36 DISEASES OF CROPS. 



Cure. — Top-dress the plants with sulphur or a mixture 

 of lime and soot. 



The Silver Y Moth {Plusia yamma) rarely causes 

 much injury in Great Britain, although the green cater- 

 pillars of this moth cause considerable damage to the 

 leaves of sugar-beets in the northern provinces of France. 

 During 1887 the author saw several crops damaged by 

 Plusia gamma in the suburbs of Paris. 



The colour of the moth is silver-grey, with brown 

 markings. The eggs are laid in clusters on the under 

 surfaces of the leaves, where they are hatched in about a 

 fortnight. 



Prevention and Cure. — See under the heading of " The 

 Parasites of the Pea " (chap. ii.). 



The Nematoid of Beetroots {Heterodera Schachti) 

 causes considerable damage to the roots of this crop. It 

 is a dimorphous worm; " the male has the usual form, the 

 female resembles a lemon " {Schacht). 



The Beet-leaf Rust {Trichobasis betce) is a fungoid 

 growth, which causes yellow spots on the leaves of beets, 

 and ultimately ruptures the epidermis of the host-plant. 

 The spores are brown, and the fungus is common in 

 August and September. 



(2) The Parasites of Carrots {Daucus carota). 



The Carrot Fly {Psila rosw, Fig. 14). The maggots 

 of this insect gnaw and burrow into the roots of this 

 crop. These burrows have the colour of iron rust. The 

 leaves of the invaded crops prematurely turn yellow, 

 owing to the diseased condition of the roots and the 

 lowered vitality of the plants. After a time the roots 

 shrivel. 



The white maggot (Fig. 14 B) is legless, and is about a 



