21 



in color, becoming black when older, and range in size from mere 

 specks to spots one-fourth to one-half inch in diameter. Two or 

 more spots may coalesce, forming large, irregular scabby areas. The 

 i-uptured skin of the apple usually persists around the margin of the 

 spot, leaving a light-colored ring at the border of the healthy tissue. 

 Young fruits affected with this disease may become pitted, one- 

 sided, and otherwise distorted, and in severe cases the fruit becomes 

 cracked, as shown in figure 4. The fungus may attack and destroy 

 the blossoms and even the unopened buds; the flctwer stalks may 

 become so weakened by the dis- 

 ease that the young fruit drops 

 off. The disease also appears on 

 both sides of the leaves and on 

 the leaf-stalks in the form of 

 smoky browTi patches, which be- 

 come swollen and blister-like. 

 It often causes the leaves to curl 

 more or less and results in the 

 premature shedding of the foli- 

 age. The scab first appears 

 early in the spring on the young 

 buds and unfolding leaves, and 

 new infections may continue 

 to take place throughout the 

 season. 



THE CAUSE. 



Apple scab is due to the pres- 

 ence of the fungus Venturia in- 

 sequalis (Cke.) Ader, which grows 

 beneath the cuticle of the leaves 

 and fruit, invading the superficial 

 cells with its branching threads. 

 In a short time the fungus gives 

 rise to groups of small stalks, 

 wliich break through the cuticle, or skin, and give forth numerous 

 minute olive-colored spores. These spores are blown about by the 

 wind, and it is by means of these wind-dispersed spores that the 

 infection takes place. The fungus is carried over winter in the dis- 

 eased leaves on the ground, where spores of the perfect stage are 

 produced, wliich are discharged in early spring as the young leaves 

 and fruit buds begin to open. These winter spores start the infec- 

 tion, wliich is further spread by the summer spores, soon produced 

 by the new scab spots. 



Fig. 4.— Apples badly affected with scab. 

 (Original.) 



