POTATO WILT, LEAF-ROLL, AND BELATED DISEASES. 25 



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-roU 

 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-roU 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 many 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-roU 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- 

 roU. 



The number of cases in Europe where myceHum 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 wliich 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 

 hyphae, 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 



