A technique for screening seedlings of Amelanchier alnifolia Nutt. for resistance to the rust pathogen Gymnosporangium nelsonii Arth.
Date
1990-04
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Degree Level
Masters
Abstract
A technique for screening seedlings of Amelanchier
alnifolia Nutt. (saskatoon berry) for resistance to the fungal
pathogen Gymnosporangium nelsonii Arth. was developed. The
rust species used in the experiments, G. nelsonii, has a
heteroecious life cycle and commonly alternates between
Juniperus horizontalis or J. scopulorum and A. alnifolia. A
reliable procedure for generating basidiospores, the inoculum
infectious to saskatoons, from telia, was developed.
Conditions allowing the expression of susceptibility in
saskatoon seedlings were investigated. Susceptibility was
evaluated on the basis of pycnial lesion density on leaves.
The average density of pycnial lesions on the youngest
three leaves was used to score disease severity. High
correlations were observed when this scoring system was
compared with other scoring systems (average of youngest two
leaves, r=0.962; average of 2nd and 3rd youngest leaves, r=0.760; average of youngest five leaves, r=0.943; maximum
density on any leaf, r=0.933), showing that the use of the
average density-of lesions on the youngest three leaves for
describing the amount of disease of saskatoon seedlings was
reasonable by comparison with the other scoring systems.
Three and four week old seedlings exhibited the greatest
disease severity when seedlings from one to seven weeks of age
were tested.
As leaves aged, their susceptibility to infection
decreased. Leaves 2-5 days old were the most susceptible and
leaves greater than 10 days old did not show substantial
infection.
The minimum leaf wetness period resulting in the greatest
disease severity was 24 hours. Shorter leaf wetness periods
resulted in fewer lesions while periods greater than 24 hours
did not induce a greater number of lesions.
The relationship between inoculum concentration and
disease severity was linear (r2=0.975) over the range of
concentrations tested.
The inoculum suspension naturally ran off the leaves
easily without the use of some method to allow the it to
adhere to the leaf surface. After testing four commercial
adjuvants (Biostick, Postick, Forevergreen, and Tween 20),
molasses, surface rubbing, and a control, it was shown that
rubbing the surface of the leaves or a 1-2% solution of
molasses were effective in sticking the suspension to the
leaves; this resulted in a high number of infections. The use
of an adjuvant required considerably less labour than the
surface rubbing treatment.
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Degree
Master of Science (M.Sc.)
Department
Horticulture
Program
Horticulture