Anoda cristata control with glyphosate in narrow- and wide-row soyabean

Experiments evaluated the effect of glyphosate rate and Anoda cristata density, on crop and weed biomass and weed seed production in wide (70 cm) and narrow rows (35 cm) glyphosate‐resistant soyabean (Glycine max). Soyabean density was higher at 35 cm row spacing as an increase in planting rate in narrow‐row soyabean is recommended for producers in Argentina. Soyabean biomass at growth stage V4 (four nodes on the main stem with fully developed leaves beginning with the unifoliate leaves) was higher when grown on narrow than in wide‐rows but was not affected by the presence of A. cristata. At growth stage R5 (seed initiation – seed 3 mm long in a pod at one of the four uppermost nodes on the main stem, with a fully developed leaf and full canopy development), crop biomass was greater in narrow rows compared with wide rows with 12 plants m^?2 of A. cristata. In narrow‐row soyabean, a single application of a reduced rate of glyphosate maintained soyabean biomass at R5 and provided excellent weed control regardless of weed density. In wide‐row soyabean control was reduced at the high weed density. Regardless of row spacing, A. cristata biomass and seed production were severely reduced by half of the recommended dose rate of glyphosate but the relationship between biomass and seed production was not altered. Glyphosate rates as low as 67.5 g a.e. ha^?1 in narrow rows or 540 g a.e. ha^?1 in wide rows provided excellent control of A. cristata. To minimize glyphosate use, planting narrow‐row soyabean are effective where A. cristata density is low.     

Visualization of the effect of a surfactant on the uptake of xenobiotics into plant foliage by confocal laser scanning microscopy

A radiolabelling method is generally used to determine the foliar uptake of xenobiotics. This technique cannot provide any information on the localization of chemicals inside leaf tissues. The influence of an alcohol ethoxylate surfactant on the uptake of three fluorescent dyes, difluorofluorescein (hydrophilic), rhodamine B (moderately lipophilic) and a naphthalimide dye (lipophilic), into the leaves of three contrasting species, bean (Vicia faba), wheat (Triticum aestivum) and cabbage (Brassica oleracea), at 16 h after treatment was measured using a conventional wash‐off method and also visualized in vivo by confocal laser scanning microscopy (CLSM). Whereas the lipophilic dyes showed greater intrinsic uptake than the hydrophilic one, the enhancing effect of the surfactant on uptake was more pronounced for the latter. CLSM revealed that the presence of the surfactant increased the transport of difluorofluorescein into the epidermal cells of bean and wheat leaves, but not cabbage leaves. Rhodamine B showed greatest transcuticular diffusion in all three species, but most of the dye moved into the vacuole of the epidermal cells. The naphthalimide dye was strongly retained by the wax–cuticle layer of all species, even in the presence of the surfactant. CLSM has proven to be an attractive tool for studying xenobiotic diffusion. The results obtained using fluorescent dyes are believed to be applicable to the foliar uptake of herbicides.     

The effect of non-inversion tillage and light availability on dispersal and spatial growth of Cynodon dactylon

The invasive ability of Cynodon dactylon is dependent on self dispersal and on cultivation practices. Tillage can seriously change patch biomass and spatial structure, spreading vegetative propagules of the weed. The objectives of this study were: (i) to quantify the effect on non‐inversion tillage on dispersal, establishment and colonization of C. dactylon and (ii) to propose a simple model considering soil cultivation effects and light availability on spatial growth of weed patches. Two experiments were carried out, exploring different soils and environmental conditions. Spatial distribution of vegetative units differed when tillage was conducted with different non‐inversion implements and could be described by simple functions. A minimum patch biomass seems necessary before vegetative structures are vulnerable to movement by cultivation. Only a small proportion of the biomass dispersed from original patches was able to establish. However, simulation showed that the area colonized by C. dactylon mostly increased by means of tillage dispersal, both with and without crop competition, in one growing cycle. It appears sensible to consider changing cultivation practices to reduce weed dispersal and to use crop competition for light to create unsuitable habitats limiting weed colonization.     

Yeasts as antagonists against fireblight

Yeasts are potential antagonists of microorganisms in the phyllosphere. Due to their osmotolerance, they should also be able to colonize apple flowers. In field experiments, we applied yeast agents against fireblight at two different sites in the southern part of Germany. They showed efficiencies 0–20% below Plantomycin (streptomycin). Co-culture experiments in liquid basal media with synthetic nectar resulted in suppression of Erwinia amylovora by yeast. This effect could not be confirmed with population studies of yeasts and E. amylovora in flower clusters. Field and laboratory experiments indicated that yeasts have antagonistic properties against fireblight but further research is needed to investigate this potential.     

Détection moléculaire spécifique de la région vir du plasmide pTi d'Agrobacterium tumefaciens dans les sols et plants au Maroc

Le meilleur moyen de contrôle du crown gall ( Agrobacterium tumefaciens ) est la prévention et donc la détection de la bactérie pathogène dans les sols prévus pour les pépinières. Pour cela nous avons essayé une approche moléculaire utilisant la PCR pour caractériser le plasmide pTi par le couple des amorces spécifiques F14/F44 dans les sols de diverses régions marocaines. Cette technique de détection représente un indicateur potentiel d'avertissement sur l'état d'infection des sols et plants par A. tumefaciens qui permet ainsi de déclencher la lutte biologique par le NoGall, produit commercial australien, hébergeant l'antagoniste K₁₀₂₆.     

Isolation of two strains of a new Tombusvirus (Havel river virus, HaRV) from surface waters in Germany

Two virus isolates from water samples — one from a small stream in South Western Germany and another one from the Havel river in North Eastern Germany c. 500 km away, proved to be strains, named S and H, respectively, of a new Tombusvirus for which the name Havel river virus (HaRV) had been suggested previously in a brief account. Immunoelectron microscopical decoration tests and sequence comparisons of the coat proteins indicated that the two HaRV strains are only distantly related to known Tombusviruses. The closest relationships were found to Cucumber necrosis virus. Nothing is known about their natural hosts. Because the S strain of HaRV was isolated in a woody area from a small stream close to its origin, they may be pathogens of trees or wild plants in such habitats.     

Footsteps from Insect Larvae Damage Leaf Surfaces and Initiate Rapid Responses

Plant resistance to insect herbivory involves gene expression in response to wounding and the detection of insect elicitors in oral secretions (Kessler and Baldwin, 2002, Ann. Rev. Plant/ Biol. 53: 299–328). However, crawling insect larvae stimulate the synthesis of 4-aminobutyrate within minutes and imprints of larval footsteps can be visualized within seconds through superoxide production or transient increases in chlorophyll fluorescence (Bown et al., 2002, Plant Physiol. 129: 1430–1434). Here cryo-scanning electron microscopy was used to demonstrate that larval feet, which are equipped with a perimeter row of hook-like crochets, damage leaf tissue and result in larval footprints. Staining for cell death shows that areas of wounding correspond to footsteps detected through increased chlorophyll fluorescence. Superoxide production in response to footsteps was inhibited by diphenyleneiodonium, an inhibitor of the plasma membrane NADPH oxidase enzyme. Inhibition of superoxide production, however, did not eliminate the detection of cell death. The results demonstrate that larval footsteps damage leaf tissue, and initiate rapid local responses which are not dependent on herbivory or oral secretions. It is proposed that superoxide production at the wound site prevents opportunistic pathogen infection.     

The salicylic acid-dependent defence pathway is effective against different pathogens in tomato and tobacco

The role of salicylic acid (SA) was investigated in basal defence and induced resistance to powdery mildew ( Oidium neolycopersici ) and grey mould ( Botrytis cinerea ) in tomato ( Lycopersicon esculentum ) and tobacco ( Nicotiana tabacum ). A comparison of NahG transgenic tomato and tobacco (unable to accumulate SA) to their respective wild types revealed that in tomato, SA was not involved in basal defence against O. neolycopersici but NahG tobacco showed an enhanced susceptibility to O. neolycopersici infection, the effect becoming more obvious as the plants grew older. In contrast, SA played no role in the basal defence of tobacco against B. cinerea , but seemed to contribute to basal defence of tomato against B. cinerea. Activation of the SA-dependent defence pathway via benzothiadiazole (BTH) resulted in induced resistance against O. neolycopersici in tobacco but not in tomato. Microscopic analysis revealed that BTH treatment could prevent penetration of the Oidium germ tube through tobacco leaves, whereas penetration was successful on tomato leaves irrespective of BTH treatment. In contrast, soil or leaf treatment with BTH induced resistance against B. cinerea in tomato but not in tobacco. It is concluded that the SA-dependent defence pathway is effective against different pathogens in tomato and tobacco.     

Growth- and leaf-temperature effects on photosynthesis of sweet orange seedlings infected with Xylella fastidiosa

The effects of growth and leaf temperature on photosynthesis were evaluated in sweet orange seedlings ( Citrus sinensis cv. Pera) infected with Xylella fastidiosa (the bacterium that causes citrus variegated chlorosis, CVC). Measurements of leaf gas exchange and chlorophyll  a fluorescence were taken at leaf temperatures of 25, 30, 35 and 40°C in healthy and infected (without visible symptoms) seedlings submitted to two temperature regimes (25/20 or 35/20°C, day/night), not simultaneously. The CO₂ assimilation rates ( A ) and stomatal conductance ( g _s) were higher in healthy plants in both temperature regimes. Values for A and g _s of infected and healthy plants were higher in the 35/20°C regime, decreasing with leaf temperature increase. In addition, differences between healthy and infected plants were higher at 35/20°C, while no differences in chlorophyll  a fluorescence parameters were observed except for potential quantum efficiency of photosystem II, which was higher in infected plants. Low A values in infected plants were caused by low g _s and probably by biochemical damage to photosynthesis. The high alternative electron sink of infected plants was another effect of reduced A . Both high growth and high leaf temperatures increased differences in A between healthy and infected plants. Therefore this feature may be partially responsible for lower growth and/or productivity of CVC-affected plants in regions with high air temperature.     

Aggressiveness and host specificity of Brazilian isolates of Phytophthora infestans

The population of Phytophthora infestans in Brazil consists of two clonal lineages, US-1 associated with tomatoes and BR-1 associated with potatoes. To assess whether host specificity in these lineages resulted from differences in aggressiveness to potato and tomato, six aggressiveness-related epidemiological components – infection frequency (IF), incubation period (IP), latent period (LP), lesion area (LA), lesion expansion rate (LER) and sporulation at several lesion ages (SSLA) – were measured on detached leaflets of late blight-susceptible potato and tomato plants. Infection frequency of US-1 was similar on potato and tomato leaflets, but IF of BR-1 was somewhat reduced on tomato. Incubation period was longer on both hosts with US-1, although this apparent lineage affect was not significant. Overall there was no host effect on IP. On potato, BR-1 had a shorter LP (110·3 h) and a larger LA (6·5 cm²) than US-1 (LP = 162·0 h; LA = 2·8 cm²). The highest LER resulted when isolates of BR-1 (0·121 cm² h⁻¹) and US-1 (0·053 cm² h⁻¹) were inoculated on potato and tomato leaflets, respectively. The highest values of the area under the sporulation capacity curve (AUSC) were obtained for isolates of US-1 inoculated on tomato leaflets (6146) and for isolates of BR-1 on potato leaflets (3775). In general, higher values of LA, LER, SSLA and AUSC, and shorter values of LP were measured when isolates of a clonal lineage were inoculated on their original host than with the opposite combinations. There is evidence that there are quantitative differences in aggressiveness components between isolates of US-1 and BR-1 clonal lineages that probably contribute to host specificity of P. infestans populations in Brazil.