Principles of Plant Genetics and Breeding

Introduction
F. Laurens
MUTAGENESIS IN PLANT BREEDING 207
The apple is the fourth largest fruit crop grown in the world (59 million tons) after citrus (108 million tons), banana (70 million
tons), and grape (65 million tons) (FAO 2004). It is widely grown throughout the world in various pedoclimatic conditions, from
the European Nordic countries to subtropical regions in Brazil or South Africa, yet it is mostly widespread in temperate regions.
Apples can be consumed as fresh fruits or after processing.
More than 10,000 cultivars have been recorded (Way et al. 1990), yet only a few are grown on a commercial scale. We can
distinguish four main categories according to the ways and times they have been released:
1 Old and local cultivars grown specifically in some regions or countries, e.g., “McIntosh” in Canada, “Belle de Boskoop”
in the Netherlands, “Reinette du Canada” and “Reine des reinettes” in France, “Cox’s Orange Pippin” and “Worcester
Pearmain” in England, and “Rome Beauty” in the USA.
2 Cultivars selected from chance seedlings in the 18th or 19th centuries. The best examples are “Golden Delicious”, “Red
Delicious”, and “Granny Smith” found in the USA. They are still among the most grown cultivars in the world.
3 Most of the cultivars released in the last 20 years, which are derived from controlled crossing programs from all over the
world, e.g., “Gala” (New Zealand), “Elstar” (the Netherlands), “Jonagold” (USA), “Fuji” (Japan), and more recently “Pink
Lady” (Australia). These cultivars derived from a very narrow genetic basis usually involving only a very few genitors, e.g.,
“Golden Delicious”, “Jonathan”, and “Red Delicious”. They show significant improvement over older cultivars; in general,
they fit the main current commercial requirements: high and regular cropping, fruit attractiveness (nice coloring, few cosmetic
defaults), good fruit size, good fruit taste, adaptation to transport, and long storage ability. These cultivars have been
propagated by grafting on a very large scale with many millions of trees planted. During the propagation process some rare
mutation events appear affecting some characters of the tree or fruit. The most interesting and stable sports are then selected
and propagated by grafting to get new cultivars that differ slightly from the original one. New sports can also be produced by
inducing mutation, mostly by irradiation (X-rays or gamma rays). Mutations have had a major economic impact on the
apple industry – in 1995, CPVR (community plant variety rights) applications for apple mutation cultivars were as important
as those for apple seedling cultivars (Semon 2004). They affect two major traits: (i) the shape and volume of the tree, with the
spur (compact) types allowing a higher orchard density (mainly for “Red Delicious”); and (ii) the coloring of the fruit. Nearly
all the most propagated bicolored apples are not from the original clone but from sports with more intense or higher
amounts of skin coloring, which make the fruits more attractive for the consumer and more profitable for the growers.
The previously cited cultivars have a common characteristic – they all are susceptible to the main diseases and pests
that affect apple orchards, namely scab (caused by Venturia inaequalis) and powdery mildew (caused by Podosphaera
leucotricha). About 15–20 specific treatments are required during the growing season to control them.
4 The latest generation of cultivars, from breeding programs that aim to combine fruit quality, good cropping, and scab
resistance.
As with most fruit trees, the apple is grown as a composite tree that combines a rootstock and a fruiting scion. The focus in this
box will be on the improvement of the dessert cultivar scion. First, I will present a short review of the main apple breeding programs
in the world and their main objectives, and then describe the specific constraints of fruit trees that limit the development of
breeding. Finally, I will describe the cycle of selection developed at the Institut National de la Recherche Agronomique (INRA),
France, to release new apple cultivars resistant to scab.
Review of the main apple breeding programs and their objectives
Industry highlights
Current apple breeding programs to release apple
scab-resistant scion cultivars
UMR Génétique et Horticulture (GenHort) (INRA/INH/UA), INRA Centre d’Angers, 49070 Beaucouzé, France
Europe is the region where the highest number of dessert apple scion breeding programs are recorded: 32 have been reviewed by
Y. Lespinasse (personal communication); 26 are developed by public institutes, but the new trend is an increasing amount of privately
funded ones. On average, there is at least one apple breeding program in each European country; in Italy, up to five have
been registered. In other continents there are fewer programs and they are generally present in countries or states where apple
production is important (Laurens 1999).