ix.] MILDEW OF ONIONS. 49 



months. Their mode of formation is described under 

 Peronospora infestans, Mont. 



Peronospora Schleideniana, Ung., is specially common 

 on the garden onion, Allium Cepa, L., and on the Eock, 

 Welsh onion, or stone leek, named Allium fistulosum, L. 

 The latter plant does not form a true bulb, but is sown 

 to form small green onions for spring salads. The 

 fungus attacks numerous other species of Allium, and is 

 even suspected on lilies and amaryllids. It is sometimes 

 extremely common and virulent, attacking the onions 

 early in the season, and so preventing the bulbs from 

 reaching perfection. The chief point of attack is the 

 leaves in an early stage of growth, and to such an ex- 

 tent is the work of destruction sometimes carried on that 

 the entire onion plant may be seen covered with one mass 

 of whitish-gray semi-transparent bloom. In bad cases the 

 onions are left as one offensive mass of putrescence. 



One form of mildew is caused by the attack of a 

 Fusisporium named F. atrovirens, Berk. This fungus is 

 not unlike F. Solani, Mart., already described, but the 

 spores are grayish -green or greenish-black in colour and 

 more decidedly curved. The disease begins with the 

 exhibition of grayish gelatinous spots, which speedily be- 

 come confluent. The Fusisporium often accompanies the 

 Peronospora. .Both mildews materially affect the crop of 

 seed. Practical growers say mildews are favoured by an 

 extremely dry season, as in the mildew of peas and roses, 

 and also by a wet and cold season, as well as by bad culti- 

 vation. Deep trenching is generally advocated for the 

 prevention of onion mildew. 



A good plan for the avoidance of mildew in onions is 

 to sow the seeds in autumn ; by following this plan the 

 onions are able to make good strong growths before the 

 appearance of the mildew in the following spring. This 

 is perhaps the only reliable plan for obtaining sound 

 onions of a large size in districts subject to attacks of 

 mildew. Autumn sowing is advocated by onion growers. 

 E 



