POTATO WILT, LEAF-ROLL, AND BELATED DISEASES. 9 



The point of view toward the genus Fusarium has changed greatly 

 smce 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 WoUenweber 

 (1910), whose monograph has laid the foundation for the separation 

 of the species by their morphological characters. Further publica- 

 tions by Dr. WoUenweber have thrown still more light on this hitherto 

 confused problem. (WoUenweber, 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 especiaUy favorable for the 

 development of conidia and sporodochia m 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. WoUenweber as 

 22741°— 14 2 



