Agriculture Reference
In-Depth Information
in Triumph barley, which is a single gene (unpublished data) that causes a 95%
reduction in the number of colonies formed by an isolate with the avirulence allele,
Avr
Ab
, compared to virulent isolates (Brown and Jessop, 1995).
(d) Environmental effects
The expression of some resistances is affected by the environmental conditions in
which tests are done. In the UK, plants used in the virulence survey of wheat brown
rust are grown, before inoculation, in standard glasshouse conditions. After
inoculation, they are transferred to either a low or a high temperature regime (10ºC
or 25ºC respectively), because some varieties are resistant to certain
P. triticina
isolates at the higher temperature but not the lower, and
vice-versa
(Jones and
Clifford, 1997). These interactions therefore depend on the environmental
conditions after inoculation.
One can therefore only make rigorous comparisons between the results of
different surveys if the resistance genes studied are expressed similarly not only in
different differential sets of varieties but also in the environmental conditions in
which each investigation is done. A standard set of isolates may be used to test
whether or not the conditions do indeed allow similar expression of resistance genes.
(e) Other variety-isolate interactions
There are many ways in which plants and pathogens may interact, other than by
the particular type of molecular interaction that characterises gene-for-gene
relationships. A particularly important and well-understood resistance which does
not have a gene-for-gene relationship with the pathogen is
mlo
in spring barley.
Non-functional alleles of
mlo
, a gene required for susceptibility of barley to
mildew (Büschges, 1997), especially
mlo11
(Piffanelli
et al.
, 2004), have provided
durable resistance in cultivars over 30 years. Variation in the ability of
B. graminis
f.sp.
hordei
isolates to infect
mlo
barley varieties has been detected in the UK
since 1998 (Slater and Clarkson, 2001). However, this has not led to
mlo
resistance becoming noticeably less effective in the field. It is not known why this
should be so; one possibility is that increased aggressiveness to
mlo
barley
reduces the pathogen's fitness in some other respect.
( f ) New differential sets
The UKCPVS differential sets for barley mildew and wheat yellow rust illustrate the
value of long-term research on a disease, in which plant pathologists collaborate
closely with plant breeders. Consequently, these sets of varieties are relevant to the
resistance genes used by breeders and are also informative in research on the
pathogen. Quite often, however, it may be necessary to construct a differential set to
study a disease despite little being known about the genetics of resistance. Yu (2000)
wished to construct a different set of wheat varieties to study the dispersal of
B.
graminis
f.sp.
tritici
in central China, where little was previously known about the
