CHAPTER IX. 



MILDEW OF ONIONS. 



Peronospora Schleideniana, Ung. 



ONE of the best known fungus pests of Onions is the 

 dreaded onion mildew caused by the fungus named 

 Peronospora Schleideniana, Ung. As its name indicates, 

 it is a close ally of the fungi of clover mildew, Perono- 

 spora trifoliorum, D.By., and P. exigua, W.Sm., already 

 described, and of the fungus of the potato disease, Perono- 

 spora infestans, Mont. 



Peronospora Schleideniana, Ung. , is named in honour of 

 the botanist Schleiden. The fungus is illustrated, enlarged 

 200 diameters, at Fig. 15 ; it is shown as growing from 

 the base of a leaf near the collar of the bulb ; at this 

 position there is but little leaf -green or chlorophyll in 

 the cells of the leaves. The spawn or mycelium of the 

 fungus ramifies amongst the loose cells of the leaf and 

 sets up decomposition in its progress. At Fig. 1 5, A, the 

 threads of the fungus may be seen emerging through an 

 organ of transpiration into the air. The minute open- 

 ings, " mouths," organs of transpiration or stomata, occur 

 abundantly on most plants, generally on the under 

 surface of the leaves, but also in various other positions. 

 The general habit of the different members of the genus 

 Peronospora is to grow within the leaves and stems, and 

 send their fruiting branches through the stomata into the 

 air. This habit is fatal to the growth of the host plant, 

 for the spawn not only causes putrefaction of the inner 

 cells of the leaf and stem by contact, but the fertile 

 threads choke up the organs of transpiration and prevent 



