Agriculture Reference
In-Depth Information
pathogenicity target in the plant (Mackey
et al.,
2002). Detection of the avirulence
protein results in induction of effective host defences and therefore causes the host-
parasite interaction to be incompatible. If the pathogen lacks a functional avirulence
gene, its presence cannot be detected by the corresponding resistance gene, while a
plant which lacks a functional resistance gene cannot detect pathogens with the
corresponding avirulence gene. In many diseases, host-parasite interactions are
controlled by several avirulence-resistance gene pairs but there need only be one
matching pair of resistance and avirulence genes for an interaction to be incompatible.
(a) Host differentials
In order to identify the virulences a pathogen isolate has (or more accurately, the
avirulence functions it lacks), it is inoculated onto a set of host varieties with
different specific resistances. A universally susceptible control variety should
always be included to check that the inoculation has been successful. Many diseases,
such as powdery mildews and rusts, have qualitative symptoms such as necrosis or
chlorosis in an incompatible interaction and conversely, growth and sporulation of
the parasite in a compatible interaction. A pathogen isolate may therefore be
classified as virulent on a differential variety if it is compatible with a high infection
type (IT), or as avirulent if the interaction is incompatible with a low IT. An ordinal
IT scale of this kind is widely used for research on cereal powdery mildew
(Moseman
et al.
, 1965).
In other diseases, the distinction between an avirulent pathogen and a virulent
one is quantitative. Here, the ratio between the amount of disease produced on a
differential variety and that on the susceptible control is much lower for an isolate
which is avirulent on the differential than it is for a virulent isolate. In such cases,
classification of isolates as avirulent or virulent requires a quantitative statistical
technique, such as median tetrad analysis, used by Brown
et al.
(2001) to investigate
septoria tritici blotch of wheat. The fact that this method (and any other method used
for the same purpose) relies on analysis of quantitative data gives rise to the usual
statistical problems: Type I error, when an avirulent isolate is identified incorrectly
as virulent, or Type II error, when a weakly virulent isolate is identified as avirulent.
Two types of differential sets of host lines are generally used. One consists of
near-isogenic lines, in which different resistance genes have been bred into a
common genetic background. Some widely used near-isogenic sets are those in
Pallas barley, with different genes for resistance to powdery mildew (
Blumeria
graminis
[formerly
Erysiphe graminis
] f.sp.
hordei
) (Kølster
et al.
, 1986),
Chancellor wheat, also for resistance to powdery mildew (
B. graminis
f.sp.
tritici
)
(Briggle, 1969) and Thatcher wheat for resistance to brown (leaf ) rust (
Puccinia
triticina
[formerly
P. recondita
]) (Samborski and Dyck, 1976).
The second important type of differential set includes cultivars, landraces,
breeding lines and other material, each of which has a different resistance specificity
but also differs from the other lines in its genetic 'background' - that is to say, genes
other than those controlling the specific resistance. Such a set offers the flexibility
