72 DISEASES OF CROPS. 



ally swedes) chiefly in the South of England. So far, it 

 has not been found in Scotland ; most probably because it 

 requires a higher mean temperature than occurs in North 

 Britain. This mildew or mould does not enter the tissues 

 of the host-plant, but lives upon the two external surfaces 

 of the leaves. When a crop of turnips is attacked by 

 Oidium Bcdsamii, the foliage appears white on both sides. 

 The mycelium (Fig. 30 A and D) spreads over the cuticle 

 of the leaves ; and gives rise to perpendicular conidio- 

 phores bearing elongated, square-ended conidia or spores 

 (Fig. 30 F). It has been estimated that there are more than 

 10,000 spores on every square inch of the infested leaves. 

 The conidia of this fungus germinate readily, and give 

 rise to mycelia. 



As the fungus covers both sides of the turnip leaves, 

 it greatly interferes with the processes of assimilation 

 and transpiration ; and thereby prevents further growth. 

 This causes a low yield of roots. The further life-history 

 of this fungus is unknown. 



Cure. — Possibly a solution of iron sulphate or copper 

 sulphate would destroy this fungus (see the " cures " for the 

 potato disease). 



The Turnip Mould {Peronospora parasitica), which 

 belongs to the same genus as the potato-disease fungus, 

 will be described under the head of " The Parasites of 

 Cabbages," as it attacks these plants as well as turnips. 



Finger-and-Toe, Anbury, or Clubbing of Tur- 

 nips {Plasmodiophora hrassicce, Fig. 31). Every farmer 

 knows that turnip and other root crops are liable to 

 degenerate through the abnormal gi-owth of nodules or 

 knobs \ upon the tubers, roots, and rootlets. Turnips so 



' See the author's book : Manures and their Uses, p. 40. 



