CHAPTER XV. 



PUTREFACTIVE MILDEW OF TURNIPS AND CABBAGES. 



Peronospora parasitica, Pers. 



THE fungus which causes this disease frequently accom- 

 panies Oidium Balsamii, Mont., already described, but it 

 differs entirely from it both in anatomy and nature. 

 When a Peronospora infected leaf is examined with the 

 unaided eye, the thick white bloom on both sides of the 

 leaves, as in Oidium Balsamiij Mont., is never seen. 

 The Peronospora appears as a thinner, more scattered bloom 

 on the under side of the leaves only, and generally 

 borders pallid, discoloured, and decomposed patches on 

 the leaf. A profuse growth of Peronospora is not to be 

 distinguished from a slight growth of Oidium, without 

 the aid of a lens. 



If a small piece of the leaf of a turnip infected with 

 Peronospora parasitica, Pers., corresponding in size and 

 thinness with the Oidium infected slice already described, 

 is placed under the microscope and examined, it will be 

 seen, if enlarged to a scale one-half that of the last, viz. 

 200 diameters, like the drawing at Fig. 29. The first 

 point to be especially noticed is, that the spawn which 

 gives rise to the fruiting threads of the Peronospora mil- 

 dew is inside the leaf, as shown between the letters A and 

 B. These letters indicate the upper and lower surface 

 of the leaf. The spawn threads are stout as compared 

 with many other mycelia, and have very few septa or 

 stops ; they are notably furnished with numerous 

 haustoria, from haustor, a drawer, or little suckers, as 

 shown at C. The suckers attach themselves to the 



