Classic Genetics and Breeding - Genetics, Genomics and Breeding of Crop Plants - page 73

Progress by such conventional breeding has provided a 60% reduction
in attack in modern varieties grown in France, compared with those grown
in 1960-70 (Vear et al. 2003; Table 2-1 ) . Nevertheless, in extremely favorable
conditions, most sunflower genotypes may show high levels of attack (Serre
et al. 2004) and further work is necessary, particularly to introduce
additional factors from perennial Helianthus species.
2.4.4.2.2 Phomopsis ( Diaporthe helianthi )
This is a new fungal species, discovered in Yugoslavia in the early 1980s. It
has since been found in neighboring countries and in France. The parasite
attacks through leaves, spreading to the stem and causing wilting, premature
drying and stem breakage. It overwinters in the remains of sunflower stems,
and is thus particularly important in areas where sunflower crop residues
are not ground and ploughed in the autumn.
Resistance sources found in naturally attacked breeding nurseries (Skoric
1985) gave good levels of resistance, and to start with, it appeared that they
might contain oligogenic resistance (Vranceanu et al. 1994). However, with
experience and observations of modern sunflower hybrids, it soon appeared
that parental lines which were not particularly resistant could give hybrids
good levels of resistance (Vear et al. 1996). Studies of factorial crosses showed
that, when measured over several years or several locations, resistance was
quantitative and strictly additive, with no interactions between different
parental lines (Viguié et al. 2000). It could thus be concluded that
combinations of inbred lines with the best levels of resistance give the best
hybrids and that it should be possible to obtain increased resistance by
selecting combinations from different sources.
Interactions between effects of parental lines have been reported but
these all come from observations of Phomopsis reactions in single locations
(Deglène et al. 1999). It has been suggested that this fungal species is still
evolving quite fast and results in France are in agreement with this (Says-
Lesage et al. 2002). In addition, when sunflower lines are infected with
mycelium of different Phomopsis isolates, some small host/pathogen
interactions appear, although these do not affect high levels of resistance
(Viguié et al. 1999). It was presumed that observations of natural attack in
several locations was the best method to assure that a given genotype would
generally show a low level of susceptibility.
In the genetic gain trials in 2000-2001, there only appeared to have
been a reduction of 30% in attack, but this may be because, in the specially
infected field trials, the old varieties had about 60% attack and the recent,
most widely grown hybrids, about 40% infection. Other varieties with higher
levels of resistance exist, but, in France, the resistance level of the highest
yielding varieties is sufficient to protect crops from yield loss and in breeding
now, the requirement is only to maintain the present level of resistance.