ii4 DISEASES OF CULTIVATED PLANTS 



E. Oospores. unknown. 



Rose mildew (Peronospora sparsa, Berk.) has only been 

 met with up to the present on cultivated roses, and the 

 injury occasioned by its presence is often of a most serious 

 nature, even when it does not assume the proportions of an 

 epidemic. Young stock often suffers severely without any 

 obvious cause, as the mildew is so minute and scattered, not 

 forming evident downy patches as in other species, but only 

 to be discovered when carefully searched for on the under 

 surface of young leaves or shoots, with the aid of a good 

 pocket-lens. The leading symptoms of its presence are as 

 follows; young vigorous leaves suddenly begin to droop, 

 and fall at once if the shoot bearing them is slightly shaken, 

 the shoot also soon becomes limp and dies back. If diseased 

 leaves or shoots are examined, blackish or reddish stains will 

 be observed at various points, and on these stains a pocket- 

 lens will reveal the presence of the parasite. If the disease 

 continues unchecked older branches become infected ; in fact 

 no part of the plant having green living bark is safe from 

 attack. I have reason to believe that the disease known as 

 ' black mildew ' by rosarians is caused by the parasite under 

 consideration. 



Conidiophores scattered, repeatedly forked; ultimate branch- 

 lets pointed, generally curved ; conidia broadly elliptical, very 

 obtuse, 1 7-2 2 x 14-1 7 p. Oospores unknown. 



I recently visited an extensive establishment entirely de- 

 voted to the cultivation of roses, the primary object being 

 to supply the London market with cut blooms. An epidemic 

 of some kind had been slowly spreading from one house to 

 another, and examination of material showed that Perono- 

 spora sparsa was the primary cause. As no previous outbreak 

 of this pest had been experienced, the means of prevention 

 was uncertain, and the use of Bordeaux mixture was advised. 

 The owner's account of his struggle and final victory is as 

 follows: 'I followed your instructions, spraying the plants 

 thoroughly with Bordeaux mixture for some weeks. This 

 appeared to have ve?y little effect, for the disease slowly spread 

 into the remainder of the houses which had been clean. 



1 Feeling thoroughly disheartened (for I had then lost the 

 crop from twenty-four houses), I decided to try very drastic 

 measures. After thoroughly drenching a house with cnpram, 



