows the affected part 

 to be covered with a furry coating. 

 On the second day these parts lose 

 their green color, turn pale, or even 

 yellow. On the third or fourth day 

 the plant is entirely collapsed. 



The diseased spots in the field may 

 each cover an area of only a few feet 

 in diameter, enlarging slowly. If the 

 weather favors the fungus, the disease 

 may sweep rapidly across the whole 

 field. Affected plants, under suitable 

 weather conditions, brilUant, dry at- 

 mosphere, slowly recover, putting out 

 new leaves to replace the dead and 

 lost. Subsequent attacks may occur 

 if the weather favors the growth of 

 the fungus. The injury from this dis- 

 ease is through the loss of leaf surface, 

 thus reducing the growing power of the plant. Usually 

 little or no increase in the size of the bulb occurs after a 

 severe attack. 



Fig. 112. — Onion plant 

 illustrating manner of 

 death of leaves affected 

 with downy mildew. 

 After Whetzel. 



