Agriculture Reference
In-Depth Information
mildew in spring barley varieties in northern Europe since 1979 (Jørgensen, 1992a;
Brown, 2002b) and, despite the evolution of
B. graminis
f.sp.
hordei
genotypes
partially aggressive to
mlo
varieties (Slater and Clarkson, 2001), continues to be
fully effective in the field. It is worth reflecting on the research that makes it
possible to write that last sentence. On the plant side, the resistances carried by
barley varieties have been determined, in the UK and several other countries, since
the mid-1960s so that information may be combined from several sources - the fact
that they are resistant to all isolates, their pedigrees and the distinctive defence
response of
mlo
plants - to identify varieties which carry
mlo
. The recent publication
of the sequence of
mlo11
(Piffanelli
et al.
, 2004), which is much the most
commercially significant
mlo
resistance allele, will in future allow the use of DNA
sequences to identify
mlo11
varieties. On the pathogen side,
mlo
varieties such as
Alexis, Atem and Apex have been used in differential sets since the resistance was
introduced (Wolfe
et al.
, 1992; Slater 2005b) and no isolate highly virulent to
mlo
has been collected among the many thousands tested since then - the level of disease
caused by
mlo
-aggressive isolates on
mlo
varieties is much lower than on non-
mlo
varieties (Slater and Clarkson, 2001; Slater, 2005b). If both these aspects of the
barley mildew survey had not been carried out, breeders would simply be left with
the observation that the resistance of some varieties - those which we in fact know
do not have
mlo
resistance - become ineffective, while the resistance of others -
those which have
mlo
- is durable. Long-term pathogen survey data, collected over
nearly 30 years, has made it possible to ascribe the durability of the mildew
resistance of
mlo
varieties to the presence of that gene, and has therefore enabled
breeders to use
mlo
systematically as a source of durable resistance.
Survey data can be used indirectly to identify varieties with partial resistance,
quantitatively effective against all pathogen isolates. Quantitative resistance is often
durable, in the sense of being effective over a wide area for a long time (Johnson,
1984), though is sometimes race-specific and non-durable - see the discussion of
Ml(Ab)
in section 3.2.1(c). Even when virulence towards a resistance gene is very
common, there is usually variation in the quantitative level of resistance of varieties
with that gene. For example, the virulence Vra, corresponding to the
Mlra
resistance
gene, has been fixed (i.e. avirulent isolates have not been detected) in the
B.
graminis
f.sp.
hordei
population since at least 1985 (Brown and Wolfe, 1990; Slater,
2005b) and probably for even longer (Wolfe, 1984). Since then, winter barley
varieties with
Mlra
resistance have had NIAB Recommended List ratings ranging
from 2 to 8 on a scale which ranges from 1 (extremely susceptible) to 9 (nearly
immune). Those at the resistant end of the scale need much less fungicide to be
applied to control mildew and are also useful in breeding for durable, partial
resistance. Many other winter barley varieties with different specific mildew
resistances also have good quantitative resistance.
Quantitative resistance can be detected, though more tentatively, in varieties with
a specific resistance which is still partially effective by means of tests with newly
detected isolates which overcome the specific resistance (Bayles and Hubbard,
2005). For instance, when the
Yr17
wheat yellow rust resistance became ineffective
in the mid-1990s through the evolution of virulence to
Yr17
(V17) in the pathogen
population, the variety Brigadier became very susceptible to V17 isolates, with a
