APPLE DISEASES 



439 



when first observed are circular and 

 about one-half inch in diameter and are 

 not sunken. If one cuts into the bark be- 

 neath one of these spots the tissues are 

 found to be discolored and to present a 

 water soaked appearance. The discolored 

 area is found soon to extend to the cam- 

 bium, where it spreads out and may in 

 some stages of the development of the 

 cankers be more extensive in that region 

 than the discolored area on the surface 

 would indicate. The spots develop very 

 slowly, if at all, during the winter months, 

 but spread quite rapidly during March 

 and April. As they enlarge they gradu- 

 ally become elliptical in outline, the sur- 

 face becomes flat, then slowly sinks. 



Sometimes the bark of developing cank- 

 ers shows bands of slightly varying color, 

 giving the appearance of concentric zones. 

 This is doubtless due to alternating 



periods of rapid and slow growth induced 

 by variations in the weather conditions. 

 In early stages of rapidly growing cank- 

 ers an irregular crack may appear in the 

 bark and drops of fermenting sap may 

 exude. 



In the early spring, when the warm 

 weather induces renewed activity in the 

 tree, the cankers develop rapidly. The 

 bark soon dries and sinks and this con- 

 dition together with the normal growth in 

 the surrounding healthy bark produces 

 a tension which results in the forma- 

 tion of a crack in the bark at the edge 

 of the canker. The spread of the fungus 

 ceases as soon as the cambium becomes 

 active in the spring. 



After the cankers reach the full size, 

 which occurs in early May, further ac- 

 tivity is confined to changes which take 

 place in the dead bark within the cank- 



Fig. 1. a, Canker of Anthracuuse (in Apple Branch. Note pustule of fungus in bark of 

 canker, b. Canker Two Years Old Showing Fruiting Bodies of tlie Perfect Stage. 



