284 CULTURE OF IRISH POTATOES 



that one spraying a week should be given. It is best to spray just 

 after a rainy period rather than before, in order to cover the new 

 growth. 



Stimulating Effect of Bordeaux. Bordeaux mixture has a de- 

 cided stimulating effect on potato foliage, and it has been frequently 

 observed to increase yield even when no disease appeared. The in- 

 creased yield usually pays the cost of spraying, even without blight. 



Insects and Insecticides. For the potato beetle and leaf -eating 

 insects it is customary to use an arsenic poison, either as Paris green 

 or lead arsenate. When spraying with Bordeaux for blight, the 

 poison may be added to the Bordeaux mixture at the rate of one- 

 half to one pound of Paris green per 50 gallons, or 3 to 5 pounds of 

 arsenate of lead. As Paris green is sometimes injurious to foliage, 

 being slightly soluble in water, it is generally preferred to use arsen- 

 ate of lead, which is absolutely insoluble. 



For spraying poison without Bordeaux, it is mixed at the same 

 rate with water. 



Potato Improvement and Breeding. Very few varieties of 

 potatoes popular 30 to 40 years ago are in use to-day. Old varieties 

 are constantly replaced with new and more vigorous sorts, or im- 

 proved strains of the old. 



Origin of New Varieties. New varieties originate in three 

 ways: (1) by growing new plants from seed balls; (2) by sports or 

 mutations, thus at once producing a new variety by bud variation ; 

 (3) by systematic selection, taking advantage of small variations. 



Potatoes from Seeds. Seed balls may be produced naturally, 

 or may be the result of cross fertilization, thus producing hybrids. 

 However, many cultivated varieties rarely produce seed balls. 



If the seed is planted, rather small plants are grown the first 

 year. Each plant will, on the average, produce two or three small 

 tubers, usually not larger than marbles, but occasionally full sized 

 tubers. 



The first crop from seed is usually quite variable and permits of 

 considerable selection, and many new varieties have been produced in 

 this way. The second or third year the tubers reach full size and 

 usually remain quite true to type. 



Sports or Mutations. Sports, now called mutations, are new 



