10 



BULLETIN 1015, U. S. DEPARTMENT OE AGRICULTURE. 



each on the wilt-infested breeding plats at the Arlington Experi- 

 mental Farm. After one to three years of continuous selection the 

 selected strains were planted beside the resistant varieties from 

 which they were selected. 



Table 2. 



-Effect of continuous selection of varieties of tomatoes on the resist- 

 ance to wilt. 



Variety and year. 



Field. 



Number of plants in 

 the experiment. 



Percentage of plants 



showing a trace of 



infection. 





Variety 

 under test. 



Selections 

 from same. 



Variety 

 under test. 



Selections 

 from same. 



Norton: 



1917 . . 



3 

 1 

 1 



2 



2 



1 



2 



1 



15 

 16 



48 

 16 

 16 



16 



48 



48 



48 

 175 

 32 

 16 

 112 



64 

 16 



25 



33 

 50 

 52 

 19 

 69 



19 

 31 



65 



63 

 39 

 63 



1917 



1918 



1918 



1919 



66 



Marvel: 



1918 



1918 



34 



76 



Louisiana Wilt-Resistant: 



1916 







Continuous selection has apparently made no improvement in the 

 wilt resistance of these strains. Whenever the real differences be- 

 come small, however, as in the present case, they are obscured by 

 fluctuations caused by environmental factors. Such a factor is soil 

 moisture. On wet spots wilt is much worse than on better drained 

 areas. Moreover, water is probably not the only variable environ- 

 mental factor that affects the development of wilt. Until the meth- 

 ods of reducing these environmental effects are sufficiently refined to 

 enable the breeder to distinguish real differences from fluctuations 

 it will be impossible to determine whether or not continuous selection 

 leads to further improvement. Under the present methods it does 

 not seem to do so. The writer has found no difference in this respect 

 between tomatoes, which are nearly always self-fertilized, and sugar 

 beets, 4 which are normally cross-fertilized. 



DURATION OF RESISTANCE. 



Wilt resistance is apparently as permanent as other characters of 

 the tomato. It fluctuates somewhat with changes in soil tempera- 

 ture, soil moisture, and other physiological factors, but on the aver- 

 age is less variable than tomato fruit characters. Its stability is 

 shown by the fact that the varieties developed by the writer have 

 maintained the same degree of resistance under both continued and 

 discontinued selection and have remained relatively constant in all 

 places tried. This is further supported by the fact that varieties 



4 Some recent Investigations in sugar-beet breeding. 

 465, 51 figs. 1916. 



In Bot. Gaz., v. 62, no. 6, p. 4S 



