Agriculture Reference
In-Depth Information
genes have been found which are race-specifi c. The boundary between major and minor
genes is not defi ned and varies across publications.
6.2.3
Multigenic/polygenic resistance
These terms have been widely used to describe resistance controlled by an unknown
number of minor genes. The former term avoids the mixing of Latin and Greek derivations.
As genetic knowledge of resistance increases, these general terms will tend to be replaced
by more precise descriptions.
6.2.4
Race-specifi c and race non-specifi c resistance
These terms sought to differentiate between resistance that was subject to loss of
effectiveness with the appearance of new virulent strains of a pathogen and resistance
that was thought would never be lost because the pathogen was not capable of developing
virulence to it. The main problem has been a lack of evidence to suggest that any particu-
lar resistance was race-non-specifi c. The assumption was generally made that genes of
large effect (major or seedling genes) were race-specifi c and genes of small effect (minor
genes or APR) were race-non-specifi c. This has subsequently and comprehensively been
shown not to be the case in many host-parasite systems.
6.2.5
Vertical and horizontal resistance
These terms were devised by Van der Plank (1968) to convey the difference between
race-specifi c and race-non-specifi c resistance. They came from bar graphs used to
display resistance to different isolates of the late blight pathogen (
Phytophthora infes-
tans
) in potatoes in South Africa. Where large differences in resistance were noted on the
resistance (vertical) axis the resistance was clearly race-specifi c. Where no signifi cant
differences were observed across the different isolates (on the horizontal axis) the resis-
tance was determined to be race-non-specifi c. These observations caused later confusion
when used too rigidly. The principal fl aw with these terms was that there was never any
good evidence to show that the minor genes that gave a horizontal response were not also
susceptible to changes in pathogen virulence.
6.2.6
Qualitative and quantitative resistance
These terms also roughly match those for seedling/APR and vertical/horizontal resis-
tance, with qualitative resistance referring to a high level of resistance controlled by a
single gene and quantitative resistance being controlled by several genes of smaller effect.
The term 'quantitative' subsequently came to be used to describe locations on chromo-
somes where genes of small effect were identifi ed through mapping studies, hence 'quan-
titative trait locus' or QTL. More recently, the use of the term QTL has come to be used
for major gene loci as well.
6.2.7
Partial resistance
This term was originally used in studies of the resistance in barley to leaf rust caused by
Puccinia hordei,
to describe resistance that reduced the rate of epidemic development
