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resistance genes present in local varieties. He found that all varieties tested had at
least one mildew resistance gene that was not present in European wheat cultivars.
He therefore chose a differential set of Chinese wheats that maximised the ability to
detect diversity in the local pathogen population while including, so far as possible,
varieties with different known resistance genes.
Septoria tritici blotch has become an important disease of wheat in Europe in the
last two decades (Bearchell
et al.,
2005) and research on this disease illustrates the
challenges involved in constructing a new differential set. In an unpublished study,
J.C. Makepeace
et al.
, investigated variation in virulence of the pathogen,
Mycosphaerella graminicola
, in the UK. Their differential set included the cultivars
Longbow and Riband as susceptible controls, the differentially resistant varieties
Hereward, Tonic, Frontana, Equinox and Chaucer and the breeding line Kavkaz-
K4500, an important source of resistance in wheat breeding. Hereward and Tonic
were subsequently found to have a single resistance gene each,
Stb6
(Brading
et al.,
2002) and
Stb9
(Chartrain, 2004) respectively, but both, especially Tonic, could be
replaced with varieties which express those resistances more clearly. Frontana
appears to have
Stb10
resistance (Chartrain
et al.
, 2004) but this is not present in any
northern European wheat studied to date. The resistance of Equinox is weakly
expressed and depends on environmental conditions, so this is not a good
differential variety. Chaucer has resistance which is clearly expressed but appears to
be cytogenetically unstable. Kavkaz-K4500 has at least five, possibly six resistance
genes (Chartrain
et al.
, 2005b) so an isolate avirulent to this line may have any one
or more of the corresponding avirulence genes. Finally, both of the susceptible
controls, Longbow and Riband (as well as many other European cultivars), were
found to be resistant to an African isolate of
M. graminicola
though not to any
European isolate tested to date. It would be desirable to use a control susceptible
variety which does not have this resistance. The survey by Makepeace
et al.
,
produced valuable new information about the distribution of virulences in the UK
population of
M. graminicola
. Given that septoria workers do not have the benefit of
a long history of research on the genetics of resistance, it is not surprising that the
choice of varieties included in the differential set could be improved for use in future
research.
3.2.2 Responses to fungicides
Fungicides are one of the major elements of modern crop protection strategies but
resistance to most modern, systemic fungicides has evolved in at least some
pathogen populations (Russell, 1995).
(a) All-or-nothing responses
In some cases, there is a very great difference between the responses of resistant and
sensitive isolates; the resistant isolates are not killed by the maximum dose of the
fungicide recommended for field use. A simple test - essentially an all-or-nothing
test of the ability to grow in the presence of a critical dose of the fungicide - is
