Pethybkidge and Laffertv — Dry-Rot of the Potato Tuber. 219 



early variety partially affected with " blight " (Phytophthora infestans) was 

 lying, still attached to the parent plant, with part of its surface exposed. 

 On this surface were two pinkish pustules made up largely of Fusarium 

 conidia. These conidia were mainly 5-septate and measured on the 

 average 40/* x 4/u. From a conidium a pure culture of the fungus was 

 obtained. 



Inoculations into healthy tubers with this pure culture caused dry-rot to 

 occur in them, rather less readily in the month of November than in February,, 

 when the rot occurred in all the cases tried (twenty-nine altogether). That 

 the fungus inoculated into the tubers was responsible for the rot was proved 

 by re-isolating it in pure culture from one of the rotted tubers. The halves 

 of a tuber inoculated at each end with this species, and cut longitudinally 

 after the rot had become established, are shown in fig. 5, Plate VI. 



An extended series of cultures was made of this species on various 

 media, but it is not proposed to go into details concerning them here. It 

 may be stated, however, that the colour of the conidial masses produced on 

 sterilized potato stems varied from buff-pink 1 to pinkish cinnamon. On some 

 media, such as Quaker Oat agar and oat extract agar, it was noted that the 

 production of masses of conidia (pseudo-pionnotes) was accelerated by pushing 

 aside the aerial mycelium and thus exposing the surface of the medium. No 

 chlamydospores were observed in cultures on any of the media used. 



Characteristic of the fungus when growing on several media was the 

 production of a surface layer of a colour varying from " spinel-red " to 

 "pansy-purjde." When the medium was made acid this colour became 

 yellow, like honey, but when made alkaline it turned reddish-purple. 



A culture on cooked potato was boiled with dilute caustic potash solution 

 and filtered, when a claret-coloured filtrate was obtained. On acidifying 

 and again filtering, a honey-yellow filtrate was obtained. This filtrate 

 made alkaline again returned to the original claret-colour. These colour- 

 changes are therefore quite different from those described in the case of 

 F. cceruleum. 



From a study of its morphological and cultural characters we believe that 

 this fungus is identical with Fusarium arthrosporioides Sherb., so that this 

 species must now be regarded as pathogenic to the potsito tuber. It is 

 interesting to note that Sherbakoff gives as the habitat of this species " In 

 discoloured tissues of tuber of Solatium tuberosum, Ireland." We have only 

 met with this species once, and only on a single tuber, and it was not clear 



1 The colours mentioned here are named according to Ridgeway's " Color Standards 

 and Color Nomenclature." 1912. 



