76 DISEASES OF FIELD & GARDEN CROPS. [OH. 



Oidium Tuckeri, B. Although several species of Oidium 

 have been recorded from Scotland, Oidium Balsamii, 

 Mont., has not yet been detected there. If it really 

 grows so far north, it could hardly have been over- 

 looked, as it is a remarkable species. 



Oidium Balsamii, Mont., first attracted attention as a 

 pest of turnips in September 1880, when Prof. James 

 Buckman, F.L.S., of Bradford Abbas, Dorsetshire, saw the 

 fungus growing in such profusion over hundreds of acres 

 of Swede turnips that the boots and clothes of persons 

 walking through the turnip fields were whitened with 

 the spores. Until 1880 the fungus was not supposed 

 to be common in Britain ; and it is remarkable that the 

 same fungus should be found growing upon three different 

 natural orders of plants, viz. the Scrophulariacece, the 

 Rosacece, and the Cruciferce. Some farmers say the plants 

 produced from early sown seeds are the most subject to 

 this mildew. It first attacks the lowermost leaves, and 

 then quickly covers every part of the affected plant. 

 The presence of this pest, which is now known to be a 

 common and injurious mildew of turnips, generally fore- 

 shadows a deficiency of roots. 



To the unaided eye the foliage of affected Swedes is 

 white on both sides when attacked by the mildew ; but 

 when seen under a low power of the microscope this 

 white coating resolves itself into a dense felted mass of 

 spider-web -like threads, dotted all over with innumerable 

 barrel-shaped spores. 



The higher powers of the microscope are required to 

 show the exact nature of the Oidium of turnips. A 

 minute fragment must be cut from an infected place on 

 a turnip leaf, and from this fragment an exceedingly thin 

 transparent slice should be cut. When placed in a dry 

 state under the microscope it must be specially noticed 

 that the fungus growth is wholly superficial, and that no 

 spawn threads belonging to the Oidium occur within the 

 leaf. In this respect the Oidium essentially differs from 



