FusARiA OF Potatoes 101 



those instances in which different action under different environment is of 

 importance in identification. 



SOURCE AND METHODS OF ISOLATION 



Most of the species of Fusarium and related organisms presented here 

 were isolated from potato tubers affected with dry or soft rot. A number 

 of isolations were made also from discolored fibro-vascular bundles of 

 potato tubers and from stems of wilted potato,^ plants. A single organ- 

 ism, Fusarium metacroum var. minus, was isolated from a spot on the 

 surface of a half-dead potato stem. 



In a few cases the isolations were made by means of poured plate 

 dilutions, but in the majority of cases they were made from affected tissues 

 of the host. The affected part of the host was first thoroughly wiped 

 with a piece of cheesecloth moistened in 0.1 per cent solution of corrosive 

 sublimate, and then the ''skin" was peeled off just above the affected 

 part, or the diseased part of the plant was broken open so that the spot 

 from which the isolation was to be made was not touched even with 

 sterile utensils.^ Four or five small fragments of diseased tissue were 

 cut out with a sterile scalpel and transferred with a sterile needle to 

 cooled poured plates of a suitable medium. When a rotted tuber showed 

 a noticeable difference, in color or otherwise, in different regions 

 of the decayed part, a separate set of plantings was made from 

 each region. The actual isolation of more than one Fusarium from a 

 single tuber shows that this precaution was, at least in some cases, 

 worth while. 



Several days after plantings were made, if any fungi were 

 present they usually had made considerable growth and often allowed 

 a preliminary macro- and microscopic comparison of the isolated 

 organism with any others of the same series of isolations or of former 

 ones. 



In case a fungus thus obtained was different in some way^ from the 

 others, two transfers into test tubes of a suitable medium were made for 

 further study. In order to make a culture from the start as pure as 



^ Small instruments and glassware may be sterilized conveniently by storing them in a jar of 80-per- 

 cent alcohol. When ready to use the excess alcohol is burned off by passing the instruments through a 

 Hame. When so treated the instruments are sterile, perfectly dry, and not too hot for immediate use. 



"Rate and character of growth, color, and type of spores were at first almost the only characters on 

 which these organisms were judged. 



