4 MANUAL OF TREE DISEASES 



Phytophthora omnivora de Bary is the most common on both 

 coniferous and deciduous tree seedlings. This latter fungus is 

 widely distributed in this country and may be found to be of 

 importance with further investigations on deciduous seedlings. 

 It is to be assumed that many other species of fungi may also 

 at times produce damping-off in tree seedlings, since Thielada 

 basicola (B. and Br.) Zopf and species of Botrytis, Colle- 

 totrichum, Volutella and other fungi have been discovered 

 producing this disease in seedlings of other crops. Numerous 

 inoculation experiments by various workers have established 

 the power of the above mentioned pathogenes to cause this 

 type of disease. 



The causal fungi represent many widely different types of 

 life history. The parasitism of these fungi is of a very primi- 

 tive sort. This is evidenced by their usual saprophytic char- 

 acter, extreme destructiveness to the host-plant, wide host- 

 range and the fact that they are limited in their activities to 

 very young seedlings, which have not developed the more com- 

 plex physical and chemical nature of older plants. These 

 fungi exist ordinarily as common saprophytes on decaying 

 vegetable matter in the soil and thus their mycelium is the 

 main infective material. Various types of spores are formed 

 by the different species of fungi concerned in damping-off, but 

 they are rarely instrumental in the inoculation of healthy 

 plants. These spores, however, are mainly useful in carrying 

 the fungus over winter and through other conditions detrimental 

 to vegetative growth. The entire life history of these fungi 

 then, so far as explaining the appearance and development of 

 damping-off in seedling-beds, is confined largely to the growth 

 of the mycelium through or on the surface of the soil, from one 

 plant to another. This manner of spreading is often well il- 

 lustrated when all the plants in a single row are destroyed and 

 only occasional plants in the adjacent rows are affected. Al- 

 though some one or several of the damping-off fungi are generally 



