FUNGI AFFECTING THE ONION 



The Onion Mildew 



Peronospora schleideniana 



This disease appears upon the tops of onions as a 

 grayish, moldlike, yels^ety coating, which is followed by 

 more or less wilting of the affected leaves (Fig. 65). 



Seed onions seem to be the most liable 

 to injnry, and crops grown on low, damp 

 ground, are more subject to attack than 

 those in higher, drier situations. The 

 fungus has been known in Europe as a 

 serious enemy, for many years, but has 

 only been recognized in America for 

 about a decade. The threads of the fun- 

 gus push about between the tissue cells 

 of the host, destroying their contents 

 and causing the collapse of the leaves. Certain branches 

 are sent out through the breathing pores of the leaf, on 

 which the spores are developed, as shown in Fig. 66, 

 The spores are pointed-oval in shajDe, and have a faint 

 purplish tinge ; they are easily detached, and wafted or 

 washed from plant to plant by wind or water. Under 

 favorable conditions they quickly germinate, entering 

 the tissues through the breathing pores by means of a 

 short germ-tube. Thus the fungus may spread rapidly. 

 Besides these conidia, or summer spores, the fungus 

 develops certain winter spores, called oospores, by 

 means of which it passes the winter. These are pro- 

 duced within the tissues, of the host-plant, and carry 



150 



FIG. 65. OXIOX 

 LEAF AFFECTED 

 WITH MILDEW 



