n8 DISEASES OF CULTIVATED PLANTS 



Conidiophores repeatedly branched, ultimate branches 

 pointed and often curved, conidia elliptical 24-28x15- 



20 fJ.. 



BREMIA (REGEL) 



Conidiophores branched, tips of branchlets expanded into 

 a saucer-shaped disc bearing sterigmata at the rim, each pro- 

 ducing one conidium ; oospores minute, brown, wall wrinkled. 

 Haustoria simple, clavate. 



Distinguished from allies by the distinctly dilated tips of 

 the Conidiophores. 



Lettuce mildew, caused by Bremia lactucae (Regel), often 

 proves very destructive to lettuce, more especially when 

 grown under glass and in a humid atmosphere. The fungus 

 appears as a very delicate white mildew on the under surface 

 of the leaves. These soon become yellow, droop, and die. 

 The disease spreads rapidly when conditions are favourable ; 

 such conditions being a fairly high temperature and a humid 

 atmosphere. 



Conidiophores 2-6 times dichotomously forked, tips 

 inflated and bearing several subglobose conidia 15 24 /x; 

 oospores globose, brownish, wrinkled, 25-35 f- diam. 



In the case of lettuce spraying or the application of sulphur 

 is out of the question. A lowering of the temperature and 

 admission of air checks the spread of the disease. Infected 

 leaves, which are easily recognised by the yellowish colour 

 and wilting, should be removed at once before the spores 

 are conveyed to other plants. 



The fungus also attacks cinerarias, artichokes, and many 

 of our wild plants belonging to the Compositae, as goat's- 

 beard, sow-thistle, thistles, hawkweed, scabious, etc. 



PLASMOPARA (SCHROT.) 



Haustoria ovate, unbranched. Conidiophores erect, com- 

 paratively sparingly branched, ccuiidia elliptical, generally 

 papillate at the apex. Oospores globose, wall coloured. 



All species are parasitic on plants. Distinguished from 

 Peronospora by the simple globose or ovate haustoria and the 

 papillate conidia. 



Grape mildew. This very destructive disease, caused by 



