CERTAIN ELEMENTS IN NURSERY PRACTICE 207 



As yet the causal nature of raspberry yellows is unknown 

 and no satisfactory method of controlling the disease has been 

 perfected. 



RASPBERRY AND BLACKBERRY ANTHRACNOSE. The anthrac- 

 nose disease is more common on the raspberry than on the 

 blackberry. In general, black raspberries are 

 more susceptible than the red varieties. 



Description. All parts of the plant above 

 ground are affected, but the disease causes the 

 greatest damage on the canes (Fig. 219). The 

 spots on the canes are elliptical, purplish in color 

 with a grayish white center. The purplish margin 

 is slightly raised and thus the healthy and dis- 

 eased tissues are sharply separated. In advanced 

 stages the spots coalesce and the cones appear 

 blotched. The disease is caused by the fungus 

 Plectodiscella wneta. 



Control. In controlling the anthracnose dis- Anthracnose 

 ease, it is advisable to set only clean anthrac- 

 nose-free plants. Spraying with bordeaux mixture * 4-4-50 

 also tends to keep the disease in check. The first treatment 

 should be made soon after the new leaves appear in the 

 spring and three or four subsequent applications should follow 

 at intervals of about two weeks. 



BLACK-SPOT OF ROSES. The disease known as black-spot, 

 leaf-blotch or leaf-spot is one of the most destructive diseases 

 on roses. In nurseries the rose plants severely affected become 

 defoliated in summer, in many cases causing the leaf-buds, 

 which should remain dormant until the following year, to 

 open late in the season. 



Description. The symptoms of the disease are black soot- 

 like blotches on the upper surface of the leaves (Fig. 220). 



219 



