Minnesota Plant Diseases. 



leaf and these spots often occur in sufficient numbers to con- 

 siderably damage the plant. The winter spores are two-celled. 



Spraying with dilute 

 bordeaux has been recom- 

 mended just as the buds 

 are opening and the leaves 

 are expanding, and at in- 

 tervals later. The fallen 

 leaves should be burned. 



Apple scab [Vcnturia ' 

 pomi (Fr.) Whit.]. Ap- 

 ple scab is by far the most 

 serious disease of apples. 

 The fungus first appears 

 in early summer on the 

 leaves of the apple tree as 



light, greyish, Circular FIG. 1S3. Apple scab on the frait After Clii 



spots which spread rapidly, often combining with neighbor- 

 ing spots to cover large areas of the leaf. The spots later 

 _ turn olive green 



and finally black. 

 The surface is 

 covered with up- 

 right threads 

 from which the 

 s u m m e r spores 

 are thrown off. 

 These spores rap- 

 ily increase the 

 spread of the fun- 

 gus from leaf to 

 leaf and tree to 

 tree. The spots 

 are frequently so 

 large and nu- 

 merous that the 



FIG. 184. Apple scab on the fruit. After Longyear. JeaVCS b C C O m C 



considerably distorted and are often shed. Whole trees may in 

 this way be stripped of their leaves. This sometimes happens un- 



