ONION MILUEW 75 



ONION MILDEW 



I^Perenospora schleideni, Ung.) 



This well-known and destructive parasite occurs wher- 

 ever the onion is cultivated. The first symptom of its 

 presence is a small yellowish patch on one or more of 

 the leaves. These discoloured patches are soon covered 

 with a delicate white bloom, looking like hoar-frost, which 

 quickly changes to a greyish-lilac colour. In the mean- 

 time, the diseased patches extend until the entire leaf is 

 usually affected, which ultimately dries up and falls back. 

 A long neck or stalk between the bulb and the base of the 

 leaves is almost constant in diseased plants. The bulb is 

 not attacked ; but if, as is usually the case, the disease 

 appears early, the bulb remains very small. 



If a small fragment of the delicate mildew from the 

 surface of a leaf is examined under the microscope, it will 

 be seen to resemble a miniature forest ; numerous branches 

 of the fungus spring from the mycelium present in the 

 tissues of the leaf, and push into the open air through the 

 stomata or pores of the leaf for the purpose of producing 

 spores. Each branch of the fungus, when it has passed 

 outside the leaf, divides into numerous branchlets, each of 

 which bears a spore, or reproductive body, at its tip. These 

 minute spores, which are produced in immense numbers, 

 are carried by wind or rain on to the surface of neigh- 

 bouring healthy leaves, where they germinate at once, enter 

 the tissues of the leaf, and give origin to a mycelium that 

 soon produces fruiting branches, which grow through the 

 stomata of the leaf and form their spores in the air, which 

 serve in turn for the infection of other plants. 



A second mode of reproduction is possessed by the 



