48 DISEASES OF CROPS. 



make their way into the stems and roots of the parsnip 

 plants. Thehyphse, or threads of the mycelium, are stout, 

 and are provided with numerous suckers, which cause 

 putrefaction to occur within the tissu.6S of the host-plant. 

 When the roots are attacked, they become spotted (Fig. 

 19 A), and finally rotten. The mycelium sends out coni- 

 diophores (Fig. 19 B), bearing conidia, through the stomata 

 of the leaves, and thereby interferes with the processes of 

 transpiration and assimilation. The protoplasmic contents 

 of the oval-shaped conidia (Fig. 19 C) divide into a number 

 of portions, each of which produces a zoospore (Fig. 19 D 

 and E) provided with cilia (filaments), which enable them 

 to be carried about by aerial currents. The zoospores re- 

 produce mycelia. 



Oospores (resting-spores) are produced, during the life- 

 history of Peronospora nivea, within the stems and roots 

 of the host-plant. This parasite of parsnips also attacks 

 parsley and other plants. 



Prevention. — (1) Burn all infested refuse from previous 

 parsnip crops. (2) Destroy such weeds as cow parsnip, 

 goutweed, wild chervil, and wild angelica. 



Cure. — See the treatment of the potato disease caused 

 by Peronospora infestans. 



(6) The Parasites of Potatoes {Solanum tuberosum). 



The Colorado Beetle {Leptinotarsa decemlineata, 

 Fig. 20) belongs to the Clirysomelidce, and is about half 

 an inch long, with wing-cases marked alternately in stripes 

 of black and yellow. The thorax is also yellow with a 

 black V-shaped mark in the centre, surrounded by several 

 smaller black marks. If the elytra (wing-cases) are opened, 

 the bright red wings will be seen ; this is one of the pecu- 

 liarities of this insect. 



