Abstract
A major consideration in the use of genetical resistance against
infection of crop plants is the ability of the pathogen concerned to
evolve genotypes with virulence to overcome such resistance. This
applies particularly to oligogenic forms of resistance with major effects
and it is Dossible that forms of partial resistance may be durable and
have economic benefit. This study was concerned with the development of screening methods and identifying possible sources of partial
resistance in barley to two fungal foliar pathogens, Evysiphe gvaminis
f.sp. hovdei3 causal agent of mildew and Rhynahospovium seoalis causing
leaf blotch.
With respect to barley mildew, lines from Ethiopia, Turkey and
Israel, as well as lines and cultivars from two European Barley Disease
Nurseries, were found in preliminary work to exhibit a wide range of
response when exposed to natural inocula of E. gvaminis. A screening
procedure was adopted to favour the selection of virulences, from the
pathogen population, for particular host genotypes and indicated those
lines which gave consistently low disease levels. When these lines
were tested in conjunction with commercial cultivars against known
isolates of E. gvaminis with various virulence combinations, different
patterns of resistance response were evidenced. Firstly, vertical
resistance was associated with commercial cultivars but not lines;
secondly, consistently high resistance was shown by some lines indicating resistance factors other than those apparently present in most
existing commercial cultivars and thirdly, some lines consistently
showed intermediate levels of horizontal resistance. In tests on
commercial cultivars grouped according to their barley mildew resistance categories, both intra- and inter-group differences were
recorded. Variations in group characteristics between years was
attributable to changing virulence combinations in the pathogen population. Variations within groups were low and inconsistent between
assessments: in some cases adult plant resistance may have been
important. The reported tolerance of Proctor may be associated with
delayed infection of emerging leaves and little necrosis resulting
from infection. Microscopic assessments indicated that leaf position
and plant age may influence fungal development, although there were no
apparent qualitative differences in pathogen behaviour on cultivars
evidencing varying degrees of partial resistance.
Studies on R. secalis were hampered by difficulties in ensuring
epidemic development in screening tests, although the development of a
system, based on automatic misting equipment eventually overcame these
and susceptible cultivars became rapidly infected. In glasshouse
trials on a range of lines and cultivars, infection above a threshold
level did not lead to increased impairment of leaf functioning: growth
habit may be of some importance in determining cultivar susceptibility
to this disease. It was demonstrated that lesion development and
level of spore production for different cultivars may not be correlated, the level of spore production being epidemiologically significant in the field.
