6 BULLETIN 1015, U. S. DEPARTMENT OF AGRICULTURE. 



green core. They are suitable varieties for crossing, but not very- 

 promising for selection. 



A comparison of the resistance of the Duke of York and several 

 selected varieties and commercial varieties may be seen in Plates IV 

 and V. In Plate IV, rows 1, 2, and 3, in the foreground, are varie- 

 ties that have been selected for resistance to wilt, while row 6 is the 

 Duke of York, a naturally resistant variety. In Plate V, rows 1, 3, 

 and 5 are the Marvel, a selected resistant variety, and rows 2 and 4 

 are susceptible commercial varieties. The photograph reproduced in 

 Plate V was taken at the end of the season after the crop was har- 

 vested. Although the Marvel vines were somewhat defoliated by 

 worms and therefore were less conspicuous than they would other- 

 wise have been, they were still green while the others were dead. 



No variety yet tested by the writer has been immune to wilt. The 

 Duke of York, Buckeye State, and several varieties selected for re- 

 sistance to wilt possess a high degree of resistance, but when grown 

 on heavily infested soil having a temperature of 80° to 85° F. most 

 of the plants show at least a trace of infection in roots or stems. 



The comparative resistance of varieties tested by the writer in 1915. 

 1918, and 1919 are shown in Table 1. As a complete record was not 

 made of the percentages of plants killed in 1916 and 1917, the results 

 for these years are omitted from the table. 



Infection was determined by cutting the stems with a knife and 

 noting the color of the tissue between the bark and the pith. Quite 

 thorough cutting was often necessary to find a trace of the disease in 

 plants of the resistant varieties; in fact, some of them were free 

 from it. Whenever the faintest symptom of the disease could be 

 found in the tissues the plant was recorded as infected. Compari- 

 sons of the data in Table 1 should be limited to varieties grown in 

 the same field and year. The conditions in different fields and in 

 different years were not similar and therefore did not produce com- 

 parable results. 



In the variety test for 1915 there was little difference among the 

 varieties in the percentage of plants infected but considerable dif- 

 ference in the percentage of plants killed by wilt. The Louisiana 

 Wilt-Eesistant made the best and the Willis variety the poorest 

 record, but in subsequent years the Duke of York and the Buckeye 

 State gave approximately as good results as the Louisiana Wilt-Re- 

 sistant variety. The Red Majestic and the Enormous made approxi-. 

 mately as good records as the Duke of York and the Crimson Cushion 

 for both percentages of plants infected and of plants killed ; but the 

 infected plants of the Red Majestic and the Enormous were small, 

 nearly dead, and destitute of fruit, while those of the Duke of York 

 produced about an average crop and those of the Crimson Cushion 

 about a tenth or an eighth of a crop. The degree of infection there- 



