Showing papers in "Annals of Applied Biology in 2000"

Effects of temperature on biology and life table parameters of the Asian citrus psyllid, Diaphorina citri Kuwayama (Homoptera: Psyllidae)

Abstract: Summary The development, survivorship, longevity, reproduction, and life table parameters of the Asian citrus psyllid, Diaphorina citri Kuwayama were evaluated at 10°C, 15°C, 20°C, 25°C, 28°C, 30°C and 33°C. The populations reared at 10°C and 33°C failed to develop. Between 15°C and 30°C, mean developmental period from egg to adult varied from 49.3 days at 15°C to 14.1 days at 28°C. The low-temperature developmental thresholds for 1st through 5th instars were estimated at 11.7°C, 10.7°C, 10.1°C, 10.5°C and 10.9°C, respectively. A modified Logan model was used to describe the relationship between developmental rate and temperature. The survival of the 3rd through 5th nymphal instars at 15–28°C was essentially the same. The mean longevity of females increased with decreasing temperature within 15–30°C. The maximal longevity of individual females was recorded 117, 60, 56, 52 and 51 days at 15°C, 20°C, 25°C, 28°C and 30°C, respectively. The average number of eggs produced per female significantly increased with increasing temperature and reached a maximum of 748.3 eggs at 28°C (P<0.001). The population reared at 28°C had the highest intrinsic rate of increased (0.199) and net reproductive rate (292.2); and the shortest population doubling time (3.5 days) and mean generation time (28.6 days) compared with populations reared at 15–25°C. The optimum range of temperatures for D. citri population growth was 25–28°C.

Green leaf area decline of wheat flag leaves: The influence of fungicides and relationships with mean grain weight and grain yield

Abstract: Summary A modified Gompertz model was derived to describe the fractional decline in green area of wheat flag leaves in field experiments where green leaf area at time t=100exp[-exp(-k(t-m))]. Curves fitted over time to visual assessments of green leaf area (% of total leaf area) throughout flag leaf life accounted for more than 98% of variation in 45 of 48 wheat cultivar × fungicide treatment (+/−) comparisons. This data set spanned 17 yr and therefore included cultivars of contrasting parentage and age. In the absence of fungicide, green leaf area decline was associated with drought or infection with a number of foliar pathogens including Septoria tritici (sexual stage Mycospherella graminicola), Erysiphe graminis and Puccinia striiformis. Fungicides applied to the flag leaf included propiconazole, propiconazole plus tridemorph, flusilazole or azoxystrobin. Fungicide effects on m (i.e. time to 37% green area) were closely related to fungicide effects (% of untreated) on mean grain weight (variation accounted for (VAF) = 80%) and grain yield (VAF = 85%).

Effect of sowing date on the optimum plant density of winter wheat.

Abstract: Pressure on financial margins in UK wheat production is driving a review of all inputs, and seed represents one of the largest financial inputs in wheat production. The potential savings through exploiting the crop's ability to compensate for reduced population are, therefore, attractive. Field experiments were carried out at ADAS Rosemaund (Herefordshire, UK) in 1996/97, 1997/98 and 1998/99 to investigate the effect of sowing date on dry matter growth and yield responses of winter wheat to reduced plant population. There were three target sowing dates (late-September, mid-October and mid-November), six seed rates (20, 40, 80, 160, 320 and 640 seeds m -2 ) and four varieties (Cadenza, Haven, Soissons and Spark). Grain yield was significantly affected by plant population with a mean reduction from 9.2 to 5.5 t ha -1 as plant number was reduced from 336 to 13 m -2 . In addition, there was a significant interaction between plant density and sowing date. There was, however, no interaction between variety and plant population in terms of yield, except when lodging affected high plant populations of lodging susceptible varieties. The experiments demonstrated scope for reducing plant populations below the current target of 250-300 plants m -2 ; however, the degree of reduction was dependent on sowing date. Over the three years, the average economic optimum plant density was 62 plants m -2 for late-September, 93 plants m -2 for mid-October, and 139 plants m -2 for mid-November sowings. Compensation for reduced population was due to increased shoot number per plant, increased grain number per ear and to a lesser extent increased grain size. Higher economic optimum plant densities at later sowing dates were due to reduced tiller production and hence ear number per plant. The other compensatory mechanisms were unaffected by sowing date.

The physiological response of winter wheat to reductions in plant density

Abstract: Summary The effects of reducing the plant density of winter wheat (cv. Haven) on canopy formation, radiation absorption and dry matter production and partitioning were investigated in field experiments in 1996/97 and 1997/98. Crop densities established ranged from 19 to 338 plants m−2. Grain yield was maintained with large reductions in plant density. At low plant densities the relative growth rate of the crop increased allowing a maintenance of crop dry matter production. An 18 fold reduction in plant density led only to a six fold reduction in green area index at the beginning of stem extension and by anthesis the difference was less than two fold. Crops grown at low plant densities increased green area per plant through increased duration of tiller production, green area per shoot and shoot survival. Main stem leaf number, phyllochron and tiller production rate were not significantly affected by plant density. Radiation use efficiency was greater at the low plant densities. We propose that better radiation distribution through the canopy and increased canopy nitrogen ratio were the causative mechanisms for this increase in RUE. As a result of increased green area per shoot and a decrease in ear production, more radiation was absorbed per shoot at the low plant densities, allowing an increase in grain number per ear from 32 to 48.

The impact of dimethoate on the spatial distribution of beneficial arthropods in winter wheat.

Abstract: The within-field spatial distribution of beneficial arthropods was assessed using two-dimensional grids of pitfall traps and suction sampling across two winter wheat fields of 4 and 16 ha, before and after an application of dimethoate. An unsprayed 6 m wide buffer zone was left around half the edge of the larger field. Arthropod numbers fluctuated to varying extents prior to spraying. Two species of Carabidae (Pterostichus madidus and P. melanarius), Linyphiidae, Lycosidae and Aphidius spp. (Braconidae) all showed their greatest reduction after spraying. For five carabid taxa, Tachyporus spp. (Staphylinidae) and Collembola the decline in numbers following spraying was no greater than any reduction found during the pre-spraying period. Within field spatial distributions of three arthropod groups were analysed using SADIE. P. madidus, present in patches across the centre of both fields prior to spraying, was removed by dimethoate and by 34 days after spraying had recovered most at the field edges. Linyphiidae were evenly distributed across both fields prior to spraying. Their numbers were reduced considerably by dimethoate and they did not recover to pre-spray levels. However, where recovery occurred this was across the centre of both fields indicating their potential to reinvade whole fields. Aphidius species were also evenly distributed across both fields prior to spraying, but did not recover after spraying. Some, but not all arthropods survived within the unsprayed buffer zone and there was some indication that reinvasion of the mid-field was more extensive where this was present. The importance of field margins with respect to insecticide treatments is discussed.

Potato yellow vein virus: its host range, distribution in South America and identification as a crinivirus transmitted by Trialeurodes vaporariorum

Abstract: Summary Sporadic outbreaks of potato yellow vein disease (PYVD) were first observed in the early 1940's by potato growers in Antioquia, Colombia. Long known to be transmitted by the greenhouse whitefly (Trialeurodes vaporariorum), the precise identity of its causal agent (presumably viral in nature) has remained obscure. Here, we present evidence that a closterovirus with a bipartite genome, potato yellow vein virus (PYVV), is associated with PYVD. Electrophoretic analysis revealed that diseased tissue contains 4–5 disease-specific dsRNAs ranging in size from c. 9 000–1 800 bp. RT-PCR reactions containing pairs of degenerate primers directed against conserved motifs in the closterovirus heat-shock protein homologue produced products of the expected sizes. Comparison of the corresponding amino acid sequences revealed striking similarities between PYVV and two bipartite, whitefly-transmitted criniviruses, Cucurbit yellow stunting disorder and Tomato chlorosis viruses. Epidemiological surveys carried out in Rionegro, Colombia identified Polygonum mepalense, Polygonum spp., Rumex obtusifolium, Tagetes spp., and Catharanthus roseus as potential viral reservoirs. PYVV is transmitted through tubers, and visual symptoms alone cannot be used to determine infection status. A sensitive hybridisation-based assay for PYVV has been developed for use in seed certification programmes.

Herbicide resistant rice (Oryza sativa L.): Global implications for weedy rice and weed management*

Abstract: Summary Rice cultivars resistant to broad-spectrum herbicides have been developed and their commercial release is imminent, especially for imidazolinone and glufosinate resistant varieties in the USA and Latin America. Glyphosate-resistant rice should follow within a few years. Rice growers throughout the world could benefit from the introduction of herbicide-resistant rice cultivars that would allow in-crop, selective control of weedy Oryza species. Other perceived benefits are the possibility to control ‘hard-to-kill’ weed species and weed populations that have already evolved resistance to herbicides currently used in rice production, especially those of the Echinochloa species complex. Weed management could also be improved by more efficient post-emergence control. Introduction of herbicide resistant rice could also bring areas heavily infested with weedy rice that have been abandoned back to rice production, allow longer term crop rotations, reduce consumption of fossil fuels, promote the replacement of traditional chemicals by more environmentally benign products, and provide more rice grain without adding new land to production. There are also concerns, however, about the impact of releasing herbicide-resistant rice on weed problems. Of most concern is the possibility of rapid transfer of the resistance trait to compatible weedy Oryza species. Development of such herbicide resistant weedy rice populations would substantially limit the chemical weed management options for farmers. Herbicide-resistant rice volunteers also could become problematic, and added selection pressure to weed populations could aggravate already serious weed resistance problems. Because of the risk of weedy Oryza species becoming resistant to broad-spectrum herbicides, mitigating measures to prevent gene flow, eventually attainable by both conventional breeding and molecular genetics, have been proposed. With commercialisation of the first herbicide resistant varieties planned for 2001, these mitigating measures will not be available for use with this first generation of herbicide resistant rice products. Release of herbicide resistant rice should depend on a thorough risk assessment especially in areas infested with con-specific weedy rice or intercrossing weedy Oryza species. Regulators will have to balance risks and benefits based on local needs and conditions before allowing commercialisation of herbicide-resistant rice varieties. If accepted, these varieties should be considered as components of integrated weed management, and a rational herbicide use and weedy rice control should be promoted to prevent losing this novel tool.

Screening of non-tuber bearing Solanaceae for resistance to and induction of juvenile hatch of potato cyst nematodes and their potential for trap cropping

Abstract: Summary Ninety accessions of non-tuber bearing Solanaceae were screened for (i) resistance to and (ii) stimulatory effect on juvenile hatch of potato cyst nematodes, and (iii) their growth under temperate climatic conditions. All plant species belonging to the genus Solanum tested induced hatching but this effect was most pronounced for plant species of the Solanum nigrum complex. Hatching of juveniles was hardly or not stimulated by other plant genera of the Solanaceae. Solanum sisymbriifolium combined a high hatching effect with complete resistance to both Globodera rosiochiensis and G pallida. Two S. nigrum varieties showed full resistance to G rostochiensis and a high level of resistance to G pallida. Moreover, S. sisymbriifolium and the two varieties of S. nigrum performed very well under Dutch field conditions and, therefore, they are suggested as candidate trap crops for the control of potato cyst nematodes.

Effects of 1‐methylcyclopropene alone and in combination with polyethylene bags on the postharvest life of mango fruit

Abstract: Experiments were conducted to determine how 1-methylcyclopropene (1-MCP) treatments influence ethylene-stimulated ripening of harvested mango cv. Zihua fruit at 20 degreesC. The ripening response of fungicide (prochloraz) treated fruit was characterised following various I-MCP treatments in sealed jars followed by storage in polyethylene bags and/or subsequent ethephon (ethylene) exposure. Exposure of fruit to increasing concentrations of 1-MCP for 12 h resulted in the reduced softening of produce when subsequently held in air for 7 days after ethephon treatment. Application levels of between 1 and 100 mul litre(-1) 1-MCP had increasing impact, while 200 mul litre(-1) 1-MCP apparently began to approach response saturation. Exposure of fruit to 50 or 100 mul litre(-1) concentrations of 1-MCP for periods from 1 to 24 h subsequently resulted in reduced softening of produce when held in air for 7 days after ethephon treatment. Increasing periods of exposure from 1 to 12 h had increasing impact, while exposure times greater that 12 h appeared to reach saturation. In the absence of ethephon-stimulation, the natural ripening of mangoes held in polyethylene bags was delayed by prior exposure to 100 V1 litre-1 1-MCP for 12 h. Extended holding of 1-MCP treated and non-1-MCP treated control fruit in polyethyene bags encouraged physiological and pathological deterioration. Following exposure to 100 il litre-1 1-MCP for 12 h, mango fruit held for 10 days in polyethylene bags showed a delay in the onset of ripening relative to bagged but non-2-MCP treated control fruit. Treatment with 1-MCP allowed storage of mango fruit in plastic bags at 20 degreesC for 30 days. Observations suggest that 1-MCP treatments do not adversely influence the quality of the post-storage ethephon-ripened fruit. Thus, application of 1-MCP in combination with the use of polyethylene bags can extend the postharvest life of mango fruit at ambient temperature. Treatments that extend postharvest life are important in developing countries, such as China, where the cold chain infrastructure is often lacking.

Community-scale seedbank response to less intense rotation and reduced herbicide input at three sites

Abstract: Summary Species diversity and abundance in weed seedbanks were measured at the beginning, and after 3 and 6 yr of the TALISMAN experiment at ADAS Boxworth, Drayton and High Mowthorpe. Fifty species were distinguished in the seedbanks of treatments comprising winter and spring rotations and full and reduced herbicide input. After 6 yr, the number of seedbank species in the main treatments ranged from 11 in the most intense to 26 in the least intense management. Principal co-ordinate analysis showed that the seedbank community diverged over time between sites and additionally between the two rotation treatments at Boxworth. Only four species were common to all sites but community-scale descriptors indicated that the change in seedbanks following reduced inputs had common features at the three sites. First, species-area relations showed a greater relative increase in number of taxa at a plot scale than in the site as a whole, indicating lower inputs encouraged the common weeds more than rare species. Second, both the mean and the standard deviation of the species-abundance distribution at 6 yr increased in response to reduced inputs. As a result, the number of species slightly more than doubled as herbicide units were reduced from four to one while the total number of seeds increased by two orders of magnitude. Some causes of seedbank change were identified: spring-germinating species tended to increase when spring cultivation was common in the rotation, while some competitive weeds had a relatively low abundance when they were targeted by herbicide. The results are discussed in relation to optimising the various functions of the seedbank.

Marker gene expression driven by the maize ubiquitin promoter in transgenic wheat

Abstract: Summary Selecting a promoter for driving transgene expression is one of the most important factors to consider in a transformation project. Information about the native regulation of the promoter activity is important, but it is also necessary to consider how that activity will be affected when integrated into the genome of the transformed plants. Study of a promoter performance in individually transformed lines provides useful information in this area. The maize ubiquitin 1 (Ubi-1) promoter has been widely used to drive constitutive transgene expression in monocotyledonous plants. However, lack of data on its activity in individual transformed wheat lines constitutes a gap in the understanding and predictability of this promoter's performance. In this paper, we began addressing this problem by examining the expression of the marker gene uidA, coding for β-glucuronidase (GUS), under the control of the maize Ubi-1 promoter in individual transgenic wheat (Triticum aestivum L.) lines from different wheat varieties. The expression of uidA driven by this promoter depended to a great extent on the specific transformation event. Whilst expression was strong and constitutive in all tissues in some of the lines analysed, there were also transgenic lines in which GUS activity was restricted to only a few tissues. In general the maize Ubi-1 promoter had strong activity in young, metabolically active tissues and in pollen grains.

Viability of plum ovules at different temperatures

Abstract: Summary The viability of ovules was studied in five plum cultivars under laboratory conditions at four constant temperatures: 5°C, 10°C, 15°C and 20°C and under field conditions over two years. During 10 days from the onset of full bloom, ovule viability in cvs ‘Cacanska Rana’, ‘Cacanska Najbolja’ and ‘Cacanska Lepotica’ was between 80–100 % at the temperatures of 5°C, 10°C and 15°C, in both years. In the same period, ovule viability in cvs ‘Wangenheims Fruhzwetsche’ and ‘Požegaca’ was lower, but never below 50%. At the constant temperature of 20°C, all plum cultivars showed a decline in longevity of ovule viability, which was pronounced in cv. ‘Cacanska Rana’. During the 10 days from the onset of full bloom, ovule viability in all five plum cultivars under field conditions showed a high viability, which approximated to the ovule viability of the cultivars at the constant temperatures of 5°C, 10°C and 15°C, in both years. Determination of the longevity of ovule viability in the mentioned plum cultivars is of great importance due to its effect on the effective pollination period and fertilisation success. This paper deals in detail with the interrelations between the temperature effects on ovule viability, pollen tube growth and fertilisation, as well as on fruit setting.

Micropropagation of three berry fruit species using nodal segments from field-grown plants

Abstract: Summary Plant micropropagation of blueberry (Vaccinium corymbosum L. cv. Berkeley), blackberry (Rubus sp. cv. Smoothstem) and raspberry (Rubus idaeus L. cv. Gradina) was carried out from nodal segments of adult field-grown plants. Hardwood and softwood cuttings were studied as explant sources. The cultures successfully established were softwood from all three species, and hardwood only from blueberry. Shoot-bud establishment from blueberry was achieved by culturing explants in WPM salts with MS vitamins for 15 days, and then 30 days in the same medium with 18 mM Zeatin. The best results of multiplication were obtained in the same medium with 25 mM 2iP. For blackberry, shoot-bud establishment was achieved by culturing explants in MS medium for 15 days, and then in the same medium with 4 mM BA and 0.25 mM IBA. This medium was also the best for blackberry multiplication. Raspberry explants (cvs Gradina and Willamette) were cultured in MS medium for 15 days and then transferred to MS medium supplemented with 4 μm BA and 0.25 mM IBA. After 30 days of culture, only ‘Gradina’ explants survived, from which shoot-bud establishment was obtained in a modified MS medium (Anderson's macronutrients except calcium, with Sequestrene as the iron source) with the same growth regulators. Multiplication was achieved by subculturing explants in the same medium either with 4 mM BA plus 0.25 mM IBA or with 8 mM BA plus 0.25 mM IBA. Shoots of at least 1 cm in length from all species were rooted ex vitro in a mixture of peat and Perlite (1:1, v/v) in a mist chamber, and 100% of rooting plants were acclimated. Bacterial, fungal and viral diseases were detected in stock plants, while tests carried out in both shoots and regenerated plants revealed the absence of any kind of disease.

Effects of potential trap crops and planting date on soil infestation with potato cyst nematodes and root-knot nematodes

Abstract: Summary In 1997 and 1998 the stimulation of hatch of potato cyst nematodes (PCN) by a trap crop was studied at various times during the growing season in a container and a field experiment. Solanum nigrum‘90-4750-188’was used as the trap crop in both experiments and was sown on 1 May, 16 June or 1 August in two successive years on different plots. Neither experiment revealed much seasonal variation in hatchability of PCN juveniles under a trap crop. In the container experiment, the hatch of the Globodera pallida Pa3 population was equally and strongly stimulated (89%) at all sowing dates in both years, except for the 1 August sowing in 1998 (when the hatch was 77% under extremely wet soil conditions). In the control treatment with non-hosts (flax followed by barley) the total spontaneous hatch was 50% over 2 yr. In the field experiment, the hatch of PCN, averaged over the four populations, was also equally stimulated (71%) at all sowing dates in both years. In the control treatment with non-hosts (flax-barley) the total spontaneous hatch was 36% over 2 yr. Total hatch under the trap crop over 2 yr varied between the four PCN populations from 63% to 80%. In 1998 and 1999, control of potato cyst nematodes (PCN) by the potential trap crops Solanum sisymbriifolium and S. nigrum‘90-4750-188’was studied in the field. Potato was also included as a trap crop. In the 1998 experiment, potato, S. sisymbriifolium and S. nigrum strongly stimulated the hatch of PCN compared with the non-host white mustard (Sinapis alba). Roots of potato and white mustard were mainly found in the top 10 cm of soil, whereas roots of S. sisymbriifolium and S. nigrum were also abundant at depths of 10–20 cm and 20–30 cm. In the 1999 experiment, soil infestation with PCN decreased markedly with potato and S. sisymbriifolium as trap crops. In plots moderately to severely infested with 2-yr old cysts (2–29 juveniles ml−1 air dried soil), potato reduced soil infestation by 87% and S. sisymbriifolium by 77%. In plots moderately to severely infested with 1-yr old cysts the reductions were 74% and 60%, respectively. The reduction was least on plots very severely infested with PCN (110–242 juveniles ml−1 soil): 69% and 52% for potato and S. sisymbriifolium, respectively. Soil infestations of plots that were initially slightly to severely infested with the root-knot nematode Meloidogyne hapla were greatly reduced under fallow and S. sisymbriifolium but increased under potato. From these and previous experiments it was concluded that, for several reasons, S. sisymbriifolium is a promising trap crop.

Oligogenic inheritance for resistance to Zucchini yellow mosaic virus in Cucurbita pepo

Abstract: Summary ‘True French’ is an open-pollinated cultivar of the Zucchini (Courgette) Group of Cucurbitapepo and is susceptible to Zucchini yellow mosaic virus (ZYMV). Using C. moschata ‘Menina’ as the source of ZYMV resistance and following six generations of backcrossing, a true-breeding line nearly isogenic to ‘True French’, designated 381e, was recovered that carried ZYMV resistance, albeit not at as high a level as in ‘Menina’. ‘True French’ and accession 381e were crossed, and their reciprocal F,, F,, and backcross progenies were grown in a chamber and inoculated with a highly virulent, non-aphidtransmissible strain of ZYMV. Nearly all F, plants and all plants of the backcross to 381e were classified as resistant. Segregation to resistant and susceptible individuals occurred in the backcross to the susceptible parent, in accordance with a 3:5 three-gene ratio of resistant: susceptible. The F, segregated in accordance with a ratio of 45 resistant : 19 susceptible, which would be obtained if there was one major gene for resistance, Zym-l (Zym), and two other genes, herein designated Zym-2 and Zym-3, both of which are complementary to Zym-1. The presence of Zym-I and either Zym-2 or Zym-3 is necessary for resistance to be expressed in young plants, but the presence of all three might be necessary for resistance to continue to be expressed during subsequent development of the plants. Evidently, Zym-2 and Zym-3 are ubiquitous in C. moschata but their susceptible alleles are much more common in C. pepo. As the level of resistance of 381e to ZYMV is not as high as that of C. moschata ‘Menina’, additional, as yet unidentified, genes must be involved in conferring high resistance to this virus.

Improving secondary pick up of insect fungal pathogen conidia by manipulating host behaviour

Abstract: Summary It is often assumed that efficient application of a mycoinsecticide involves hitting the target pest insect directly with a lethal dose of conidia. However, secondary pick-up of conidia from surrounding vegetation may be a more important source of inoculum. We have investigated ways of increasing conidia acquisition by enhancing host movement. The aphid alarm pheromone, E-β-farnesene, significantly increased mortality among peach potato aphids. Myzus persicae Sulzer, that were exposed for 24 h to discs of green pepper leaf sprayed with conidia of Verticillium lecanii (Zimmerman) Viegas then transferred to fresh untreated discs to allow disease development. A more practical approach to increasing conidia pick-up appears to be the use of sub-lethal doses of the chloronicotinyl insecticide imidacloprid. One percent of the recommended dose, applied systemically, dramatically increased aphid movement; quantified by image analysis of videotaped aphid behaviour. This resulted in greater mortality from mycosis in experiments where aphids were exposed to insecticide-treated leaf discs that had been sprayed with fungal conidia. A comparison with results from an experiment where conidia were sprayed directly onto aphids which were feeding on insecticide-infused pepper discs established that synergy was due to an indirect effect of the insecticide, i.e. through increased movement, rather than a direct effect viz. predisposition of insecticide-weakened insects to disease.

Responses of the sweetpotato whitefly, Bemisia tabaci, to the chinaberry tree (Melia azedarach L.) and its extracts.

Abstract: Summary Host preference bioassays for adults of the sweetpotato whitefly were performed with leaves of the chinaberry tree Melia azedarach L., tomato, cucumber and bean. Fruit and leaf extracts of the chinaberry tree were tested against adults of the sweetpotato whitefly. Fruit extracts were tested against eggs, first and second instar nymphs, and pupae of the insect. Treatments included aqueous, methanol, and acetone fruit extracts of 200 mg ml−1 and serial dilutions of 20.0 and 2.00 mg ml−1, ether extract, the botanical insecticides Azatin® and Margosan® -O and the control, water or water with Triton®. Mortality data was collected at 6, 7, and 8 days after treatment of the eggs, nymphs and pupae, respectively. Results of the host preference bioassays indicated a significantly lower number of live insects on leaves of the chinaberry tree vs leaves of bean, cucumber, and tomato after 24 h. This indicates that M. azedarach is not a good host for the sweetpotato whitefly. Adults of the insect were significantly more repelled from tomato plants treated with the undiluted extracts when compared to the control after 72 h. There were significant differences in percent mortality of nymphal instars when exposed to the undiluted extracts compared to other extracts and the control. However, there was no significant effect of the fruit extracts on the egg and pupa instars. Thus M. azedarach extracts were found to be repellent to the whitefly adults, while the fruit extracts have shown a significant detrimental effect against early nymphal instars. In general, the methanol extracts were more active against B. tabaci than extracts with other solvents.

Density, distribution and dispersal of the carabid beetle Nebria brevicollis in two adjacent cereal fields

Abstract: Summary The distribution of the carabid beetle Nebria brevicollis was monitored in the summer during a period of declining activity associated with aestivation in a hedgerow. After emergence from aestivation, population density, distribution and dispersal of N. brevicollis were studied during autumn 1994 in a mark-recapture experiment. 3560 beetles were marked and 1887 were recaptured in a grid of pitfall traps spanning a hedgerow and extending approximately 32 m either side into two recently harvested cereal fields. Population size, estimated from a Lincoln index, increased slightly with time with a mean population density of approximately 0.9 beetles m−2. Activity-density varied during the experiment and was significantly related to maximum temperature. The population was aggregated within the hedgerow during aestivation and in several spatially stable hot-spots of activity-density within the field during autumn. There was considerable movement within fields but the hedgerow was a significant barrier to dispersal between fields, with potential effects on the metapopulation structure of the species.

The measurement of multiple spray deposits by sequential application of metal chelate tracers

Abstract: Summary Chelates of the transition metals copper, cobalt, manganese and zinc, available as foliar feeds, were identified as the most suitable spray tracers for comparisons of deposits arising from up to four sequential applications to a single target. Their use minimised many of the disadvantages associated with other commonly used tracers such as visible and fluorescent dyes, and it was possible to quantify each tracer when present on the target in ratios of up to 100:1. Each spray solution contained a single chelate at a metal concentration of 1 g litre−1. The recovery of metal chelates from plant tissue may not be quantitative, as a consequence of absorption or adsorption by leaves for example. The food colorant tartrazine, at a concentration of 10 g litre−1, was shown to be a suitable internal standard for estimation of percentage recovery. Combined spray deposits were recovered readily from apple leaves and fruit by aqueous extraction and analysed by atomic absorption spectroscopy. The spray deposit of each metal tracer was calculated from the raw analytical data using procedures for generating calibration curves designed to optimise the accuracy of the analysis.

Physiological ageing of potato tubers: a review.

Abstract: Summary Numerous theories have been proposed to describe the complex process of ageing in biological systems. Two general groups of ageing theories currently exist: 1) stochastic where the accumulation of random molecular damage leads to loss of information vital to the cell; and, 2) systemic where an organised, genetically based sequence of metabolic activities leads to programmed ageing. Whether these are acceptable models of ageing in potato tubers is unknown although the tuber could provide a useful experimental system for studying ageing. An initial requirement for advancing the concept of ageing in potato tubers must centre on the development of a suitable ageing index. A review of the literature suggests that a modified approach to ‘sprouting capacity’ and ‘incubation period’ may allow tuber ageing to be described in mathematical terms that would, in turn, facilitate the development of a physiological ageing index as well as temperature sensitive predictive models. Although a number of biochemical studies of ageing have been pursued, the development of adequate biomarkers has yet to achieve a coordinated level of development as found in a range of organisms. For example, ageing in other biological systems may be viewed as an outcome of an accumulation of random molecular damage and may be primarily caused by a changing balance between reactive oxygen species and diminishing levels of protective agents such as superoxide dismutase, alpha-tocopherol or vitamin C. The exploration of these and similar problems in the context of appropriate modelling approaches should allow a better understanding of physiological ageing in potato tubers.

Effect of soil type, salinity, and allelochemicals on germination and seedling growth of a medicinal plant Lepidium sativum L.

Abstract: Summary The present work was conducted to elucidate the effect of soil type, salinity stress and allelochemicals from alfalfa on the germination efficiency, seedling growth and photosynthetic pigments of Medicago sativa L. The highest germination rate (69.9%) was recorded for seeds cultivated in sandy soil followed by that of clay soil (42%) and then loamy soil (19%). The maximum germination rate was attained at 50 mM NaCl concentration level, after that the values were decreased as the salinity levels increased. The maximum lengths (cm) of plumule and radicle and their growth rates (cm/day) were obtained at control level and found to decline significantly as the salinity increased. Aqueous extracts of alfalfa reduced the germination rate, radicle and plumule growth and seedling dry weight of L. sativum. However, chlorophyll and carotenoid content was increased, their maxima were recorded at 50% concentration level. The chlorophyll a/b ratio attained its maximum at the full strength concentration level.

Identification of resistance to Peanut bud necrosis virus (PBNV) in wild Arachis germplasm

Abstract: Eighty three wild Arachis germplasm accessions, belonging to 24 species of five sections and one natural hybrid derivative of a cross between the cultivated and a wild Arachis species, were evaluated along with a susceptible groundnut cultivar for resistance to Peanut bud necrosis virus (PBNV) in a replicated field trial at ICRISAT, Patancheru, India. Thirty days after sowing, the percentage of infected plants were recorded for all the accessions and subsequently young leaflets from all these accessions were tested for the presence of the virus by enzyme linked immunosorbent assay (ELISA). One accession each of A. benensis and A. cardenasii, and two accessions of A. villosa, in the section Arachis, two accessions of A. appressipila in the section Procumbentes, and one accession of A. triseminata under section Triseminatae were not infected by PBNV. These seven field-resistant accessions were tested under glasshouse conditions for virus resistance by mechanical sap inoculations. One accession of A. cardenasii and two accessions of A. villosa did not show systemic infection. Similarly, in another glasshouse test, where 13 A. cardenasii accessions of section Arachis were evaluated, two accessions did not show systemic infection. In all these resistant accessions, the inoculated leaves showed infection, but the systemic leaves did not show the presence of virus in spite of repeated mechanical sap inoculations. So, the resistance in these accessions appears to be due to a block in systemic movement of the virus. To our knowledge this is the first report on the identification of resistance to PBNV in wild Arachis species. Since both A. cardenasii and A. villosa are the progenitors of cultivated groundnut and can be hybridised with the latter, the resistant accessions are being utilised in conventional breeding programmes to transfer PBNV resistance to widely adapted groundnut cultivars.

Relationships between phoma leaf spot and development of stem canker (Leptosphaeria maculans) on winter oilseed rape (Brassica napus) in southern England

Abstract: Models were constructed to describe the relationships between incidence of phoma leaf spot at different growth stages in autumn/winter or early spring and incidence of stem canker (basal canker or stem lesions) in summer on winter oilseed rape in southern England. Model 1, describing the phoma leaf spot/basal canker relationship, was y(1) = beta (0) + beta (1)x(1) + beta (2)(x(2) - x(1)) if x(2) > x(1), and y(1) = beta (0) + beta (0) + beta (1)x(1) if x(2) less than or equal to x(1), in which y(1) was the incidence (Ic plants affected) of basal canker at harvest,x(1) was the maximum incidence of phoma leaf spot during the period from sowing to growth stage (G.S.) 1,6-1,7 (about 100 days after sowing) and x(2) was the maximum incidence of phoma leaf spot between G.S. 1,7 and G.S. 2,0 (start of stem extension). Model 2, describing the phoma leaf spot/stem lesion relationship, was y(2) = alpha (0) + alpha (1)x(3) + alpha (2)x(4), in which y(2) was the incidence of stem lesions at harvest, x(3) was the incidence of phoma leaf spot at G.S. 3,3-3,5 (flower buds visible) and x(4) was the incidence of phoma leaf spot at G.S. 4,5-5,5 (flower buds opening). Data from field experiments with four winter oilseed rape cultivars at Boxworth or Rothamsted in the 1992/93, 1993/94, 1996/97, 1997/98 or 1998/99 seasons were used to test the models. The values of R-2 for the regression equations testing model 1 for the phoma leaf spot/basal canker relationship were 0.75, 0.93, 0.91 and 0.89 for cvs Apex, Bristol, Capitol and Envol, respectively. The values of R-2 for the regression equations testing model 2 for the phoma leaf spot/stem lesion relationship were 0.58, 0.57, 0.54 and 0.71 for cvs Apex, Bristol, Capitol and Envol, respectively. The phoma leaf spot/basal canker relationship (model 1) could also be fitted to the combined data set for all four cultivars (R-2 = 0.65), whereas the phoma leaf spot/stem lesion relationship (model 2) could not to be fitted to the combined data set for the four cultivars. The relationships between incidence and severity of stem canker were examined and the values of R-2 for the regressions of severity on incidence were 0.91 for basal canker and 0.89 for stern lesions.

Biological properties of necrotic and non‐necrotic strains of bean yellow mosaic virus in cool season grain legumes

Abstract: Summary A collection of 51 bean yellow mosaic virus (BYMV) isolates was transmitted from infected Trifolium subterraneum (subterranean clover) to Lupinus angustifolius (narrow-leafed lupin) by Myzus persicae (green peach aphid). Depending on isolate and L. angiistifolius genotype used, two distinct responses developed in L. angustifolius plants, either systemic necrosis and plant death or non-necrotic reactions of varying severity. Ten isolates caused necrosis and plant death in cv. Danja. However, when nine of these were inoculated to breeding line 90L423-07-13, seven induced non-necrotic reactions, while two caused necrosis and plant death. Thirty seven isolates always produced non-necrotic reactions regardless of genotype of L. angustifolius inoculated. Non-necrotic and necrotic isolates originally came both from lupins and other species, and the non-necrotic isolates were no less efficiently transmitted by M. persicae than the necrotic ones. When one isolate of each type was inoculated together to T. subterraneum and nine months later this culture was used as an acquisition source for aphid transmission to L. angustifolius, only the necrotic type was detected. Previous infection of L. angustifolius plants with a non necrolic isolate prevented subsequent infection by a necrotic one. All necrotic and non-necrotic isolates reacted with BYMV antiserum in ELISA but only two cross-reacted with antiserum to clover yellow vein virus (CYVV). When selected necrotic and non-necrotic isolates were inoculated to differential hosts, all behaved like BYMV and not CYVV. When three isolates of each type were transmitted to 11 other cool season grain legume species, except in Cicer arietinum (chickpea), there were no necrotic reactions, but symptom severity varied with the isolate and species inoculated. The two isolates that caused necrosis in C. arietinum did not do so in L. angustifolius. The six isolates from Vicia faba (faba bean) all caused non-necrotic reactions in L. angustifolius cv. Danja and 90L423-07-13. These and two necrotic isolates readily infected five genotypes of V. faba always causing severe symptoms. However, three non-necrotic isolates from L. angustifolius and a further necrotic isolate were poorly infectious on V. faba in which they generally induced mild symptoms. These results show that at least three strain groups of BYMV can be distinguished by their reactions in different L. angustifolius genotypes, one causing necrosis and death in cv. Danja and 90L423-07-13, one causing necrosis and death in Danja but not 90L423-07-13, and one causing non-nccrotic reactions in both. These strain groups could not be distinguished when representative necrotic and non-necrotic isolates were inoculated to other grain legume species. However, inoculation to V. faba distinguished two other BYMV strain groupings differing in severity of symptoms and ability to infect this species.

Growth and development of plants with potential for use as trap crops for potato cyst nematodes and their effects on the numbers of juveniles in cysts

Abstract: Growth and development of three plant accessions with potential for use as trap crops for potato cyst nematodes (PCN), Solanum sisymbriifolium and two varieties of S. nigrum, were studied under 12 h and 17 h photoperiods. In pot experiments, rate of plant emergence, plant height, and shoot and root mass were greater for the S. nigrum varieties ‘90-4750-188’ and ‘88-4750-061’ than for S. sisymbriifolium and markedly greater than for a S. nigrum variety found as a weed of arable fields in The Netherlands. However, the last mentioned S. nigrum variety produced the most berries. Plant height and shoot weight of all the S. nigrum varieties were greater under the longer photoperiod, whereas the root mass was hardly affected. Plant height and shoot weight of S. sisymbriifolium also were greater under the longer photoperiod but the root weight was less. Under field conditions, with sowing dates from the end of March to mid August, S. sisymbriifolium and S. nigrum ‘90-4750-188’ grew better than S. nigrum ‘88-4750-061’. In contrast to S. nigrum, S. sisymbriifolium appeared resistant to night frosts in autumn. The stubbles of both S. sisymbriifolium and S. nigrum showed good regrowth after cutting the plants 5 or 10 cm above the soil surface 11 wk after sowing. In a pot experiment, all the plant accessions strongly reduced the numbers of juveniles in cysts compared with flax. Tolerance to Globodera rostochiensis of S. sisymbriifolium and S. nigrum‘90-4750-188’ was investigated in pot experiments under glasshouse conditions in sandy soil at pH 4.8 and 6.0. At soil infestation levels ranging from 0 to 56 juveniles ml−1 soil, S. sisymbriifolium appeared much more tolerant than S. nigrum ‘90-4750-188’. Shoot yield of S. nigrum decreased markedly with increasing soil infestation and root weight also decreased, except at pH 4.8 and light infestation levels. Both S. sisymbriifolium and S. nigrum grew better at soil pH 4.8 than 6.0. The proportion of lateral roots in the total root mass increased in both species with increasing PCN infestation and soil pH. However, although the proportion of lateral roots in plants grown at soil pH 6.0 was greater at PCN infestations up to 14 juveniles ml−1 soil, the proportion of laterals in S. nigrum was considerably less at PCN infestations of 56 juveniles ml−1 soil. The proportion of PCN juveniles hatching was similar for the two species and decreased slightly with increasing initial nematode population densities.

Effect of potato used as a trap crop on potato cyst nematodes and other soil pathogens and on the growth of a subsequent main potato crop

Abstract: A field experiment in which main-crop potatoes were grown every other year was conducted on a sandy soil from 1994 to 1999. The aim of the experiment was to control soil-borne pathogens of potato with ecologically sound methods. Potato grown as a trap crop from the end of April to the end of June (8 wk) was used to control potato cyst nematodes (PCN) (Globodera pallida), and its effects on other important soil pathogens and on the growth of a subsequent potato crop were also assessed. Additional experimental treatments were a potato crop from which the haulm was removed and a green manure crop. Three potato cultivars with different degrees of resistance to PCN were grown as the main crop. Duplicate sets of the experiment were run concurrently. The PCN were effectively controlled by the potato trap crop. When a highly resistant potato cultivar was grown as a main crop after the trap crop, the post-harvest soil infestation was very low. When a moderately resistant cultivar was grown after the trap crop the soil infestation also remained low. When the trap crop was alternated with a susceptible potato cultivar as a main crop, soil infestation increased slightly, but the degree of control when compared with no trap crop averaged 96%. Soil infestation with root-knot nematodes (mainly Meloidogyne hapla) increased when potato was grown as a trap crop, but soil infestation with the root-lesion nematode Pratylenchus crenatus was not affected. Stem canker caused by Rhizoctonia solani was not affected by the trap crop but black scurf (sclerotia of R. solani) on tubers was reduced. Soil infestation with Verticillium dahliae declined in one of the duplicate sets of the experiment but not in the other. However, stem infections by V. dahliae were significantly decreased in both sets, although the effect depended on the PCN-resistance level of the potato cultivar. When a highly resistant potato cultivar was grown Verticillium stem infections were not significantly affected, they were decreased with a moderately resistant cultivar but the decrease was most pronounced with a PCN-susceptible cultivar. Senescence of a following potato crop was not influenced by the trap crop when a highly PCN- resistant cultivar was grown, but it was delayed in the case of a moderately resistant or a susceptible cultivar, resulting in higher tuber yields for those cultivars. The experiment proved that a trap crop can be an alternative to chemical soil disinfection but, for several reasons, the potato itself is not an ideal crop for this purpose; a trap crop other than potato must be developed.

Resistance of roses to pathotypes of Diplocarpon rosae

Abstract: Summary Colonies of Diplocarpon rosae derived from single conidia were isolated on malt extract agar, multiplied (at 23°C) and stored (at -20°C) on surface-sterilised leaf discs of a universally susceptible rose, ‘Frensham’. The resistance of 16 species and cultivars of Rosa to different isolates ofD. rosae was assessed using surface-sterilised leaf discs. Four pathotypes of D. vosae were distinguished on the basis of host range. One species and one hybrid were resistant to all pathotypes. Two species and two cultivars were susceptible to all pathotypes. Four species and six cultivars were interpreted as having vertical resistance because they were strongly resistant to some but not all pathotypes. Only species and hybrids of the section Cinnamomeae were resistant to the pathotype identified as CW1 whereas only roses of other origins were resistant to the pathotype DA2.

Evaluation of methods of screening strawberry cultivars for resistance to crown rot caused by Phytophthora cactorum.

Abstract: Summary Four methods were evaluated in measuring resistance of strawberry cultivars to crown rot caused by Phytophthora cactorum. Meristem propagated plants grown in vitro were inoculated with mycelial discs. Four to five days after inoculation, it was possible to distinguish between cultivars with large differences in susceptibility to the disease. Ten days later, all plants were totally necrotic making it impossible to distinguish between cultivars. When detached leaves were inoculated by inoculating a plug of mycelium into the petiole, disease symptoms developed more slowly in resistant cultivars, but leaf age greatly affected the rate of symptom development. When plug plants (not cold stored) were lightly wounded in the rhizome with a scalpel and inoculated with either zoospores or mycelium, differences in disease development between cultivars were mainly as would be expected from previous information on susceptibility, but both age and size of plants influenced the rate of disease development. Unwounded, inoculated plants did not develop symptoms. When cold-stored plug plants were either unwounded or lightly wounded with a scalpel in the rhizome and inoculated with zoospores, the relative rates of disease development consistently reflected the susceptibility to crown rot. At the time of final assessment, disease was much more severe in wounded plants, but the relative susceptibility of cultivars was not affected by the wounding.

Integrated and biological control of gladiolus corm rot and wilt caused by Fusarium oxysporum f.sp. gladioli

Abstract: Summary An isolate of Trichoderma virens Miller, Giddens & Foster, carboxin and a combination of both were evaluated for the control of gladiolus corm rot and wilt caused by Fusarium oxysporum f.sp. gladioli in glasshouse and field experiments. All treatments significantly reduced disease incidence in both glasshouse and field conditions. T. virens gave control at least as good as carboxin in all experiments. Control was significantly improved in two field experiments by combining the biological and chemical treatments.