POTATO WILT, LEAF-ROLL, AND RELATED DISEASES. 2§ 
the four weeks during which they were under observation there was 
no apparent influence of the diseased part on the healthy. Similar 
experiments were performed by Schander (1912) with substantially 
the same results. 
The evidence in the seedling collection of this bureau is also 
strongly negative. Certain varieties with clearly marked leaf-roll 
have stood surrounded by healthy varieties without any indication 
of the spread of the disease. 
RELATION OF FUNGI TO LEAF-ROLL. 
The first investigations of leaf -roll made by Appel in 1905 led him 
to believe that it was due to a Fusarium similar to that described 
by Smith and Swingle. Mycelium was found in the vascular bundles 
of diseased plants, and cultures were derived from the stem ends of 
tubers. The species of Fusarium was not determined with cer- 
tainty, for at that time the identification of Fusaria by morphologi- 
cal characters was not possible. The findings of Appel were verified 
by many other workers, and for a time leaf -roll was generally at- 
tributed to a Fusarium. Some good authorities are even now strongly 
inclined to this theory (Kock and Kornauth, 1912). 
It has, however, been abundantly proved that in man}^ cases of 
leaf-roll no fungus is present, and that these include the most ad- 
vanced stages of the disease. The theory has been advanced by Appel 
that these fungus-free cases represent the second stage of a disease, 
the first stage having been due to Fusarium infection, and the weak- 
ness caused by the fungus transmitted to the progeny. This hypoth- 
esis has not been supported by the observed facts. It is greatly 
weakened by the results of the writer's seedling studies, which show 
the earliest typical stages of leaf-roll to be fungus free and by the 
fact that no inheritable leaf -roll follows Fusarium oxysporum in- 
fections in America. The subject has been somewhat obscured by 
the mass of polemic discussion, but it is now quite generally admitted 
that the presence of fungous mycelium is not a characteristic of leaf- 
roll. 
The number of cases in Europe where mycelium has been found in 
diseased plants is so great that some explanation is required. In 
the opinion of the writer, the Fusaria that have been found in con- 
nection with leaf-roll in Europe are of nonparasitic types which have 
invaded diseased or weakened tissues. Where mycelium is reported 
in the bundles, and especially where it is found up to the tips of the 
stems, the first inference must be Verticillium albo-atrum, whose 
hyphse, though thinner, may easily be mistaken for that of Fusarium. 
Mixed infection with Verticillium may account for most of the present 
confusion. This fungus is widespread in Europe, while it is now 
quite definitely established by Dr. Wollenweber that the Fusarium 
