Nov. II, 1918 Sweet-Potato Storage-Rots 343 



hours at a temperature of 38° to 41° C. No germination occurred at 

 42°. In general, she found that the length of time required to germinate 

 the spores increased as the temperature decreased. At 25° and 20°, 

 temperatures more nearly approximating room temperature, 13 and 

 16 hours, respectively, were required to germinate the spores. 



In a large number of experiments the writers have been unable to 

 verify the results of Taubenhaus (55) and Taubenhaus and Manns (57) 

 by the method they used. Neither have satisfactory results been 

 obtained by inserting spores and hyphae deep into healthy tissue and 

 confining the potatoes in a damp chamber. Potatoes inoculated by 

 wounding and wrapped in wet filter paper and then in oiled paper to 

 retain the moisture would not give positive results. Potatoes cut in 

 two lengthwise and the spores and hyphae confined between the two 

 halves would not rot any more than the controls, even when wrapped 

 in moist filter paper and oiled paper and subjected to the environment 

 of a moist chamber. Spores suspended in water in a well made in the 

 potato and sealed over by a cover slip would not give consistent 

 results. Chilling the potatoes or bruising before inoculating likewise 

 failed to give anything Hke conclusive results. Experiments of this 

 type in large numbers have been made on potatoes of all sizes taken 

 from storage houses at different times during the storage period and 

 always with negative results. The writers have even failed to get results 

 by cutting ofT the rotted end from a softrotted potato and inserting bits 

 of the diseased tissue from the rotted part into the sound portion. 



Similar results have been obtained independently by Dr. Heinrich 

 Hasselbring, of the Bureau of Plant Industry, who has permitted the 

 writers to use his results. He made literally hundreds of inoculations 

 from pure cultures of Rhizopus nigricans into jagged wounds made with 

 a scalpel. The potatoes were then kept in moist chambers or under 

 bell glasses at room temperature. Now and then a potato rotted, but 

 not enough to justify the conclusion that consistent infection had been 

 obtained. 



Not imtil the writers had developed the "well method" described on 

 page 339 could they rot the potatoes at will. A 24-hour growth of Rhizopus 

 nigricans in sweet-potato decoction poured into a "well" and protected 

 against too rapid drying out would almost always result in rotting the 

 potato. By this method rot has been produced in large as well as small 

 potatoes and in sprouted as easily as unsprouted. In view of the fact 

 that stored sweet potatoes could be rotted with equal ease at any time 

 of the storage period by this method, it would seem that the sugar con- 

 tent of the potatoes, if contributory to infection, certainly was not a con- 

 trolling factor. Hasselbring and Hawkins (24), in an exhaustive series 

 of experiments, have shown that at digging time the starch content 

 is high and the sugar content of the sweet potato is low. From the 

 beginning of the storage period the percentage of starch gradually 



