Volume 1 Number 2 2007

CONTENTS AND ABSTRACTS

Beatriz M. Diaz, Alberto Fereres (Spain) Ultraviolet-Blocking Materials as a Physical Barrier to Control Insect Pests and Plant Pathogens in Protected Crops (pp 85-95)

Full Text [PDF]

Invited Mini-Review: New types of ultraviolet (UV)-blocking materials, such as polyethylene films and nets, have been developed in recent years as a very promising tool to control insect vectors of plant disease threatening greenhouse crops. UV-blocking materials have properties to filter the UV radiation (280-400 nm) interfering with the vision of insects and in consequence, their behaviour related with movement, host location ability and their population parameters. The exclusion of part of the UV radiation within the greenhouse environment has a dramatically incidence on insect orientation, movement and on the spread of insect-transmitted viral diseases. In the same way, the impact of UV-absorbing materials on population dynamics of natural enemies, pollinators and crop yield needs further investigation. The level of protection of the different UV-blocking materials may vary among different designs of greenhouses and the geographic location that determines different internal climatic conditions and the amount of UV and visible light absorbed and transmitted within the covered structures. In this review, we will discuss the current knowledge about the impact of UV-absorbing films on insect pests, plant pathogens and beneficial organisms, with special attention to insects acting as vectors of plant disease. Also, the new perspectives, limitations and advantages of using UV-blocking materials together with other control strategies under Integrated Pest Management (IPM) production systems will be discussed.

Ellen De Geyter, Ellen Lambert, Danny Geelen, Guy Smagghe (Belgium) Novel Advances with Plant Saponins as Natural Insecticides to Control Pest Insects (pp 96-105)

Full Text [PDF]

Invited Mini-Review: Saponins are a class of secondary plant metabolites with diverse biological properties. They occur in a great number of plant species (mainly Angiosperms), both wild plants and cultivated crops. Triterpene saponins are mostly found in dicotyledonous species, while many of the major steroidal saponins are synthesized by monocots, such as members of the Liliaceae, Dioscoraceae and Agavaceae families. Many legumes contain saponins, such as soya beans, beans, peas, tea, spinach, sugar beet and quinoa, oats, capsicum peppers, aubergine, tomato seed, alliums and asparagus. Saponins possess clear insecticidal activities: they exert a strong and rapid-working action against a broad range of pest insects that is different from neurotoxicity. The most observed effects are increased mortality, lowered food intake, weight reduction, retardation in development and decreased reproduction. According to the main hypotheses in literature, saponins exert a repellent/deterrent activity, bear digestive problems, provoke insect moulting defects or cause cellular toxicity effects. As a consequence these interesting plant components open new strategies to protect crops in modern agriculture and horticulture with integrated pest management (IPM) programs against pest insects, either by spraying, or by selecting high-saponin varieties of commercial crops.

Eduardo S.G. Mizubuti, Valdir Lourenço Júnior (Brazil), Greg A. Forbes (Peru) Management of Late Blight with Alternative Products (pp 106-116)

Full Text [PDF]

Invited Review: Controlling highly destructive plant diseases such as potato and tomato late blight, caused by the oomycete Phytophthora infestans, with non-fungicidal alternative products is a difficult but necessary task that needs to be accomplished. Given the rapid development of potato and tomato late blight epidemics, for many years control strategies relied solely upon the application of fungicides. There are many reports of alternative strategies for managing several plant diseases, however reports on late blight control with non-fungicidal products are recent and appeared in the last 20 years. The most commonly used strategy to control the disease has been the prevention of establishment of P. infestans in the host plant, mainly by using organisms capable of producing chemical compounds that inhibit spore germination. Nevertheless, the epidemiological characteristics of late blight makes the adoption of a “silver bullet approach” risky. To enhance the chances of success of alternative products, a combination of compounds and microorganisms with different modes of action should be employed beginning at the early stages of the host-pathogen interaction. Specifically, when effective options are available, one should consider the use of phylloplane and endophytic organisms combined with resistance induction mediated by plant growth promoting rhizobacteria. Late blight control with alternative products is likely to positively impact both conventional and organic production systems.

Adrián A. Vojnov (Argentina), J. Maxwell Dow (Ireland), Kamal Bouarab (Canada) Recent Progress in Understanding the Roles of DSF-regulated Virulence Factors in Xanthomonas campestris Pathogenicity (pp 117-126)

Full Text [PDF]

Invited Mini-Review: Xanthomonas campestris pathovar campestris (Xcc) is the causal agent of black rot disease of cruciferous plants. A cell-cell signaling system encoded by genes within the rpf cluster is required for the full virulence of this plant pathogen. This system has been implicated in regulation of production of extracellular enzymes, cyclic glucan and the exopolysaccharide xanthan and in the regulation of biofilm formation in Xcc. Cell-cell communication is mediated by the diffusible signal factor (DSF), an unsaturated fatty acid whose synthesis requires RpfF and RpfB. Here we review current progress on our understanding of DSF signal transduction and of the roles of xanthan, cyclic glucan and biofilm development in the interaction of Xcc with plants. Recent observations have shown that the perception of the DSF signal requires the sensor kinase RpfC and is linked to the degradation of the intracellular second messenger cyclic di-GMP by the HD-GYP domain regulator RpfG. The mechanisms by which cyclic di-GMP exerts its regulatory influence on xanthan, cyclic glucan and biofilm formation remain obscure however. It is now established that DSF signaling has to be finely balanced for the formation of structured biofilms in static cultures in minimal medium and for virulence to plants. New roles for xanthan and cyclic glucan in disease through suppression of plant immune responses have been uncovered. Xanthan induces susceptibility to Xcc in Nicotiana benthamiana and Arabidopsis thaliana by suppressing callose deposition. Unlike xanthan, which acts only locally, the effects of cyclic glucan on plant defense suppression and callose deposition occur in a systemic fashion. These advances contribute to the increased understanding of the molecular basis of bacterial disease, which is a major aim in the post-genomic era in plant-bacterial interactions.

Aly A. Aly, Mohamed A. Abdel-Sattar, Moawad R. Omar, Kamel A. Abd-Elsalam (Egypt) Differential Interaction between Isolates of Macrophomina phaseolina and Egyptian Cotton Cultivars (pp 127-132)

Full Text [PDF]

Original Research Paper: Pathogeniciy of 20 isolates of Macrophomina phaseolina was tested on six cotton cultivars under greenhouse conditions. Preemergence damping-off, postemergence damping-off, survival, plant height, and dry weight were used as criteria to evaluate pathogenicity. Analysis of variance showed that the main effects of both cultivars (p=0.0001) and isolates (p=0.0001) were very highly significant sources of variation in all the tested parameters as was cultivar × isolate interaction (p=0.0001). Statistically significant cultivars, isolates, these significant main effects and interactions suggest that physiologic specialization exists within M. phaseolina isolates pathogenic on cotton. It also implies that the resistance of the tested cultivars is a mixture of both vertical and horizontal resistance and there are significant differences among cultivars in both types of resistance. Similarly, pathogenicity of the tested isolates is also mixture of virulence and aggressiveness, and the isolates significantly differ in both types of pathogenicity. Cluster analysis differentiated the isolates into 4 pathotypes based on their virulence on the 6 cotton cultivars.

S. Nithya Meenakshi, P.R. Jeyaramraja, Rajesh Manian (India) Degradation of the Fungicides, Azoxystrobin and Difenoconazole in Soil and their Influence on Soil Microbial Activity (pp 133-138)

Full Text [PDF]

Original Research Paper: The non-target effects of Amistar^TM (azoxystrobin) and Score^TM (difenoconazole) were studied in terms of changes in the soil microbial populations, soil respiration and activity of soil enzymes. Both the fungicides significantly reduced the fungal population in the soils even at the lowest concentration used (0.44 mg. (a.i.) g^-1 soil) and totally eliminated it at the recommended dose for field application (2.2 mg. (a.i.) g^-1 soil). On the other hand, the populations of bacteria and actinomycetes in the treated soil increased with increasing concentrations of the fungicides. Soil respiration decreased significantly with increasing concentrations of Azoxystrobin and Difenoconazole. However, there was an increase in the rate of soil respiration when up to 0.44 mg. (a.i.) g^-1 soil of Difenoconazole was applied. The activities of soil enzymes cellulase, xylanase and protease increased with increasing concentrations of the fungicides Azoxystrobin and Difenoconazole. On the other hand, the activities of urease and acid phosphatase decreased with the application of these fungicides. Both the rate of soil respiration and the activities of the tested soil enzymes reached the same level as controls after prolonged incubation. Degradation of Azoxystrobin and Difenoconazole in two different soils was estimated using HPLC. Azoxystrobin was degraded by 95% in a period of 96 hours in the slightly alkaline Coimbatore soil, whereas in the acidic Valparai soil, the degradation was only 70% during the same period. Difenoconazole, however, was quickly degraded in both Coimbatore (95%) and Valparai (97%) soils after 96 hours.

S. Nithya Meenakshi, S. Manian, P.R. Jeyaramraja (India) Influence of Azoxystrobin and Difenoconazole on N₂-Fixing and Antagonistic Organisms (pp 139-144)

Full Text [PDF]

Original Research Paper: The non-target effects of Amistar^TM (azoxystrobin) and Score^TM (difenoconazole) on soil microbial populations were expressed in terms of changes in the microbial populations and effects on symbiotic associations. The effects of Azoxystrobin and Difenoconazole on the growth of the free-living N₂-fixing bacteria Azospirillum chroococcum and Azotobacter brasilense and the antagonistic microbes Pseudomonas flurorescens and Trichoderma spp. were assessed following disc diffusion and poisoned food methods. At or below the recommended doses, the fungicides did not inhibit all these microorganisms. At higher concentrations, the sensitivity varied according to the organism and fungicide. The extent of rhizobial root nodulation and arbuscular mycorrhizal (AM) root colonization as influenced by the foliar spray with different concentrations of Azoxystrobin and Difenoconazole was studied in Arachis hypogea Linn. Foliar spray of both the fungicides at recommended (2.2 mg (a.i.) ml^-1) or lower doses generally enhanced root nodulation. Higher concentrations, however, resulted in marked decreases in the number of root nodules. A similar trend was observed in terms of AM root colonization and incidence of arbuscules in plants treated with Azoxystrobin. On the other hand, Difenoconazole spray significantly decreased both these parameters with an increasing concentration, from the lowest concentration (0.44 mg. (a.i.) ml^-1). The vesicular structures were however not significantly altered by different fungicide concentrations.

D.E. Okwu, A.N. Awurum, J.I. Okoronkwo (Nigeria) Phytochemical Composition and In Vitro Antifungal Activity Screening of Extracts from Citrus Plants against Fusarium oxysporum of Okra Plant (Hibiscus esculentus) (pp 145-148)

Full Text [PDF]

Original Research Paper: Phytochemical studies of five varieties of citrus species, sweet orange (Citrus sinensis), tangerine (Citrus reticulata), lemon (Citrus limonum), lime (Citrus aurantifolia) and grape (Citrus vitis) revealed the presence of bioactive compounds comprising alkaloids (0.22-1.60%), saponin (0.30-0.98%), flavonoids (0.30-0.89), phenols (0.02-0.64%) and tannins (0.23-1.45%). The growth of Fusarium oxysporum which causes damping-off diseases of okra (Hibiscus esculentus) was inhibited in vitro by the extracts of citrus species. The extracts from the peels of C. sinensis, C. aurentifolia and C. reticulata showed 83.55%, 71.10% and 68.14% inhibition activity, respectively. An analysis of chemical composition showed that the most active geranoxycumarine, triclosan, benzetonine, limonin and nomilin contained in grapefruit (Citrus vitis) and sweet orange (C. sinensis) have high alkaloids and phenolic phytoconstituents. The fungitoxicity of the extracts from the peels of C. sinensis was the same as that of benomyl, a synthetic fungicide.