44 DISEASES OF CROPS. 



putrefaction among the microscopic cells of the leaves, 

 etc., as well as greatly interfering with the processes of 

 transpiration and assimilation. The onion ■ mildew pro- 

 duces moist, greyish spots on the leaves of various species 

 of Allium, or, in other words, causes, by contact, the 

 complete decomposition of the leaves. 



The mycelium (Fig. 17 B) ramifies amongst the living 

 cells of the leaves. It produces conidiophores (Fig. 17 A) 

 which bear aerial conidia (spores) Oospores or resting- 

 spores are produced by sexual reproduction, i.e., by the 

 union of the antheridium with the oogonium, similar to 

 those already described in connection with the clover 

 mildew (see chap. ii.). The oospores are found in de- 

 caying onions. 



Prevention. — (1) Burn all decomposing onion refuse. 

 (2) Good cultivation, with the seeds sown in the autumn, 

 produces strong healthy plants capable of withstanding 

 the attacks of this fungus in the following spring. (3) 

 Deep trenching. 



Ctt7'e.— Manure the land, when the plants are a few 

 inches above ground, with half-cwt. of iron sulphate per 

 acre. The sulphate may be mixed with five to ten times 

 its weight of sand or dry earth, in order to obtain regular 

 distribution. 



The Smut of Onions {Urocystis cepulce) is allied to 

 the " smuts," etc., of corn, potatoes, and other plants. It 

 is more common in the United States of America, and in 

 France, than in this country. It produces amongst the 

 onions a blackish-brown dust after the harvest. 



The Onion Mould {Mucor subtilissimus) grows on 

 Alliumcepa, and possibly on allied plants. Another species 

 of the Mucorini is the mould of preserved fruits etc. 



The onion mucor produces a very fine mycelium which 



