282 PLANT DISEASES 



Strawberry. Leaf-spot or leaf-blight (Mycosphcerella fragarioe). — 

 Small purple or red spots ajipearing on the leaves. They increase 

 in size and make the leaf appear blotched. The fungus passes 

 the winter in the old diseased leaves that fall to the ground. 



Control. — In setting new plantations remove all diseased 

 leaves from the plants before they are taken to the field. Soon 

 after growth begins, spray the plants with bordeaux mixture, 

 4_4-50. Make three or four additional sprayings during the 

 season. The following spring spray just before blossoming, and 

 again in ten to fourteen days. If the bed is to be fruited again, 

 mow the plants and bum over the bed as soon as the crop is off. 

 Mildew (SphcFrothcca cuMagnei). — A whitish cobweb-like mildew 

 on fruit and leaves, causing the latter to curl. 



Control. — Spraying as for leaf -spot ; dusting with sulfur flour. 

 Sweet-potato. Black-rot {Ceratocystis fimbriata) . — Causing black 

 shank of the plant and a black rot of the tuber. The spots on 

 the tuber are greenish black, from a quarter of an inch to four 

 inches in diameter and extending for some distance into the 

 tissue. 



Control. — Never use affected potatoes from which to grow 

 sprouts. Steam-sterilize the soil in the hotbed. Practice rotation. 

 Rots. — The sweet-potato is susceptible to a large number of rots, 

 soft, dry, hard, white, etc. In practically^ all cases the organism 

 producing the disease is an inhabitant of the soil. The best 

 method of preventing these diseases is to use perfectly sound 

 potatoes for sprouts and plant on soil which has not grown sweet- 

 potatoes for several years. 

 Tobacco. Root-rot {Thielavia hasicola). — A rot of the main root 

 and dwarfs the plants. Occurs both in seed-bed and field. 



Control. — Steam-sterilize the seed-bed by the inverted pan 

 method. (Soe discussion on p. 253.) Rotate crops. Avoid lim- 

 ing, and add acid fertilizers. 

 Wilt {Bacterium solanacearum). — A wilt of the plants caused by 

 bacteria. 



Control. — Very difTicult to control, as the organism lives in the 

 soil for years. Never jilant on land known to be diseased. Do 

 not cultivate related plants, as potato, tomato, egg plant, or pepper, 

 on the same soil. Transplant early, and avoid breaking the 



