pd THOMAS: INFECTION OF APIUM GRAVEOLENS 
adapted to the living host, having completely lost the ability to 
grow on dead tissue even that of its most common host. In the 
more highly specialized forms, the relation may become specific 
to such a degree that a comparatively slight change in either host 
or fungus will completely change the virulence of the parasite or 
the effect on the host. It is to be expected, after the long period of 
association necessary for the close adaptation of fungus to. host, 
that both would be more or less similarly influenced by their 
environmental conditions. I shall present data to show that the 
infection of Apium graveolens by Septoria A pii is favored by con- 
ditions which accelerate the growth of the host. 
HISTOCICAL INTRODUCTION 
The late blight of celery was first reported in the United 
States in 1891 by Chester (4) and Humphrey (15). It had been 
described the previous year from Italy by Briosi and Cavara (3), 
probably the earliest record of the disease. Prillieux and Dela- 
croix (29) reported it in France in 1894, and Sorauer (35) recorded 
its occurrence in Germany in severe form in 1896. It came to 
the notice of Salmon (34) in England about 1906. By 1897 the 
disease in America had spread to California and was reported by 
Rogers (31) as a serious pest in that state. An interesting sug- 
gestion as to the possible source of origin of the disease on culti- 
vated celery comes from Pethybridge (26). This author found 
on wild celery a fungus which appeared to be identical with the 
form on the cultivated crop. He was able to transfer it to culti- 
vated celery and produce typical infection. The wild form was 
growing in comparatively complete isolation and Pethybridge 
believes it very unlikely that the fungus could have spread from 
cultivated plants to this host. Hence he concludes that the wild 
form is probably the source of the pest. Dorogin (9) in Russia 
reports a new species of Septoria on celery, S. A pii graveolentis, in 
addition to the common species. The former is said to be more 
destructive than the latter. During the early and middle parts 
of the growing season no severe infection of celery is usually 
noted, although Salmon (34) found that serious damage was done > 
throughout the growing season. Further study of the relation of . 
this disease to temperature and other seasonal conditions is 
needed. 
