Cereal aphids and the infectivity index for barley yellow dwarf virus (BYDV) in northern England

Suction traps at Leeds University Farm, N. Yorkshire, monitored aerial populations of cereal aphids over three autumns. Different migration patterns were observed between the four main species, Sitobion avenae, Metopolophium dirhodum, Rhopalosiphum padi and R. insertum. The relevance of these patterns to the epidemiology of barley yellow dwarf virus (BYDV) is discussed. Transmission tests revealed S. avenae to be the major vector of BYDV, rather than R. padi, which is responsible for disease outbreaks in the south and west of Britain. An Infectivity Index (II) of 50 has been advocated for R. padi-transmitted BYDV, above which economic damage is likely to occur. This value is shown not to be applicable to the Vale of York, and methods of adapting the data are proposed. Such adjusted II values depend on the behaviour and reproduction of the aphids during the transmission tests, and produce II values that correlate well with levels of field infection in the area.     

Effect of plant variety, plant age and photoperiod on glandular pubescence and host-plant resistance to potato moth (Phthorimaea operculella) in Lycopersicon spp.

The effect of plant age and daylength on glandular pubescence was determined for two lines of tomato derived from Lycopersicon hirsutum (BTN 979 and LA 1777A) and a variety of L. esculentum (N 91‐1‐1‐1‐1). Densities of type I, IV, VI and VII glandular trichomes were lowest in N 91‐1‐1‐1‐1 and, over all varieties, were more dense on plants aged greater than 6 wk. Daylength interacted with variety to significantly affect densities of type VII trichomes only. Host‐plant resistance to Phthorimaea operculella was determined in preliminary tests using insects cultured from founders from a potato crop and in confirmatory tests using (less readily available) insects recovered from foliage of a tomato crop. Mortality of ex‐potato neonates on LA 1777 A and BTN 979 foliage was higher 18 h after placement than for N 91‐1‐1‐1‐1, with no effect of day length or plant age. Mortality for ex‐tomato neonates followed a similar trend. Ten days later, two‐thirds of ex‐tomato larvae had established mines on N 91‐1‐1‐1‐1 but fewer (16.7%) were live on other varieties. Stepwise multiple regression using variety as the sole factor was significant (P<0.001) in accounting for 61.4% of the variation in ex‐tomato larval survival but addition of other factors to the regression model was not significant. BTN 979 supported fewer, smaller adults to develop than did N 91‐1‐1‐1‐1, whilst no adults developed on LA 1777 A. In a non‐choice test using ex‐potato adults, significantly more eggs were laid on N 91‐1‐1‐1‐1 than on L. hirsutum varieties and 9‐wk‐old plants were preferred over plants three weeks older or younger. The same variety and plant age trends were evident in a free‐choice test using ex‐tomato adults.     

Changes in trace metal species and other components of the rhizosphere during growth of radish

Changes in the properties of soil solution in the rhizosphere of developing radish plants were investigated. Variations in these properties were expected to affect the distribution and speciation of metals in the soil and soil solution. Applications of essential nutrients were linked to plant transpiration rates and prevented excess addition of nutrient ions, so that subtle changes in soil solution composition would not be obscured. Soil solution pH, the concentration of dissolved organic carbon (DOC) and the concentrations of major and trace elements in solution were found to vary over time. Strict control of fertilizer additions led to the maintenance of a relatively low ionic strength in the soil solution, and under such conditions trace metal solubility appeared to be highy influenced by the concentration of DOC. A chemical speciation analysis was performed which showed that, while dissolved Cd and Zn were largely uncomplexed in unplanted soil, Cd and Zn in the rhizosphere existed mainly as complexed forms. It is hypothesized that this is partly a result of Ca-metal-ligand equilibrium in solution, with higher Ca concentrations in unplanted soil leading to more of the Cd and Zn in solution existing in the uncomplexed state. Changes in the concentrations of uncomplexed Cd and Zn with time gave the best correlations with changes in plant uptake of these metals over time, supporting the hypothesis that plants mainly absorb the free metal ion from soil solution.     

An independent method of deriving the carbon dioxide fertilization effect in dry conditions using historical yield data from wet and dry years

Accurate estimates of the fertilization effect that elevated carbon dioxide [CO₂] has on crop yields are valuable for estimation of future crop production, yet there is still some controversy over these estimates due to possible CO₂‐by‐water‐status interactions in chamber studies and the difficulty of conducting field experiments with elevated [CO₂]. This study presents a new method to estimate the CO₂ fertilization effect (CFE) in dry conditions (CFE_dry), based on a combination of historical yield and climatic data and field experiments that do not require elevated [CO₂]. It was estimated that approximately 50 years of increasing [CO₂] (i.e., a 73 ppm increase) resulted in a 9% and 14% improvement of yield in dry conditions for maize and soybean, respectively, which are similar to estimates derived from free air CO₂ enrichment (FACE) studies. The main source of uncertainty in this approach relates to differential effects of technology trends such as new cultivars in wet vs. dry years. Estimates of this technology–water interaction can be refined by further experimentation under ambient [CO₂], offering a cost‐effective path for improving CFE estimates. The results should prove useful for modeling future yield impacts of climate change, and the approach could be used to derive estimates for other species using relatively simple yield trials.