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" Damping off " of Tomato Seedlings. 



[Oct., 



"DAMPING OFF" OF TOMATO 

 SEEDLINGS/ 



W. F. Bewley, 



Mycologist at the Experimental and Research Station, 

 Cheshunt, Herts. 



The disease of tomatoes known as " Damping off " is of 

 common occurrence in nurseries in this country during the 

 propagating season. At times it causes considerable damage, 

 giving rise to serious financial losses. Cases are known where 

 nurserymen have lost as many as 50,000 young plants in a 

 season owing to this disease, and most growers have experi- 

 enced anxiety as to it at some time or another. 



Description. — The symptoms of the disease are readily 

 recognised. The seedlings are attacked at the soil level or a 

 little above it ; the organism readily pierces the external skin, 

 and destroys the internal tissues. The latter collapse, assume 

 a brown colour at the point of attack, and the seedling falls 

 over. The action of the parasite is very rapid, 18 hours only 

 being sufficient to cause death. Often the plant is attacked at 

 a later period in its life, when it has been " pricked out " into 

 pots or even planted in the houses; the symptoms are practi- 

 cally the same as in the seedling stage. This later appearance 

 of the disease is often termed ' ' Blackleg " or " Foot rot ' ' by 

 growers. The disease is usually caused by pathogenic 

 organisms, but some discrimination is necessary, because the 

 presence of injurious chemical factors in the soil may produce 

 similar symptoms. Among the latter factors may be men- 

 tioned free ammonia and certain acids such as cresylic acid. 

 The investigations here discussed deal only with the diseases 

 caused by pathogenic fungi. 



Three different organisms have been found to produce 

 " Damping off " of tomato seedlings in the Lea Valley, viz., Phy- 

 tophthora parasitica (Dastur), Phytophthora cryptogea (Pethy- 

 bridge) and Rhizoctonia solani (Kuhn). The disease organisms 

 exist in certain soils as a definite infection, but are absent in 

 others. The infection may be carried from one season to the next 

 in the seedboxes and pots. Another frequent source of infection 

 is the water supply. Examination of a large number of 

 nursery waters showed that the above mentioned fungi were 



* An abridged account of an investigation to be published in full in the Annals 

 of Applied Biology, Vol. VII., No. 2, 1920. 



