i6o 



DISKASKS <)!' PLANTS 



The fungus. The niycvlium of this fungus is typical of the 

 i'umily. The haustoria have very much the form of those pre- 

 viously described for the grape mycelium. The sporophores arise 



through' the stomates, 

 singly or in small clus- 

 ters, and they are 

 considerably branched, 

 somewhat more flexu- 

 ous than those of the 

 grape mildew, and the 

 fruiting tips are less 

 rigid and more widely 

 separated one from 

 H another, corresponding 

 more nearly to separate 

 branches. The spores 

 or zoosporangia are 

 slightly violet tinted in 

 mass and generally 

 ovate in outline. The 

 germination of the spore 

 has been followed care- 

 fully and is known to 

 take place by means of 

 the characteristic motile 

 zoospores. Clinton 

 alone has illustrated the 

 various stages of germi- 

 nation (Fig. 54, c and d\ 

 FIG. 55- PERONOSPORA ON YOUNG CABBAGE LEAK Occasionally the normal 



sporophores are accompanied by short, swollen hyphae (Fig. 54, /;), 

 which may also bear spores. It is believed that the latter are pro- 

 duced under unfavorable conditions, and similar modifications have 

 been noted in other specie's. The oosporic form of this species has 

 not yet been found, and it is doubtful if it occurs in this country. 



Control. Very careful experiments have been made with a view 

 to holding in check the ravages of this fungus, and it has been 

 found that the greater part of the damage can be prevented by 



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