Agriculture Reference
In-Depth Information
Unfortunately, small-scale, short-term field plot experiments may be inadequate
for answering questions about field selection because of different factors including
plot interference and a limited range of genotypes in the starting inoculum. For
example, Dileone and Mundt (1994) found that the increase of complex pathotypes
in mixtures was related inversely to the number of other pathotypes occurring in the
mixture.
Rates of change in population structure varied in different experiments and are
obviously dependent on many factors. One of the most important is the genetic
composition of the pathogen population surviving between seasons (Schaffner
et al.
,
1992). This cannot be tested in small trials because the populations produced have
little influence on pathogen survival between crops, except, perhaps, on an
extremely small area. Consequently, a sequence of trials, even if they are carried out
on the same plots, cannot provide an estimate of selection over years on a specific
mixture used in agriculture.
10.6.3 Results from large-scale mixture production
Experimental evidence that the useful lifetime of resistance genes can be extended
through diversification, is practically impossible to gather. However, Mundt (1994)
observed that a wheat cultivar released as a component of a multiline maintained its
resistance to yellow rust for several years. When released as a pure line cultivar, the
resistance was overcome within two or three seasons.
A more detailed analysis became possible during the 1980s in the former GDR
(Schaffner
et al.
, 1992; Wolfe and McDermott, 1994). Mixtures of malting quality
barley cultivars were introduced in 1984 and, by 1988/9, the whole of the spring
barley crop (ca 300,000 ha) was grown in this way. The strategy was effective, with
mildew held consistently at a low level and fungicide use considerably reduced.
There was also a restriction of leaf rust, even though this was not a target disease in
the choice of mixture components. Crop yields remained high and the grain was
used for high quality malt and beer production (Wolfe
et al.
, 1991; Wolfe, 1992).
These results were obtained despite little effective diversity: only three resistance
genes,
mlo, Mla12
and
Mla13
were distributed among most of the cultivars used and
field size was large, often 50 to 100 hectares. Thus, the pathogen needed to
recombine only three virulences to produce a super race. The virulence against the
mlo
-gene has not yet occurred in Europe (see 10.5) but an increase in the
recombinant genotype with
Va12
and
Va13,
occurred eventually in the GDR in
1990. However, common pathogen isolates with this gene combination were not
generated locally but had immigrated from Poland and the former Czechoslovakia
(Wolfe
et al.
, 1992) and complex races did not dominate the pathogen population
every year (Schaffner, 1993). The important inference from these observations is
that, despite the limited mixture diversity in this large-scale application, progress in
the pathogen population towards dominance of complex or super races was
remarkably slow (see 10.6.1 and 10.7).
A second large-scale investigation followed the pathogen response to the use of
cultivar mixtures of spring barley in Poland, which increased to a significant scale
