118 



SCIENTIFIC AGRICULTURE. 



December, 1921. 



Some Difficulties in Fruit Breeding 



By M. J. DORSET 



West Virginia Experiment Station, 

 Morarantown, West Virginia. * 



It is the purpose of this paper to discuss 

 briefly some of the difficulties in fruit 

 breeding which appear to be inherent in 

 the material with which we have to deal. 

 The time element has entered into some of 

 the breeding attempts sufficiently to bring 

 many of the difficulties to the foreground. 

 A careful consideration of these may now 

 indicate the limitations of the present me- 

 thod of attack from the genetic point of 

 view. 



Since the process of crossing is so basic 

 to all breeding attempts the subject at 

 hand may be approached from that point 

 of view. The genetic relationship of var- 

 ieties and species may be summarized under 

 four headings — self-sterility, cross-ster- 

 ility, self-fertility, and cross fertility. 



In horticidture, self-sterility and self- 

 fertility may be viewed from the stand- 

 point of t^e individual flower, the variety, 

 or the clone. This condition, strictly 

 speaking, may be limited by some to the 

 relationship between the pollen and pistil 

 of the same flower or, under the monoe- 

 cious condition, to the pollen and pistils 

 borne on the same plant but in different 

 flowers. The self-sterile or the cross-sterile 

 condition is of chief interest in this con- 

 nection because of the limitations placed 

 . upon breeding. Many apparently desir- 

 able points of attack are closed by self — 

 and cross-sterility, and many more have to 

 be abandoned after careful crossing at- 

 tempts fail. If a large progeny is necessary 

 or desirable even partial self — or cross- 

 sterility may be as effective in Ijlocking the 

 way. 



iiut .self and cross-sterility are not the 

 only limitations in breeding horticultural 

 plants. In some self-pollinated progeny 

 there is sometimes such a great redu(;tion 

 in vigor as to practically prohibit frui'^ful- 

 ness. This condition has been especially 

 oustanding in selfed seedlings of the apple 



* Formerly in charf t of the Section of 

 Fruit Breeding, Dep& tment of Horticul- 

 ture, University of ) (innesota, St. Paul, 

 Minnesota. 



and grape. While theoretically when these 

 selfed seedlings of reduced vigor are in- 

 ter-crossed at least some of their progeny 

 should again regain vigor, actually, the 

 breeding work with the long time crops 

 has as yet not been carried that far. If in 

 succeeding inbred generations there is still 

 further loss of vigor the point may event- 

 ually be reached when lessened fruit pro- 

 duction W'ould interfere with further in- 

 breeding. This may not happen, however, 

 and, as in some of the cereals, a point may 

 be reached where there is practically no 

 further reduction in vigor. Furthermore, 

 it should be kept in mind that selfed pro- 

 geny of all of the F. seedlings may not ex- 

 hibit the same reduction in vigor. 



Emphasis is placed upon this poini here 

 because one of the most effective ap- 

 proaches to breeding problems is by means 

 of homozygous parents for the characters 

 of greatest importance or interest. This 

 can be done only through inbreeding. The 

 formula in common use in determining the 

 percentage of homozj-gous individuals in 

 any selected generation following a cross is 

 {2n — 1/2??)'" where n = the number of 

 segregating generations, and m = the 

 number of separately inherited alleomor- 

 phic pairs. 



It will be seen therefore that in sterility 

 or fertility and in loss of vigor we have 

 characteristics of horticultural plants 

 which must be taken into consideration. It 

 should not be inferred, however, from the 

 above discussion that the loss of vigor en- 

 countered in many crosses must of necessitj' 

 be general. This point lias not been ade- 

 (lua'ely tested in horticultural plants, 

 largely becau.se any breeder must consider 

 vigor as one of the most important cliar- 

 acters in -selecting new types. This being 

 the case it would require some courage 

 and confidence in the ultimate outcome to 

 contiinie inbreeding api)lcs with, say ten 

 years between generations for- any number 

 of years wlien great reduction in vigor is 

 encountered. It sliould be .stated liere, in 

 view of the above, tliat most of our im- 

 portant commercial vaiieties of fruits are 



