18 DISEASE RESISTANCE OF POTATOES. 



The onl}^ remedies proposed or practiced for blackleo- in Europe 

 consist in selection of sound seed, careful avoidance of rotten tubers 

 or those taken from a diseased crop, attention to rotation of crops, 

 since the germs ma}^ persist and accumulate in the soil, and the use of 

 disease- resisting- varieties. None of these methods has been tested 

 carefully enough to establish its merits. 



The chief evidence as to varietal susceptibilities or resistance to 

 blackleg is that of Appel." He states that while no varieties have 

 been shown to be entirely free from the disease, the evidence is that 

 thick-skinned, starch-rich, late varieties are in general more resist- 

 ant than thin-skinned, starch-poor, early varieties. The Dabersche he 

 tinds to be the most resistant of the widel}^ used German sorts, while 

 the Rose varieties are especially liable to blackleg. Contiguous plats 

 of the Dabersche and White Rose in Doctor AppeFs grounds showed 

 this difference very clearl3^ at the time of the writer's visit in July, 

 1904, the Rose being badly diseased, the Dabersche but slightly. At 

 the Deutschen-Kartoffel-Kultur-Station distinct evidence of variations 

 in disease resistance was found. It was stated at this station that in 

 the trial grounds the red varieties were in general found to be more 

 resistant than the white. 



A considerable development of this disease was noticed at the Uni- 

 versity Farm. Cambridge, England, where extensive variety trials were 

 being conducted. The observations of Mr. H. Henshaw, who had imme- 

 diate charge of these, were in agreement with those of Doctor Appel 

 as to the association of the disease with the rot of the seed tuber, and 

 he regarded it as a bacterial malady. There was a difference in the 

 amount of the disease on different varieties, but Mr. Henshaw found 

 none wholly free from it. At the time of this visit Factor and Up-to- 

 Date were noticeably freer from the trouble than any other varieties 

 there under trial. They were about alike in this, as, indeed, in all 

 other characters. Doctor Delacroix cited as especially resistant to his 

 bacterial disease in France the variety La Czarine. He also stated 

 that the variety Geante Bleue is resistant, but to a less degree. Lau- 

 rent observed in his work on bacterial diseases that the maximum of 

 resistance was shown by the varieties Chardon, Pousse-debout, and 

 Chave. 



OTHER STEM DISEASES. 



Several other potato stem diseases have been reported in Europe, 

 but since the writer did not study them in the field and learned noth- 

 ing beyond what is recorded in literature they will be passed with brief 

 mention. Prillieux and Delacroix have described another bacterial 

 stem disease in France attributed to Bacillus cmiUvorus. Rhizoctonia 

 is conmion on potato stems as well as tubers, but none of the patholo- 



aArb. aus Biol. Abt. Gesundheitsamte, 3: 408 (1903). 



