ONION MILDEW 235 



ONION MILDEW (Peronosfora schleideni). . 



This destructive disease appears upon the tops of 

 Onions, at first as a small yellowish patch on one or 

 more of the leaves, and on these discoloured patches 

 presently appears a delicate white coating, not unlike 

 hoar-frost, which speedily changes to a greyish-lilac 

 colour. The diseased patches usually extend until the 

 entire leaf or leaves are affected and ultimately the 

 diseased leaf or leaves fall back and dries up. The 

 disease usually appears early and does not attack the 

 bulb, but this remains very small, and diseased plants 

 generally have a long or " thick " neck. Sometimes, 

 however, the mildew attacks the full-sized bulb, both 

 before and after it is harvested, and commences from 

 the outside. 



The threads of the fungus push about between 

 the tissue cells of the Onion leaf, 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 B, p. 237. These pointed-oval spores are 

 carried by wind or rain on to the surface of neighbour- 

 ing healthy leaves, where they germinate at once, enter 

 the tissues of the leaf, there give origin to mycelium, 

 that soon produces fruiting branches, and the spores 

 from these in turn infect other plants. 



Besides the conidia or summer spores, the fungus 

 develops resting spores or oospores within the tissues 

 of the diseased leaves. These resting spores have a 



