juiy 24 ,i9i6 Disease of Potatoes Known as "Leak" 629 



a moist chamber and in from two to three days about 50 per cent showed 

 evidences of the disease by brown coloration of the skin around the wound. 

 In a week after inoculation the infected potatoes were usually entirely 

 rotted. The skin was brown, and the interior of the potato was soft and 

 watery. They were apparently typical leaky tubers. The fungus 

 was readily reisolated from the rotten potatoes. From the investi- 

 gations of Orton and the experiments of Mrs. Tillotson and of 

 the present writer it is evident, then, that R. nigricans causes a rot of 

 the Irish potato typical in appearance of the disease known as "leak." 

 This work did not prove, however, that all cases of leak were due to 

 R. nigricans, as it was very possible that other fungi acting in the same 

 way might produce very similar results. 



PYTHIUM DEBARYANUM 

 ISOLATION OP THE FUNGUS 



Isolations of the fungus from potatoes were made by transferring 

 portions of the partially rotted tubers obtained in the field to sterile 

 tubes of slanted corn-meal agar and beef agar. In making these 

 transfers the outer surface of the potato which had been washed 

 in a 1 to 1,000 solution of mercuric chlorid was sliced away with 

 a flamed knife and bits of the rotten portion of the potato farthest 

 from the apparent point of infection were removed and placed in 

 the culture tubes. In 24 hours a rather coarse hyalin mycelium was 

 evident on the surface of the agar. After the cultures had grown for 

 three days a microscopic examination of the fungus showed abundant 

 fruiting bodies which much resembled the conidiospores of some species 

 of Pythium. Occasionally structures were found which seemed to be 

 oogonia and antheridia, though these were more frequently seen after a 

 longer period. Transfers were made to the agar slants from 61 typical 

 leaky tubers from a number of different fields. Of these transfers 49 

 proved to be cultures of this fungus, 5 of which were contaminated 

 with bacteria. Six were cultures of bacteria only, and 6 were sterile. 

 R. nigricans was not obtained in any of the cultures. 



MORPHOLOGY OP THE FUNGUS 



The fungus obtained from the leaky tubers was studied and found to 

 be apparently a species of Pythium. The mycelium (fig. i,c) of the fungus 

 is rather coarse, irregularly branched, granular, usually nonseptate, 

 though sometimes becoming septate when old. The conidia are borne 

 either terminally or intercalarly. They are usually nearly spherical 

 when mature and are from 12 to 26/i.in diameter, averaging about 22/1. 

 They germinate immediately with one or more germ tubes when they are 

 placed in water at ordinary room temperatures (fig. 1, d). The oogonia 

 are spherical and borne like the conidia either terminally or intercalarly. 



