Agriculture Reference
In-Depth Information
NIAB rating of 1, whereas Hussar appears to have some additional, so-called
background resistance, with a rating of 5 (Anon., 1998). The latter variety therefore
needed much less expenditure on fungicide treatments than the former and in fact,
the use of Brigadier in commercial wheat production in the UK ceased because of its
susceptibility to yellow rust.
(d) Use of resistance genes
Priestley and Bayles (1980) proposed the use of variety diversification schemes as a
means of reducing the risk of disease. They suggested that when a farmer grows
several fields of a crop species, they should be of varieties with different resistances,
particularly in neighbouring fields, to restrict the ability of a pathogen to spread
between fields on a single farm. NIAB publishes diversification schemes for wheat
yellow rust and barley powdery mildew each year, to assist the choice of varieties
for this purpose. The schemes are based on the frequencies of virulences matching
the specific resistances of commercially important varieties and on associations
between those virulences.
The value of diversification schemes in reducing the spread of disease on the
scale of a farm depends on the mobility of the pathogen population. O'Hara and
Brown (1998) showed that very few
B. graminis
f.sp.
hordei
spores move more than
a very few metres in a field of spring barley, while O'Hara and Brown (1996)
showed that almost all immigration of
B. graminis
f.sp.
hordei
spores into a plot of a
susceptible variety occurs early in the season. It is therefore unlikely that
diversification of varieties among fields which are typically a few hundred metres
across would restrict the development of disease within each field, because
relatively little inoculum moves between fields.
However, diversification schemes have considerable value at a national or
regional level, because, if farmers do indeed choose several varieties with different
genes for resistance to a disease, the acreage covered by varieties with any one
resistance gene will be limited, which reduces the opportunity for pathogen clones
virulent to that resistance to spread.
3.6.2 Effective use of fungicides
Information about fungicide resistance is important and valuable to enable farmers
to implement cost-effective disease control. It also enables the agrochemical
industry to adapt the marketing of fungicides to the altered response of the pathogen
population.
(a) Choice of a fungicide
If resistance has evolved to a fungicide, the simplest decision for a farmer to make is
to choose to spray a different fungicide to which the pathogen population is still
sensitive, if one is available. The emergence of resistance to QoI fungicides in
several important plant diseases during the late 1990s and early 2000s was a major
