' DAMPING-OFF * 



693 



"^^V^ 



' Damping-off.' 



Symptoms. — When certain kinds of seeds are sown thickly and 

 kept very moist the young plants, especially in shaded places, 

 turn yellow, and begin to die off in patches as soon as they 

 appear above ground. Each affected seedling, when examined 

 in an early stage of the disease, is seen to possess a weak, thin, 

 and somewhat shrivelled place on the stem near the surface of 

 the soil. On account of this weak point the upper part of the 

 plant bends, or topples 

 over, in a characteristic 

 manner {£, Fig. 232). 



It is noticed that soon 

 after one seedling is at- 

 tacked, others near it 

 become similarly 

 affected, and the 

 disease spreads out- 

 wards in all directions 

 until all the plants in a 



1 J u J Fig. 232. — A, Normal healthy seedling. B, seedling 



tne Seeu - DeCl are re- which has ' damped-off ' : jr thin, shrivelled portion of 

 J J . ., its hypocotyl (natural size). 



duced to a rotten mass, 



on which small patches of white 'mould' may be seen. 



A new batch of seedlings raised on soil which has already 

 carried a crop of diseased plants, almost always becomes at- 

 tacked, and experience teaches that the cause of the infection 

 remains active in such soil for many months. 



Cruciferous plants, such as mustard and cress, are especially 

 liable to ' damp-off,' and the disease is sometimes prevalent on 

 spurrey, maize, mangel and clovers. 



Some seedlings, such as peas, barley, poppy, potato and 

 others appear to be exempt from the disease. 



Cause. — The disease is caused by the fungus Pythiufn de 

 Baryanum Hesse. The mycelium of the parasite is readily 

 observed within the tissues of a seedling which has damped-off; 



