Rorj.(lle1.cr. )jl 



BRET'IJING FOR RrSTSTANCE TO POTATO I«OT EKLi-ZOlM, 

 HET^':itCDEKj\ ]tf)STOCHIENSIS VJOLL. 



F. G. VJ. Jones 



An expedition went from Great Britain in 1938 to collect tuber-form- 

 ing species of Solanum from mountainous areas in south and central 

 America. Many of the plants brought back were screened by i*;ilenby 

 (195U), who was able to demonstrate resistance in 3. vernei (=balsii) 

 and in five tetraploid lines of S. tuberosum var. andigena . Work in 

 Holland, Great Britain, and Germany (Toxopeus & Huijsman 193^3, Jones 

 195U, Huijsman 1955, 1956, Howard 1955, Williams 1956) has confimied 

 Ellenby's results, and the resistant material has been used as a basis 

 for the breeding of new varieties. Resistance has also been reported 

 in S. sucrense (Mai k Petersen 1952) and in S. capsicibaccatum , S. aff - 

 famatinae , S. microdontum , and 3. suaveolens (Goffart & Ross 195II) . 

 S. vernei and S. catarthrum are reported to be used as resistant 

 parents in Russia (Hawkes 1956) . Table 1 summarises the material used 

 for breeding in Breat Britain. 



Breeding from the tetraploid S. andigena is easy, since the cultivated 

 S. tuberosum varieties are all tetraploids. S. vernei and the other 

 resistant species are diploid and must first be treated with colchi- 

 cine to obtain plants with iui chromosomes. Crossing in S. ve rnei is 

 then satisfactory. Certain complications arise from tetraploidy 

 (Haldane 1930). Assuming that resistance is due to a single dominant 

 gene (H), then five types of plants are possible, as shown in column (1) 

 of Table 2. The usual symbols and the nomenclature adopted for these 

 types are shown in columns (2) and (3), while the expected gametes and 

 the results of crossing with the recessive (susceptible = O) or selfing, 

 are set out in columns (U), (5), and (6) respectively. Small deviations 

 from these expected ratios would be expected on a random clxromatid 

 rather than random chromosome hypothesis. Deviations are small for 

 crosses with simplex and duplex plants, but are chiefly of interest, 

 because they explain the appearance of susceptible plants in triplex 

 crosses where only resisters are expected. 



Breeding from resistant lines of S. andigena is now well advanced. 

 Resistance is apparently due to a single dominant gene (H), (Table 3), 

 which, when simplex, gives approximately 50/' of resistant plants in 

 crosses with susceptible plants and approximately 80^;^ of resistant 

 plants when duplex, column (5) Table 2 and Table 3. Toxopeus (1956) 

 has indicated that the gene is knowi from a limited area in the Andes 

 (S. Peru, Bolivia, and N. Argentine; absent from Central and N. Peru, 

 Eucador, and Colombia). 



Types of resistance have been reviewed by Ellenby (19U5), Jones (19^5), 

 and .Jilliams (1956). 



