Agriculture Reference
In-Depth Information
despite having a susceptible reaction type (Parlevliet & Van Ommeren, 1975). In studies
on a number of varieties, resistance to leaf rust was found to be governed by up to 6-7
minor genes with additive effects and was correlated with increased latency period and
reduced infection frequency, pustule size, infectious period and spore production. The
term is now used more generally to describe any resistance that is only partially effective
in reducing disease expression and is usually synonymous with minor gene resistance.
6.2.8
Hypersensitive resistance
This refers to highly effective resistance that produces small necrotic spots caused by
a strong reaction at the site of infection resulting in localised death of challenged plant
cells. This is typically aligned with seedling major gene resistance and has been exten-
sively studied to understand the physiological interaction between host and pathogen.
These studies are gradually revealing the complex biochemical interactions that mediate
the defence response.
6.2.9
Non-host resistance/immunity
In these responses, no symptoms are seen and the pathogen is not capable of growing on
the host.
6.2.10
Durable resistance
This is now one of the most widely used terms in disease resistance breeding, as it describes
the resistance that plant breeders and farmers want in their varieties without trying to
defi ne the genetic mechanism. The term was suggested for use by Johnson & Law (1975)
to avoid problems created by previous terminology and to refer to rust resistance in wheat
that in practice had provided stable resistance in varieties that had been grown over a large
area for many years. It specifi cally avoided identifying the resistance with particular phe-
notypes or suggesting that the resistance would never be lost to a change in pathogen viru-
lence. Specifi c well-studied examples are the
Sr
2 and
Rpg
1 genes for partial resistance to
stem rust in wheat and barley respectively, the genes
Lr
34 and
Yr
18 which provide partial
resistance to leaf and stripe rust in wheat and which may in fact be the same gene and the
mlo
locus which provides resistance to powdery mildew in barley.
6.2.11
Other terms used
In some host pathogen systems, minor gene, APR or partial resistance has been observed
as resistance that reduces the rate of epidemic spread of a disease. This has been also
described as 'slow rusting' or 'slow mildewing' and so on. Other researchers have
noted that some minor gene resistances may be more affected by temperature than
major or seedling resistance genes. These have been described as 'temperature sensitive'
resistances. In reality, resistance provided by genes of small effect are likely to operate by
a very wide range of mechanisms including pathogen avoidance, physical barriers, resis-
tance to infection, reduction in pathogen growth and reduced spore production. Reduction
in pathogen growth and sporulation in turn are likely to be affected by a wide range of
