ONION MILDEW 77 



these resting -spores completely decay, but the spores 

 remain unchanged in the soil until the return of spring. 



Preventive Means. — In the early stage of the disease 

 its spread may be checked by dusting with powdered quick- 

 lime and sulphur, using twice as much lime as sulphur. 

 This is best applied with bellows, and when the plants are 

 damp. Spraying with sulphide of potassium — half an ounce 

 to a gallon of water — will also answer the purpose. 



Prevention rather than cure, however, should be the 

 constant aim, and this can be assured if the known means 

 for securing it are systematically and thoroughly carried 

 out. 



It has already been stated that the first appearance of 

 the disease in the spring depends on the presence of 

 resting-spores of the fungus, which are produced in the 

 leaves previously killed by the summer form of the 

 fungus. Therefore, relative immunity from disease depends 

 on the amount of care exercised in collecting and burning 

 all such diseased leaves. If allowed to lie on the ground 

 and rot, the resting-spores are set free in the soil ; and as 

 they retain their vitality for at least tw^o years, there is a 

 constant and practically a certain danger of the disease 

 breaking out afresh. This may appear a tedious, and some 

 will say impracticable, method of preventing the disease ; 

 nevertheless it is certain. If practicable, onions should 

 not be grown on the same land more than once in three 

 years, as at the expiration of this period all resting-spores 

 present in the soil are dead. Damp and shaded situations 

 favour the spread of the summer, or mildew, form of the 

 disease. 



Prillieux, Ma lad. des Plantes Agric, vol. i. p. 143, figs. 



