DISEASES OF CROPS 219 



parsnips, onions, celery, beets and many other plants. The only satis- 

 factory treatment lies in the rotation of crops. 



PARSNIP 

 Blight (Cercospora apii, Fr.). See Celery. 



POTATO 



Late Blight or Downy Mildew (Phytopthora infestans [Mont.], De 

 By.). This disease usually starts near the tip or margin of the leaf, but 

 causes the infected area to die and blacken. In cool, wet, cloudy weather 

 it spreads very rapidly and causes an offensive odor. The diseased tubers 

 may show slightly depressed, dark-colored areas and a dirty brown color 

 within. The disease is frequently the cause of heavy losses by rotting. 



Treatment. Spray with Bordeaux mixture, beginning when the plants 

 are about six inches in height and repeat about every two or three weeks 

 throughout the growing season. 



Early Blight (AUernaria solani [E. and M.], J. and G.). This disease 

 appears earlier in the season than the late blight. It causes brown, brittle, 

 irregular, more or less circular leaf spots with rather definite concentric 

 circles. These spots frequently unite and the plant dies very much as 

 though from natural causes. 



Treatment. Same as for late blight. 



Wilt, Stem Rot and Dry Rot (Fusarium oxysporum, Schlecht). The 

 plant assumes an unhealthy appearance, the leaves roll and curl and the 

 plant falls and dies prematurely. The stems are partly or entirely black and 

 dead near the base and frequently show a white or pink mould. When 

 stems are cut across below the ground they show discolorations just below 

 the surface. This field form of the disease is known as " wilt " or " stem rot." 



In storage the tubers undergo a "dry rot" beginning at the stem end, 

 which causes them to shrivel and become light in weight. When cut 

 across, these tubers show black discolorations just below the surface. The 

 disease can be carried on the seed and will also persist in the soil. 



Treatment. Select seed potatoes which are free from surface cankers 

 and are perfectly white when cut. When the soil becomes infected use 

 rotation of crops for from three to five years. 



Black Leg (Bacillus phytophthorus, Appel). This disease causes the 

 plants to be dwarfed, erect, pale in color and to die early. The stems 

 become brown or black near the ground and the disease works downward. 

 It is carried in the seed. 



Treatment. Soak the seed in formaldehyde or corrosive sublimate as 

 recommended for potato scab. 



Scab (Oospora scabies, Thaxt.). This well-known disease is readily 

 recognized by the rough, pitted character of the tubers and is the cause of 

 heavy losses. It can be carried on the seed and will persist in the soil for 

 several years. 



