CHAPTER XXXV 



DETAILED ACCOUNT OF SPECIFIC PLANT DISEASES 

 (CONTINUED) 



Hemlock (Tsuga canadensis Carr) 



Heart-rot (Polyporus horealis (Wahl.), Fr.). — This bracket fungus is 

 distributed widely in North Temperate regions. As a wound para- 

 site, it occurs on hemlocks, pines and spruces, entering these trees 

 through the stubs formed by the breaking off of branches. The 

 mycelium gradually grows into the heart of the trees and from there 

 downward into the roots and upward into the tops. It advances in 

 definite directions through the wood in the form of cords, or strands, 

 which run radially, or tangentially, in the channels dissolved by the 

 action of the enzyme, which is formed by the living hyphae. The 

 wood shrinks and the mycelial strands begin to dry up, and the wood 

 is separated into cuboidal blocks marked off by the channels formed 

 by enzyme action. If the myceUum attacks the cambium, the trees 

 die. The bracket-like fruit bodies are soft and spongy and last only a 

 season. They are, according to Atkinson, lo to 20 cm. (4 to 8 inches) 

 by 6 to 15 cm. broad. Several of these sporophores may be joined 

 together. The upper surface is rough, shaggy and has a sodden ap- 

 pearance. The pores on the under side are quite regular with rounded 

 openings in some specimens, or irregular, elongated and sinuous in other 

 samples. 



HoUyhock (AUhaa rosea Cav.) (Fig. 187) 



Rust {Puccinia malvacearum, Mont.). — This fungus was introduced 

 into France about 1868 from Chili, where it is native, and in the 

 summer of 1915, the writer found it very destructive to the hollyhocks 

 in the gardens on the Island of Nantucket off the southern coast of New 

 England. It spread rapidly over Europe and came to the United 

 States in 1886 upon infected seed. The leaves are spotted with the 

 yellowish-brown sori slightly raised above the leaf surface (Fig. 72), or 

 they are found on the stem in the form of small wart-like elevations. 

 The leaves dry up, as if blighted, and during August of 1915 on Nan- 

 Si? 



