The relative susceptibility of Soay and Blackface sheep to natural infection with Dictyocaulus filaria.

Simultaneous studies by faecal larval counts on a mixed flock of Blackface and Soay ewes, yearlings and lambs suggested that the Soay animals were less able to develop resistance to Dictyocaulus filaria. This susceptibility resulted in a greater proportion of the Soay ewes developing heavy burdens of these lung worms, and in a greater proportion of the yearlings and ewes excreting D filaria larvae in their faeces.     

Evidence for a relationship between certain soil arthropods and acid scab development

Springtails and mites have been found consistently in acid scab lesions and studies have shown that these arthropods carryStreptomyces spp. both on and in their bodies. Tests with several soil-applied, systemic insecticides have shown significant control of acid scab despite having little effect on the acid scab pathogen in culture. Diazinon, a long residual insecticide, gave acid scab control as good as the best fungicides thus far tested as seed or soil treatments. This insecticide also was found to be almost nontoxic to the acid scab pathogen. We suggest that soil arthropods appear to be a factor in acid scab development and present circumstantial evidence suggestive of their involvement in all potato scab development.     

Aphid abundance and potato virus y transmission in imidacloprid-treated potatoes

Imidacloprid is a commonly used insecticide that is highly effective in controlling insect pests of potato. The relatively high cost of imidacloprid provides commercial growers with a strong incentive to reduce field application rates. In the present study, we investigated the impact of reduced imidacloprid rates on aphid abundance and potato virus Y transmission within potato fields. Infurrow applications of imidacloprid provided better aphid control even at a reduced rate when compared to threshold-based foliar applications, but did not suppress the spread of the non-persistently transmitted potato virus Y. Virus transmission was significantly decreased in foliar-sprayed plots, possibly because foliar applications incapacitated aphid vectors probing or feeding on infected plants. Despite these generally encouraging results, we still observed 2.3 to 2.7 fold increase in virus infection at foliar-treated plots at the end of the growing season.     

Dynamics of a declining amphibian metapopulation: Survival, dispersal and the impact of climate

Climate can interact with population dynamics in complex ways. In this study we describe how climatic factors influenced the dynamics of an amphibian metapopulation over 12 years through interactions with survival, recruitment and dispersal. Low annual survival of great crested newts (Triturus cristatus) was related to mild winters and heavy rainfall, which impacted the metapopulation at the regional level. Consequently, survival varied between years but not between subpopulations. Despite this regional effect, the four subpopulations were largely asynchronous in their dynamics. Three out of the four subpopulations suffered reproductive failure in most years, and recruitment to the metapopulation relied on one source. Variation in recruitment and juvenile dispersal was therefore probably driving asynchrony in population dynamics. At least one subpopulation went extinct over the 12 year period. These trends are consistent with simulations of the system, which predicted that two subpopulations had an extinction risk of >50% if adult survival fell below 30% in combination with low juvenile survival. Intermittent recruitment may therefore only result in population persistence if compensated for by relatively high adult survival. Mild winters may consequently reduce the viability of amphibian metapopulations. In the face of climate change, conservation actions may be needed at the local scale to compensate for reduced adult survival. These would need to include management to enhance recruitment, connectivity and dispersal.     

Phenotypic variation and quantitative trait locus identification for osmotic potential in an interspecific hybrid inbred F2 poplar pedigree grown in contrasting environments

Elucidation of the mechanisms of dehydration tolerance in poplar (Populus sp.) trees will permit development of biochemical and molecular indicators to identify dehydration-tolerant genotypes during genetic selection. The objectives of this study were to characterize the degree of phenotypic variation in osmotic potential (a determinant of dehydration tolerance), determine the relationship between osmotic potential at full turgor and relative growth rate, and identify quantitative trait loci (QTL) for osmotic potential in an advanced-generation, interspecific poplar pedigree established in contrasting environments. A three-generation, sib-mated black cottonwood (Populus trichocarpa Torr. & Gray) and eastern cottonwood (P. deltoides Bartr.) segregating F(2) family (Family 331) was analyzed at a dry site east of the Cascade Mountain Range (Boardman, OR) and at a wet site west of the mountains (Clatskanie, OR). At the Boardman site, 2-year-old trees (59 clones) were either irrigated everyday (wet) or every other day (dry), whereas 3- and 4-year-old trees (58 clones) at the Clatskanie site were unirrigated. At the Boardman site, the typically narrow range of osmotic potentials exhibited by grandparents and parents was greatly expanded in the F(2) population, spanning from -1.38 to -2.35 MPa under wet conditions, with a similar range under dry conditions (-1.40 to -2.15 MPa). Clones that had osmotic potentials < or = -1.90 MPa generally displayed full maintenance of stem relative growth rates under dry conditions in contrast to clones with osmotic potentials that were < or = -1.60 MPa, in which stem relative growth rates were reduced by an average of 38% in the dry treatment relative to the wet treatment. Although osmotic adjustments of 0.13 to 0.36 MPa were observed in nine out of 59 clones, adjustment typically occurred from relatively high baseline osmotic potentials. The range in osmotic potential at the wetter Clatskanie site at age three was higher (-1.27 to -1.84 MPa) and was further expanded the following year (-1.14 to -1.94 MPa), which had a wetter spring than the previous year, followed by a typically dry July. Seven QTL for osmotic potential were identified that each explained > 7.5% of the variation in osmotic potential. Given that four clones (7%) had osmotic potentials of -2.00 MPa or less and that QTL for osmotic potential have been identified, we suggest that there are opportunities to extend the limit of dehydration tolerance in Populus.     

Resistance Trials of Some Apple Rootstock Varieties to Phytophthora Cactorum (L. & C.) Schroet.

Expression of the relationship between the yield of a plant part (e.g. tap-root, leaf or seed) per unit area and plant density as an equation has hitherto been possible only when a knowledge of the yield at k considerable number of plant densities has been obtained. New equations are presented which enable this relationship to be calculated on the basis of the yields at any two plant densities. The conditions under which the equations are valid are discussed and illustrated with experimental data on carrots and radish. The practical value of the equations in horticulture is indicated.