204 



Diseases of Economic Plants 



in this country, in Europe, and in Bermuda. In the latter 

 place it was so injurious that the legislature applied to the 

 mother country for investigation and 



)i aid. The first reference to it in the 



I , United States was in 1872, It is now 

 1 / reported from coast to coast. 

 B / The blight was described fully by 



Thaxter in 1889, again by Whetzel in 

 1904, during which latter year it was 

 particularly destructive in New York, 

 causing a loss of half the yield, even 

 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 peculiar 

 violet tint especially easy to discern 

 through the dewy covering. Close ex- 

 amination shows the affected part to 

 be covered with a fuzzy 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 rajiidly across the whole 

 field. Affected plants, under suitable 

 weather conditions, brilliant, dry atmo- 

 sphere, recover slowly, putting out 

 new leaves. Subsequent attacks may occur if the weather 

 favors the growth of the fungus. The injury from 

 this disease is through the loss of leaf surface, thus re- 

 ducing the growing power of the plant. Usually little or 



Fig. 108. — Onion 

 plant illustrating 

 manner of death 

 of leaves affected 

 with downy-mil- 

 dew. After Whet- 

 zel. 



