CHAPTER XXVL 



DISEASES OP GEBENHOUSE PLAlirTS. 

 FUKGOtrS DISEASES OF THE EOSE. — ^BLACK SPOT. 



{Actinonema rosm, Fr.). 



This disease, which is the cause of the black spots 

 that are so commonly seen upon the leaves of moss and 

 hybrid roses in wet seasons, frequently invades the green- 

 house and causes the leaves of the tea roses to take on an 

 unhealthy appearance and finally to drop from the stems. 

 Its development here seems to be invited by the same 

 conditions as in the open ground. If the bed is poorly 

 drained, or has been over-watered, a drop 

 in the temperature below 60 degrees is 

 likely to cause the fungus to appear. The 

 "spot" when first seen is of a dark brown 

 -color, with an irregular margin (Fig. 

 112); it rapidly enlarges and in a short 

 time the portion of the leaf around the 

 spot takes on a sickly yellow color and 

 the leaf drops. A magnified section of 

 the leaf is seen in Fig. 113. The dark 

 bodies (A) are the outer layer of epider- 

 RosE SPOT. mal cells, the contents of which have been 

 changed by the fungus into a dark brown granular sub- 

 stance, which can be seen through the transparent 

 cuticle of the leaf, and gives it a brown or black ap- 

 pearance. The mycelium also penetrates the underlying 

 cells and draws its nourishment from them, thus break- 

 ing down the tissues and causing the surrounding por- 



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