236 VEGETABLE FOES AND DISEASES 



thicker wall than the conidia, are spherical rather than 

 oval, and do not germinate until the spring following 

 their production, when they inoculate the young Onions 

 and start the disease anew. 



Prevention. The collecting and burning of all 

 diseased tops in which the resting spores may pass the 

 winter as, if left on the ground they rot, and are 

 set free in the soil is imperative, for the resting spores 

 retain their vitality for at least two years, hence Onions 

 should not be grown on the same land more than once 

 in three years. Where this is attended to the Onion 

 crop cannot be affected as regards infection from the 

 land. 



Crops grown on low damp ground are more subject 

 to attack than those in higher, drier and more open 

 situations. 



As the disease appears early, it has been advised to 

 sow the seed in autumn so that the Onions are able to 

 make a good strong growth before the appearance of 

 the mildew in the spring following. This plan, how- 

 ever, does not secure absolute immunity from the 

 disease ; besides, the bulbs of autumn-sown Onions do 

 not keep through the winter nearly so well as spring- 

 sown, and we have known autumn-sown Onion bulbs 

 collapse entirely from mildew when they should have 

 been maturing. 



In the early stages of the disease it may be checked 

 by dusting with a mixture of two parts quick lime and 

 one part flowers of sulphur, applying by means of a 

 bellows apparatus when the plants are damp. For 



