368 FIELD CROP PRODUCTION 



ing chickens to run on a newly plowed field, are means used to 

 control them. 



DISEASES 



394. Early Blight (Alternaria solani). This is an old dis- 

 ease which has probably been more or less destructive for many 

 years. It begins to make its appearance on the plants about the 

 time they are in blossom or sometimes when the plants are quite 

 small. It is distinguished by small round grayish patches which 

 later turn brown. The spots increase in size and many of them 

 may coalesce and form large irregular patches. The punctures 

 made by the flea beetles and other insects seem to favor the 

 entrance of the fungus into the tissues of the plant. While the 

 disease is not followed by the rotting of the tubers, it decreases the 

 yield greatly. Some seasons it lessens the yield by many millions 

 of bushels. 



395. Late Blight (Phytophthora infestans). This disease has 

 proved to be one of the most destructive and widespread of all 

 diseases of the potato. It is found in all countries of the world 

 where the potato is grown. The disease appears as an irregular 

 dead area on any portion of the leaf, but commonly it first appears 

 at the tip or on the margins of the leaf. There is often a notice- 

 able and offensive odor from a field affected with the blight, 

 especially if the air is filled with moisture. In order to control 

 the disease the plant should be sprayed thoroughly with Bor- 

 deaux mixture. 1 Spraying should be begun when the plants are 

 six to eight inches high and repeated every ten days to two weeks 

 throughout the season, making in all about five applications. 



396. Scab (Oospora scabies). This disease is quite com- 

 mon in all potato growing sections. The irregular, cankered- 

 looking spots which may more or less cover the tubers are famil- 

 liar to every one who has ever grown potatoes. This disease is 

 known to live over in the soil for several years and a change of 

 soil is necessary ; no potato crop following for four or five years 

 would be advisable. It is not difficult to keep this trouble in 

 check if precautions regarding seed and rotation are taken each 

 year. The seed should be treated before planting, with one pint 



1 This spray is prepared by dissolving 4 Ibs. copper sulphate and 4 Ibs. 

 stone lime and adding water to make 50 gal. 



