14 PLANT DISEASES 



they produce some form of reproductive body which 

 inoculates the young host-plant. In many instances 

 sclerotia are formed in living parts of the host, especially 

 in bulbs, where they remain latent until the bulb pro- 

 duces a new growth, which is infected in due course. 



From the above account it will be seen that sclerotia 

 perform the same function as winter-spores, remaining 

 passive during the resting condition of the host, and pro- 

 ducing fruit w^ien the latter commences a new growth. 



How Parasitic Fungi Infect their Victims. — The 

 conidia or spores of numerous parasitic fungi, carried by 

 wind, rain, or other agents, are deposited on the leaves, 

 fruit, or other parts of the host-plant, germinate at once, 

 and enter the tissues. This may be considered as the most 

 direct and general method of infection. 



Armillaria mellea, an exceedingly common gill-bearing 

 fungus, or ' toadstool,' very destructive to forest and 

 orchard trees, illustrates a second mode of attack. 



The fungus often lives as a saprophyte, growing in 

 dense clusters on decaying stumps, etc. Its mycelium 

 forms thin, cordlike strands which radiate in all directions 

 in the soil ; and if one of these strands of mycelium comes 

 in contact with the living rootlets of a tree, its tissues are 

 attacked, the mycelium of the fungus enters the rootlet, 

 adopts a parasitic mode of life, spreading further and 

 further into the tissues, until finally the whole root-system 

 of the living tree is enveloped in a white, felty mycelium, 

 which gradually extends up the trunk, ending in the death 

 of the tree. 



In the meantime, numerous strands of mycelium origin- 

 ating from the diseased tree are traversing the soil in 

 every direction in search of other victims, and by this 



