Principles of Plant Genetics and Breeding

208 CHAPTER 12
Fruit quality and disease resistance are the most important breeding targets. Adaptation to climatic conditions is also of prime
interest for the countries located in marginal areas, either in Nordic or subtropical regions. Many apple breeders also try to
improve the tree habit to obtain productive and regular cropping trees.
Fruit quality
Releasing apple cultivars with high fruit quality is obviously the major aim for each breeder. All around the world, the same criteria
are taken into account to assess fruit quality. For fruit appearance, the first aim is to select big size and nicely colored fruits.
Selections with cosmetic defaults (russeting) are discarded. Many traits are involved to build the fruit taste. The most important
are flesh texture, firmness, juiciness, and sugar and acidity content. Flavor is also very important but more difficult to assess.
Assessment of fruit quality is very subjective, it varies between people and countries. Breeding programs in China, Japan, Brazil,
and India focus on sweet-tasting fruits with “Fuji”, “Gala”, or “Red Delicious” as references. People from northern countries
prefer more acidic fruits such as those from “Elstar”, “Jonagold”, or “Braeburn”.
Many apple breeders aim to improve storage ability but there are very few detailed studies on this topic.
The ongoing European project HiDRAS (High Quality Disease Resistance Apples Project) (2003–2006) is a collaborative effort
between 11 European groups aimed at the identification of genetic factors controlling fruit quality based on a multidisciplinary
approach, including apple breeding, genetics, molecular biology, statistics, and bioinformatics (Gianfranceschi & Soglio 2004).
Breeders will be provided with new tools, such as molecular markers linked to fruit quality and pathogen resistance, to develop
marker-assisted selection (MAS).
Disease and pest resistance
Scab, caused by Venturia inaequalis, and powdery mildew, caused by Podosphaera leucotricha, are the two main diseases in
apple orchards. About 15–20 specific treatments per year are required to control them.
Scab For the last 60 years, apple breeders all over the world have aimed to release new apple cultivars resistant to scab. The V f
gene has been widely used in all scab-resistance programs with more than 80% of the scab-resistant cultivars released today
carrying the V f gene. All over the world, crossing designs followed the same strategy: a series of modified backcrosses between
scab-resistant hybrids and commercial scab-susceptible cultivars. At each step of selection, scab-resistant hybrids with the
highest fruit quality were selected and crossed with other commercial cultivars. But, in the 1990s, the first strains of V. inaequalis
able to overcome the V f gene were found in Europe (Parisi et al. 1993; Roberts & Crute 1993) and have been progressively
spreading in Northern Europe. These events have led apple breeders to change crossing strategies: the aim is now to combine
several types of resistance in one genotype. Molecular markers are necessary to detect in progenies those seedlings that carry
more than one gene of resistance.
The European project DARE (Durable Apple Resistance in Europe) (1998–2002) was partly set up to control this issue. Decisive
results were obtained characterizing the pathogenicity and diversity of V. inaequalis (Parisi et al. 2004) and in determining the
resistance status of a large range of apple cultivars carrying various levels of durable resistance (Laurens et al. 2004). The central
part of the project studied the genetic bases of apple scab resistance (Calenge et al. 2004; Tartarini et al. 2004) through the
development of genetic maps on various progenies. Major resistant genes and quantitative trait loci (QTLs) were localized and
tested for their durability against the new virulent strains of V. inaequalis. New molecular markers usable by the breeders were
proposed for MAS (Kellerhals et al. 2004).
Powdery mildew Powdery mildew is also an important disease worldwide. Resistance to it is one of the major aims in most
breeding programs. Up to now, all the cultivars released with improved mildew resistance have been based on partially resistant
parents. But some major genes originating from wild Malus species have now been combined with scab-resistant ones. The most
used are Pl 2 from M. zumi (Knight & Alston 1968), Pl 1 from M. robusta, and Pl w from “White Angel”, an ornamental crab apple
from northern America.
Other disease and pest resistance Fire blight (Erwinia amylovora) is one of the worst plagues in most of the apple-producing
countries. No major resistance has yet been detected. Genetic studies are ongoing to localize QTLs of partial resistance (F.
Calenge, personal communication). Some markers should soon be available to further develop MAS strategies.
Other fungi that can cause severe damage in some regions (Nectria galligena, Valsa ceratosperma, Gymnosporangium
juniperi-virginianae, etc.) are also studied but only at a small scale (Laurens 1999).
Some breeding works are also being developed for aphids – the rosy apple aphid (Dispahis plantaginea) and wooly apple aphid
(Eriosoma lanigerum).