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severity ratings, showed mean lesion size to be the best indicator of field response
(Table 5.2).
Kong et al. (1997) found little difference in incubation periods between the
accessions used. Consequently, a poor correlation with field results was obtained. As
they point out, it may be that small differences in incubation period have little
epidemiological consequence compared with other components. They did, however,
find a high degree of correlation between infection frequency and mean lesion size
which, they suggest, could be due to these components being controlled either by the
same gene(s) or by having at least some gene(s) in common.
Table 5.2. Matrix of correlation coefficients (r) for components of horizontal resistance to
Alternaria helianthi in 17 sunflower accessions (from Kong et al., 1997)
Infection
frequency
Mean
lesion size
Incubation
period
Spore
production
IF x MLS
index
Infection
frequency
1.00
Mean lesion
size
0.75
1.00
Incubation
period
0.12
-0.03
1.00
Spore
production
0.26
0.22
-0.26
1.00
Field 1994
0.55
0.80
-0.03
-0.03
0.73
Field 1995
0.73
0.76
-0.01
-0.18
0.83
Field (both
years)
0.58
0.74
-0.02
-0.07
0.72
Components were regressed with one another, with severity ratings obtained from field trials
conducted in 1994 and 1995, the pooled ratings for both years and with a severity index
calculated from infection frequency (IF) and mean lesion size (MLS).
Parlevliet (1992) pointed out that infection frequency is the most likely of all
components to be independent of the others, even though the apparent association of
two or more components of resistance does make potential exploitation through
breeding more feasible. In reality, it may be desirable for components not to be
controlled by linked genes as durability is likely to be reduced if this were to be the
case.
Plant breeders in the past have mainly avoided horizontal resistance due to its
more complex inheritance. Indeed, according to Van der Plank (1968), no identifica-
tion of horizontal resistance is possible since it can only be acquired indirectly
through the selection of linked genes. For the successful identification of horizontal
resistance, therefore, breeders are faced with difficulties in the demand for adequate
and meaningful assessments of disease progress, the influence of environmental
interactions, inoculum potential, changes to the physiological status of the host plant
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