352 



Minnesota Plant Diseases. 



cler conditions favorable to the fungus and such conditions are 

 realized in cold damp summers. The shedding of the leaves, of 

 course, impoverishes the tree not only for one sum- 

 mer, but may weaken it for several successive years. 

 The fungus also attacks the fruit and forms here 

 even more characteristic spots than on the leaf. The 

 fruit spots are dark brown to black, lined with a 

 whitish rim, and are scab-like in appearance. They 

 are usually not over one-half inch in diameter and 

 are more abundant toward the further end of the 

 fruit, though they may occur anywhere on the latter. 

 When abundant the scabs may deform and dwarf 

 the fruit and they always disfigure it, so that its 

 market value is lowered. When the young fruit is 

 seriously attacked the whole fruit may fall. In ad- 

 dition to these injuries, the attacked portions of the 

 fruit become hard and often crack open, allowing 

 the apple to dry out. The cracks also open the way 

 to the soft rots, which soon destroy the apple. The 

 fungus lives through the winter in the sac-spore cap- 

 sule stage. The sacs each enclose eight spores, and 

 are contained in a pored capsule which is formed in 

 the tissues of the apple plant and bursts out at the 

 surface at maturity, ejecting its spores through a 

 pore opening to the exterior. These sac spores are 

 thrown out in the spring, are carried to the lower 

 branches of adjacent trees and here cause the first 

 infection in the spring. 

 In combating this disease a number of recommendations have 

 been made. The disease can be very successfully fought by 

 means of spraying with bordeaux. Several sprayings are usu- 

 ally necessary. A winter spray, with strong copper sulphate, be- 

 fore the buds open should be applied. This should be followed 

 by bordeaux just before blossoming and again just after blos- 

 soming, and two or three other sprayings at intervals of two or 

 three weeks. The number of sprayings must be governed by 

 the amount of rainfall and coolness of the season. Good ventila- 

 tion and spacing of trees and proper pruning will aid in avoid- 

 ing conditions favorable to the fungus growth. The fallen 



FIG. 185. Ap- 

 ple scab on a 

 twig. After 



Clinton. 



