66 PLANT DISEASES 



PREVENTIVE MEANS. Spraying with dilute Bordeaux 

 mixture. 



Thaxter, Bot. Gazette, vol. xiv. p. 273, 1889. 



BEECH SEEDLING MILDEW 



(Phytophthora omnivora, De Bary. 

 Phytophthora cactorum, Schrot.) 



Hartig has investigated this fungus more especially as a 

 parasite attacking seedlings of the beech (Fogus sylvatica], 

 although, as one of its specific names would lead one to 

 expect, it attacks many different plants, more especially in 

 the seedling condition, among which may be enumerated, 

 Cactus, Acer, Fraxinus, Robinia, Fogopyrum, Sempervivum , 

 Clarkia, and various conifers. 



In seedlings the appearance of dark-coloured blotches 

 on the cotyledons or the primary leaves indicates the 

 presence of the fungus ; dark lines also frequently occur 

 on the stem below the cotyledons, and when this is the 

 case, recovery is impossible ; whereas if the upper leaves 

 only are attacked, the plantlets may recover. 



Gaps are often made in seed-beds by this fungus, which 

 spreads rapidly when once introduced. The spread of 

 the disease is favoured by damp weather, retarded by 

 drought. 



In this species the mycelium is furnished with minute, 

 roundish haustoria, which pierce the cells and absorb 

 nourishment. Lemon-shaped, papillate conidia are pro- 

 duced on the surface of diseased portions. These are 

 carried by wind or rain to neighbouring plants, where they 

 germinate, enter the tissues, and thus extend the disease. 



