Screening out ‘poor’ juvenile seedlings saves the space, time and money spent growing them to adulthood. Discussions of breeding strategies usually emphasise the importance of discarding ‘losers’ as early and ruthlessly as possible (e.g. Janick et al. 1996; MAIA, 1999).
Equally useful is the ability to identify promising adult characteristics on the basis of juvenile traits. According to Janick et al. (1996), correlations between juvenile stage attributes and desirable features in the mature tree must fulfil two requirements if they are to be useful in pre-selecting during the juvenile phase. Firstly, the degree of correlation must be fairly high and, secondly, the attribute in the juvenile seedling must be easy to measure. The juvenile characteristics described below have been used to predict adult traits.
Trees with small leaves nearly always produce small apples; hence juvenile seedlings with the smallest leaves can be discarded during their second year of growth. Conversely, large leaves generally correlate with large apples. However, selections based only the largest leaved seedlings may be weighted towards triploids.
Leafing out date
The date of leafing out of juvenile seedlings is highly positively correlated with flowering date in the adult trees (Tydeman,1964). If early flowering is regarded as an undesirable trait in adulthood, early leafing seedlings should be discarded during the second season of growth (15 months after seed germination). There is also evidence that early germinating apple seeds are associated with early leafing seedlings, hence early flowering adults.
Length of juvenile period
Length of the juvenile period is correlated with parent characteristics, such as season of flowering and ripening date. A greater proportion of summer ripening apples are found among seedlings with a relatively short juvenile period, and vice versa with regard to winter ripening apples (Visser, 1965).
Winter chilling requirements
Early leafing out date of seedlings correlates with low winter chilling requirement in the adult.
Undesirable tree forms
Various growth attributes can be evaluated during the juvenile phase, such as tree weakness and the production of long spindly shoots. We noted the latter in seedlings from crosses made with Tydeman’s Late Orange, a variety prone to spindly extension growth under our growing conditions.
Stem diameter of apple seedlings is inversely correlated with the duration of the juvenile period (the thicker, the shorter) and initial productivity of the seedling (the thicker, the higher).
Anthocyanin pigmentation of shoots and leaf petioles on one-year-old trees is sometimes correlated with red fruit colour. However, Janick et al. (1996) discount its general use for pre-determining fruit colour, arguing that anthocyanin distribution depends on many factors and that there are too many exceptions to the correlations.
The seedling stage is an appropriate time to select for resistance to diseases like scab where infection occurs on young leaves (Janick et al.,1996). Susceptible seedlings can be discarded a few weeks after inoculations with spore suspensions. According to these authors, ‘selection for scab resistance alone can reduce the number of progeny by 50 - 80% depending on selection criteria.’ Similar discard rates can be expected for resistance to Fire blight (Ervinia amylovora). Resistance to other diseases such as mildew (Podosphaera leucotricha) is best assessed during the second year of seedling growth. Mildew resistance of parent varieties provides a reasonably reliable guide to their breeding behaviour.
Janick J, Cummins JN, Brown SK and Hemmat M. 1996. Apples. In: Janick J, Moore JN (eds), Fruit Breeding, Volume I: Tree and Tropical Fruits. John Wiley & Sons, Inc. 1-77.
MAIA, 1999. Midwest Apple Improvement Association News Letter 1999, No. 1.
Tydeman HM. 1964. The relation between time of leaf break and of flowering in apple seedlings. In: Report of East Malling Research Station, 1963. p. 70-72.
Visser T. 1970. Environmental and genetic factors influencing the juvenile period in apple. In: Proc. Angers Fruit Breeding. Symp. p. 101-115.