194 PLANT DISEASES 



rather soft, and easily broken. The entire fungus is brittle, 

 has a very disagreeable smell, and is poisonous. 



In addition to the usual means of reproduction by spores, 

 two other methods of reproduction are known ; one by 

 means of sporelike bodies called conidia, which are pro- 

 duced in great abundance from the mycelium growing in 

 cavities in the wood destroyed by the fungus, or sometimes 

 in receptacles in the flesh of the fungus itself. The second 

 form of reproduction in addition to that of the spores also 

 consists of conidia produced in the interior of special 

 receptacles, which resemble the young stage of the ordinary 

 spore-bearing pileus, from which they differ in not producing 

 pores bearing basidia. These last-named structures were 

 at one time considered to be an independent fungus be- 

 longing to a genus called Ptychogaster, and the form we 

 now know as forming a link in the life-history of Polyporus 

 sulphureus was called Ptychogaster aurantiacus. 



The fungus is an annual, grows rapidly, and decays in 

 the autumn. It is a wound-parasite, gaining access to the 

 living part of the trunk by means of mycelium which 

 originates from the germination of a spore or conidium on 

 the surface of a wound, such as a broken branch, or an 

 imperfectly protected surface exposed by pruning. The 

 heart- wood is first attacked, the mycelium gradually extend- 

 ing towards the sap-wood. During decay the wood 

 changes to a clear reddish-brown colour, cracks, patches 

 of white mycelium appearing in the crevices. 



PREVENTIVE MEANS. From what has been described 

 respecting the mode of attack of the fungus, it will be seen 

 how important it is to trim the ends of broken branches, 

 and to protect cut surfaces with some fungicide. If the 

 fungus has gained a footing on the trunk, it should be 



