1919.'] THE FUNGOUS DISEASES OF BOSES. 81 



RED RUST. 



The Red Rust of roses is caused hy the parasitic alga Cephaleuros 

 virescens which attacks a wide variety of plants in the tropics. This 

 disease is met with on roses only in fairly damp situations, but in such 

 places does an imaiense amount of damage, killing the plants right 

 down to the ground. 



The disease begins as a very small black or purplish spot on the 

 stem, which increases very rapidly in size until eventually the whole 

 stem, from top to bottom, becomes almost black and the bark cracks 

 slightly. Very soon, owing to the development of the fruiting stalks 

 of the alga, the affected parts assume a reddish velvety appearance, 

 and when examined under a hand lens of low power, it will be seen 

 that they are covered with minute red stalks standing very close 

 together, and each surmounted by a little round red ball. Though 

 this alga does not penetrate very deep into the bark, it kills the outer 

 layers vcrj- quickly'. 



Treatment. 



Where plants are badly affected with this disease, the only thing to 

 do is to cut them back very severely and, as they make new growth, 

 spray continually with Bordeaux mixture. In damp situations, perhaps 

 fifteen or twenty applications a year would be • necessary to protect 

 the plants from this trouble 



The alga causing this disease must not be confused with the 

 so-called " Red-headed " fungus, which attacks scale insects, and 

 which is very often found on rose plants in damp situations. The 

 Red-headed fungus is much brighter in colour than the alga and does 

 not cover the whole plant, but if examined closely, it will be seen 

 that it is growing out from scale insects. 



This is a beneficial fungus, as it kills the scale insects, but can only 

 develop abundantly in damp situations, and would be considered on 

 the whole a successful control for rose scales. 



■ ROSE CANKER. 



Another fungous disease of roses, which has been found occasionally, 

 may be called " Rose Canker." 



This disease is caused by a species of Stilbum. Small purplish 

 spots appear on the rose stems which gradually increase in size, 

 sometimes even girdling the branch. The bark becomes shrivelled and 

 sunken, and in the later stages is covered with bright red stalks, 

 surmounted by red globules. This disease can be controlled by cutting 

 out and burning the diseased stems. 



If one watches the rose plants carefully, it will be seen that certain 

 varieties or individual plants are immune or partially immune to some 

 of the fungous diseases. An effort should always be made to select 

 resistant varieties and propagate from resistant plants, wherever 

 jjossible. 



