Chapter 11. 



DISEASES OF LEAVES AND SHOOTS (contd.). 



Black-Leg or Wilt Disease of Asters. Tomato-leaf rust or 

 mould Leaf-blotch of Cucumber. Mildews of Roses, Goose- 

 berries, etc. 



The Black-Leg or wilt disease of asters is extremely 

 prevalent wherever asters are grown. It is caused by a 

 species of Phyto-pkthora, which differs in several respects 

 from Phyto-phthora infestans, the cause of the Late-Blight 

 disease of potatoes. The disease may manifest itself at 

 any stage in the life of the aster, but the initial attack is 

 always upon the seedlings. In severe cases the seedlings 

 collapse, as in the " Damping-off " disease caused by 

 Pythium, but more frequently they harbour the fungus 

 without showing any outward sign of disease at this stage. 

 Such seedlings may wilt and collapse when transplanted, 

 but many succeed in surviving even to the flowering stage 

 without showing external signs of injury. In the latter 

 cases the plants succumb quite suddenly, almost without 

 warning. The leaves wilt, hang limp and flaccid, and in 

 a few days the whole plant shrivels and dies. Even, 

 though affected plants show little direct sign of the disease 

 until the wilt sets; in, they are often much dwarfed in size 

 and produce fewer flowering branches than healthy plants. 

 The "fact that this disease may be present in apparently 

 healthy asters, which only wilt after they come into flower, 



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