PERONOSPORA 117 



a few days the entire surface of the leaf is often covered. 

 The disease spreads very rapidly when once introduced, and 

 unless preventive measures are promptly applied the crop 

 suffers severely, or may be completely destroyed. Schrenk, 

 speaking of an epidemic caused by this fungus on cauliflowers 

 grown under glass, says : ' The fungus made its appearance 

 very suddenly, and within a week had spread over all the 

 plants in the affected greenhouse, attacking both the old and 

 the young leaves. So virulent was the attack that it looked 

 for a time as if the whole crop would be destroyed.' It has 

 also caused serious loss to growers of cauliflowers under glass 

 in France. When wallflowers are attacked the leaves are 

 somewhat stunted, rather fleshy, and inclined to grow erect. 



This disease often accompanies white rust (Cystopus 

 candidus, Lev. ). 



The sporophores are stout and flaccid, irregularly 5-8 

 times divided into two or three branches, the branches 

 repeatedly forked, the last lot of branchlets slender and 

 curved; spores broadly elliptical, 20-22X16-20 /x, white. 

 Resting-spores globose, smooth or becoming wrinkled, 26- 



43 l x - 



Diseased plants should be sprayed with a solution of 



potassium sulphide. When spraying cabbages or cauliflowers 



it is important that the spray should come in contact with the 



under surface of the leaf. To secure this a pipe curved just 



below the nozzle should be used so that the spray can be 



directed upwards. All diseased leaves should be burned, 



otherwise the resting-spores will be set at liberty somewhere 



and at some time, and endanger other crops. Infected soil 



should be dressed with lime. Weeds of the crucifer family 



should not be tolerated in the neighbourhood of cultivated 



crops belonging to the same family. 



Pero?iospora cytisi (Rostrup). This fungus causes brown 



specks to appear on the leaves of laburnum. The conidio- 



phores are 4-5- times forked ; conidia elliptical, clear brown, 



20-28 X 15-20 /x. Oospores in the tissue of the leaf, 35-38 /x 



diam., wall 7-8 /x thick. 



Rostrup, Pflanzenkrankh., 2, p. 1 (1892). 



Peronospora sordida (Berk.) forms broadly effused, dingy 

 lilac patches of mildew on the under surface of leaves of 

 species of Scrophitlaria, Verbascum, and Digitalis. 



