180 



LEAF AND SEEDLING DISEASES 



with a suitable seedling, it settles down and germinates with 

 a germ tube which forces its way into the seedling at some 

 point where two ej^idermal cells touch each other. The 

 germt ube then branches and quickly forms a mycelium 

 whicli maj^ spread through a considerable part of the host. 

 Sporangiophores and sporangia are produced in a few days 



and a new cycle is commenced. 

 In this way the disease maj^ 

 spread very quickly and 

 whole seed-beds may be com- 

 pletely exterminated in *a 

 very short time. 



The fungus has a special 

 means of perennation, i. e. 

 living over the winter. Certain 

 hyphae inside the seedling 

 become swollen at the ends 

 and form female organs. 

 Other hyphae by the side of 

 these become converted into 

 A fusion takes 

 a large spore, 

 called the oospore, is formed 



Fif). 73.— A, 



Sporangio])liore of 

 Phi/to phi hora omnivora : a, stalk ; h, 

 young sporangium ; c, sporangium 

 ready to fall, b, C'onidiophore of . 



Fusoma parasiticum. : a, short ^^^^^^ organs 

 hyphae bearing conidia ; b, young, place, and 

 unseptatc conidium ; c, mature 

 septate conidium. 



inside the 



female organ. 

 This oospore puts on a very thick wall, and contains 

 sufficient food material to live through the winter. In the 

 spring it divides up into a number of swarm -spores like 

 those which are set free from the sporangium. 



Attacked seedlings show dark patches, especially near 

 the surface of the ground, and quickly collapse. Remedial 

 measures will be considered after the other dam ping -off 

 fungus lias been described. 



Fusoma parasiticimi, Tub., also attacks the seedlings of 

 nearly all species of conifers. Like the last fungus, it causes 

 dark patches and the collapse of the seedlings. In damp 

 weather a greyish mycelium appears on the outside of 

 attacked parts, and this mycelium bears slightly curved 

 septate conidia (fig. 73, b). These are genuine conidia, and 



I 



