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Response of hop cultivars to two-spotted spider mite infestation


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Lawanprasert, A 1994 , 'Response of hop cultivars to two-spotted spider mite infestation', PhD thesis, University of Tasmania.

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The two-spotted spider mite (TSSM), Tetranychus, urticae Koch, is
one of the most serious pests in regions where hops,Humu/us lupulus
L., are grown. Resistant cultivars would be ideal to reduce plant damage
caused by this pest. Studies on the biological interaction between
cultivated hop cultivars and TSSM were conducted over three growing
seasons to gain a greater understanding of plant mechanisms involved in
the response to mite attack under both natural and controlled
environmental conditions.
In field observations, the results revealed a generalised pattern of
growth and decline in naturally occurring mite populations infesting
commercial hops during the growing season. The mites peaked after
hops had formed the visible bases of inflorescences. Eggs were the
predominant stage for most of the season and stage-specific percentages
changed as the season progressed. The spatial and vertical distributions
of the mites for each stage also varied with time. Natural enemies did
not appear at levels sufficient to give control.
Studies on the susceptibility of different hop genotypes to artificial
and natural infestations by the mites indicated that all the genotypes
tested expressed an intermediate to highly susceptible reaction, with M26
(Huller Bitterer) being the most susceptible. Despite no outstanding
evidence of antibiosis, there were significant differences in susceptibility
among hop genotypes in terms of tolerance, non-preference and plant
avoidance in which plant growth exceeds the mite dispersive capacity.
Significant differences in mite densities between the hop cultivars of
European and American parentage were also detected in that the
European cultivar was more susceptible than the American.
Morphological variation in external and internal characteristics of
hop leaves were found among the genotypes studied. Significant
differences were found in ventral gland size, trichome density, trichome
size, length of trichomes, stomatal density, stomatal size and moisture
content. The morphological characteristics of the hop leaves collected
from the same genotype may vary significantly according to leaf age and
growing conditions. Under controlled conditions, none of these
morphological characteristics were found to influence TSSM population
reproductive parameters. In addition to genotypic effects, the findings demonstrated that
environmental factors did influence life history traits of TSSM. These
factors included temperature, relative humidity, light intensity and plant
Physiological and chemical responses of hop genotypes to mite
feeding damage were examined in field and glasshouse experiments.
Feeding damage by mites increased hop leaf resistance to CO2 uptake
through stomatal closure and decreased photosynthetic rates. The
stomatal response to infestation differed between cultivars. Analysis of
chemicals extracted from hop leaves indicated that phenolics, alkaloids
and volatile compounds in infested leaves were qualitatively and
quantitatively similar to those detected in undamaged control leaves.
Hop-canopy microenvironments do play an important role in
seasonal population development of two-spotted spider mites. This role
was amplified by hop susceptibility to the mites. For the hop genotypes
studied; the populations of mites, and 'their predators, tended to increase
more rapidly on sparsely leaved canopies than on densely leaved
canopies. The overall results gtrongly suggest that TSSM performance
was mainly regulated by the microenvircinments that exist within the
canopies of different cultivars with (1) sparsely leaved canopies favouring
population increase and (2) reduced temperature and light and increased
humidity within dense canopies limiting population increase.

Item Type: Thesis - PhD
Authors/Creators:Lawanprasert, A
Keywords: Hops, Two-spotted spider mite
Copyright Holders: The Author
Copyright Information:

Copyright 1994 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1995. Includes bibliographical references (leaves 392-431)

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