Agriculture Reference
In-Depth Information
well-designed pathogen surveys should ultimately be of benefit to farmers but the
primary users, whose work supports that of farmers, may be plant breeders and
fungicide manufacturing companies. Agronomists often mediate between those two
groups and farmers, helping to translate technical information into practical recom-
mendations.
3.6.1 Breeding resistant varieties
Pathogen surveys are used in many countries to support the breeding and use of
resistant varieties. Results of surveys enable breeders to assess the risk to existing
sources of resistance, validate potential new sources of resistance and identify
sources of durable resistance. In addition, several proposals have been made to
manage the deployment of resistance genes in time or space, generally on a scale of
several years and tens of thousands of square kilometres. Such deployment schemes
would make use of data on frequencies of virulences matching resistance genes
which are already present in commercial varieties and have been overcome by part
of the pathogen population. With one exception, see section 3.6.1(d), none of these
schemes are achievable without regulation by government and are therefore not
considered further here (see review by Brown, 1995).
The examples below are drawn from obligate pathogens of cereals in the UK.
They could equally well have been taken from other countries, crops and diseases
in which long-term surveys have been carried out.
(a) Existing sources of resistance
A major application of survey data is in identifying current sources of resistance that
are threatened by the emergence of new pathotypes. If a differential set includes
varieties which are being used currently by breeders as sources of resistance, new
pathogen strains, virulent towards that resistance, can be detected while they are still
at a low frequency in the population. Perhaps the most important aim of the
UKCPVS is to provide such an early warning system. The current differential sets
for powdery mildew of wheat and barley and for yellow rust of wheat include
several such varieties (Bayles and Hubbard, 2005; Slater, 2005a, 2005b). When
survey data indicate that a hitherto effective resistance gene or combination of genes
is at risk from new pathotypes, breeders may wish to cease using a threatened
variety in their breeding programmes. They may, however, wish to continue using
such a variety if it has other desirable properties (Brown, 1995). Farmers may use
such information either to choose not to grow a variety or - more usually in Western
Europe - to budget for additional fungicide applications.
An isolate with a new pathotype, virulent towards a previously effective
resistance, may be useful to plant breeders. If the source of the resistance which is
now at risk has been used in a breeding programme, tests with the new, virulent
isolate may give a better prediction of a line's future disease performance than field
trials in which lines are exposed to the current, natural population of the pathogen, in
which the new pathotype is still at a low frequency. Trials of varieties with selected
