RUST-FUNGI. 243 



above those infected. On parts attacked saprophytically small conidia- 

 cushions break out of the bark, upon which the vermilion and dark-red 

 spore-bearers appear in autumn and winter. But the mycelium can 

 extend parasitically from wound - surfaces into living branches, soon 

 spreads quickly in the woody tissue, kills the cambium, and prevents 

 the upward flow of sap. Remedy. Infected parts should be cut off and 

 burned before the spores scatter in autumn and spring. 



The Spruce-bark Canker, Nectria curcubitula (Fig. 59), occurs chiefly 

 as a wound-parasite on young Spruce poles, also Silver Fir, Pines, and Larch, 

 and is common in Britain, though generally only as a saprophyte. On 

 young trees in vigorous growth it remains saprophytic, but in weakly 

 young trees the canker extends, kills the cambium, and penetrates the 

 sapwood. The first signs of the disease are bleaching of the needles, and 

 drying and browning of the bark and cambium, especially near wounds 

 caused by insects, &c. The mycelium spreads quickly in the bark during 

 the winter. Infected parts should be cut and burned in autumn or early 

 winter, before the spores ripen and are shed. 



The Maple and Sycamore leaf-blotch, Rhytisma acerinum, appears in 

 damp summer weather as small round yellow spots, about to \ in. broad 

 on the lower side of leaves, that gradually enlarge and turn jet-black in 

 autumn. The spores produced in these black sporophores are scattered 

 from the dead leaves in the following May or June, and are borne by 

 wind to the new foliage, which they attack in the same way as before. 

 It can only be checked by collecting and burning the infected dead leaves 

 in autumn in parks and gardens. It is not a serious disease in woods. 



The Rust -fungi (Uredinece), so called from their sporophores often 

 assuming a reddish-yellow rusty colour, are all parasitic, and mostly have 

 a change of generation with some other kind of host-plant, upon which 

 they appear like entirely different diseases and have other quasi-generic and 

 specific names. Five different genera of metoxenous (heteroecious) rust- 

 fungi attack our trees, Melampsora, Melampsorella, Coleosporium, Cronar- 

 tium, and Chrysomyxa, one species of which is autoxenous (autoecious). 



In the genus Melampsora the intermediate form is called Cceoma, and 

 the chief disease of this kind is the Poplar-rust and Pine shoot-twisting 

 fungus, Melampsora pinitorqua + Cceoma pinitorquum, of which the 

 Melampsora stage, that producing resting - spores, is passed as yellow 

 patches on the foliage of Aspens and White and Grey Poplars, while the 

 much more destructive intermediate stage is the Cceoma pinitorquum (Fig. 

 60, 61), that breaks out in spring after hibernating on the dead Poplar 

 foliage, and chiefly attacks Pine plantations up to 10 years old. The 

 yellow Cceoma pustules, about 1 in. long, burst lengthways, and generally 

 kill young shoots, while older twigs get C- or S-shaped when two pustules 

 break out on the same or on different sides of a twig. The mycelium 

 can perenniate, each year forming new pustules that shrivel up in dry 



