Abstract:
Hop downy mildew is a devastating disease affecting hop requiring expensive
fungicide applications throughout the growing season. Plant resistance is highly
desirable and theorized as being decidedly quantitative with dominance and epistasis
involved in resistance. An association mapping approach using a mixed-model was
used to identify AFLP markers associated with the hop downy mildew resistant
phenotype. New protocols for the extraction, isolation and recovery of oospores from
plant tissue and soil were developed to aid in the study of the hop downy mildew
oospore. In addition, logic regression was used in a mathematical model in order to
test the statistical procedure’s potential for modeling epistasis. Our results suggest the
hop downy mildew resistant phenotype has a broad-sense heritability of 76% with an
estimated narrow-sense heritability of 49%. Mixed-model results revealed 9% of the
AFLP markers to be associated with the hop downy mildew resistant phenotype. The
association mapping results suggest resistance to hop downy mildew is quantitatively
inherited with moderate heritability, which can be successfully investigated using
mixed-models. The concentration of soil-borne oospores was 14 oospores/g soil.
Germination of oospores occurred between two and eight weeks after preparation of
the slides. Observations of the oospore suggested the sexual stage of this disease may
play a role in over-wintering as oospores were capable of in vitro germination. In
addition, MTT as a stain for downy mildew spore viability comes into question due to
the possibility of a chemical reduction in the presence of NADH. Logic regression
correctly identified the model which best describes the epistatic interaction of VRN-H1
and VRN-H2 and the model was synonymous with the hypothesized genetic model for
vernalization in barley. The mathematical simulation using logic regression software
suggested Boolean logic may be more robust when compared to general linear
modeling for the identification and modeling of epistasis.
