POTATO WILT, LEAF-ROLL, AND RELATED DISEASES. 9 
The point of view toward the genus Fusarium has changed greatly 
since 1904. At that time Smith and Swingle reviewed all species of 
Fusarium that had been described as occurring ,on the potato and 
concluded that as far as the description went it was impossible to 
distinguish these from each other or from the cause of wilt. 
They therefore took the oldest name, Fusarium oxysporum, for 
their species and considered the others, including F. solani, to be 
synonyms. A little later, when the relation of Fusarium to leaf-roll 
was taken up by Dr. O. Appel, of the Kaiserliche Biologische Anstalt 
in Dahlem, Berlin, he caused to be inaugurated some morphological 
studies based on his conclusions that, up to the present, mycologists 
had not described Fusarium species in a way that permits their 
reidentification; that the insufficiency of the characters utilized for 
systematic description had led to a widespread belief that the genus 
was exceedingly variable, and that they had been differentiated by 
their host or substratum to too great an extent. 
Appel therefore concluded that before a proper study of Fusarium 
diseases could be made it would be necessary to learn more about 
the fungi and to be able to distinguish the species with certainty 
through their morphological characters. Much progress has been 
made in this direction through the work of Appel and Wollenweber 
(1910), whose monograph has laid the foundation for the separation 
of the species by their morphological characters. Further publica- 
tions by Dr. Wollenweber have thrown still more light on this hitherto 
confused problem. (Wollenweber, 1913.) 
It has already been found that the Fusaria are not so variable as 
was formerly thought. Artificial cultures lend great help in the 
work of identification, in which many characters are utilized which 
had been previously neglected, viz: The character of the curvature 
of the conidia, constancy of septation, development of the basal and 
apical cells of the conidia, etc. 
It has also been shown that the pure cultures must be grown on 
such media as will produce a normal development of the fungus. 
Agar, for instance, is poorly adapted for the culture of Fusaria, as it 
tends to produce constricted conidia. Vegetable media are best, 
as a rule, and many plant stems are especially favorable for the 
development of conidia and sporodochia in a normal manner. 
A culture derived from mycelium gives a less normal culture than 
one from conidia and must be grown till spores are produced to start 
new and typical cultures. Young cultures are not favorable for 
morphological study, as they contain many abnormal forms. Neither 
are old cultures good, as they exhibit hunger forms not typical of the 
species. 
For these reasons many substrata must be tried and the cultures 
grown to their best development, designated by Dr. Wollenweber as 
22741°— 14 2 
