Unsprayed Plots Research Articles

A Comparison of the Infestation ofAproaerema simplexella(Walker) on Groundnut and other Known Hosts forAproaerema modicella(Deventer) (Lepidoptera: Gelechiidae)

A groundnut pest occurring in South Africa and widely reported as Aproaerema modicella (Deventer) has recently been identified by mtDNA CO1 analysis as Aproaerema simplexella (Walker), a strain native to Australia. Two experiments were conducted during 2011–2012 growing season at five sites in South Africa. The first experiment examined the preference of A. simplexella between groundnut, soya bean, lucerne, pigeon pea and lablab (common known host crops for A. modicella). The second experiment examined the effect of cypermethrin on the control of the pest on soya bean and groundnut at two sites. Among the host crops tested, soya bean was the most preferred by the pest followed by groundnut at all sites. The pest was also observed on pigeon pea at all sites, but the infestation was very low. Lucerne was observed as the less preferred host for this species. No infestation was observed on lablab bean across the sites. Spraying cypermethrin in groundnut and soya bean reduced infestation in both crops to very low levels. In the unsprayed plots, the high infestation levels significantly reduced crop yields.

Effects of Preceding Cowpea on the Performance of Maize in Cowpea-Maize Sequential Cropping

Abstract Field experiment was conducted at the Federal University of Agriculture in 2004/2005, 2005/2006 and 2006/2007 cropping seasons to investigate the effects of cowpea varieties on succeeding of maize crop. The experiment was laid out in split-plot design and the treatment replicated three times. The main plot treatment was sprayed and unsprayed cowpea (Vigna unguculata L.) while variety constituted the subplot treatment (IT90K-76, IT90K-277-2, Drum, Olo, Oloyin, Mallam and Sokoto varieties). Maize variety cv TZESR-W was planted as the test crop in the early cropping season of 2005, 2006 and 2007 on each subplot of the preceding cowpea. The biomass of cowpea in the spray plots were higher than those of unsprayed at 8, 10 WAP in 2004. Olo variety had significantly lower biomass compared to others in 2004. The grain yield of cowpea from the sprayed plots was significantly higher than the unsprayed plots in all the years. IT90K-76 variety had the highest grain yield whereas Mallam and Drum had the lowest in all the years. Maize grain yields from the preceding cowpea plots were significantly higher than that of 0 N kg/ha. The fertilizer equivalent of the preceding varieties of cowpea ranges between 24 and 38 N kg/ha. Thus, preceding cowpea enhances the performance of succeeding maize.

The Effects of Extracts of Lantana camara (L.) and Azadirachta indica (A. Juss) on the Population Dynamics of Plutella xylostella, Brevicoryne brassicae and Hellula undalis on Cabbage

<p>The effects of ethanolic extract of neem, <em>Azadirachta indica</em> (Meliaceae), seeds and petroleum ether extract of <em>Lantana camara</em> leaves (Verbenaceae) on the populations of three cabbage pests, <em>Plutella xylostella</em>, <em>Brevicoryne brassicae</em> and <em>Hellula undalis</em> were studied. The study was conducted between January and April 2008. Extracts of the two plants were sprayed on cabbage plants to control these pests. A standard synthetic chemical insecticide (Mektin) was used as reference product. The experiment was conducted in a randomized complete block design and each treatment was replicated four times. The effects of plant extracts on the population dynamics of the pests’ species, the level of infestation and yield were assessed. Significantly more of the pests infested the control plants than the treated plants (<em>P</em>< 0.01). The mean weight of cabbage heads on the sprayed plots was significantly heavier than that of the control unsprayed plots. The use of <em>A. indica</em> seeds and <em>L. camara</em> leaf extracts increased yield by 37.05% and 25.80%, respectively. Spraying the cabbage plants with the plant extracts significantly reduced the numbers of pests compared with the control plants. The use of these plant extracts can be incorporated into an overall control programme of these pests.</p>

Yield Response to Pea (Pisum sativum L.) Genotype, Population and Sowing Date

This research objective was to examine the effect of herbicide, genotype, population and sowing date on crop yield and weed growth in Pisum sativum. In 2007/08, cyanazine treated peas had a mean seed yield of 508 g m -2 , 19% more than in unsprayed plots. There was a significant sowing date by pea genotype interaction which showed that in the August sowing genotype had no effect on seed yield. However, in September Pro 7035 yielded 559 g m -2 , which was 40% more than Midichi. By the October sowing, it was 87% more. There was a distinct variation in weed spectrum, over time. It can be concluded that fully leafed peas and semi-leafless can be sown at similar plant populations and give similar yields under weed free conditions and that increased pea sowing rates increased total dry matter and seed yield in weedy environments. Fully leafed peas yielded more than semi-leafless peas when both were late sown. Increased pea sowing rate improved weed suppression.

Microbial control of cotton pests. Part I: Use of the naturally occurring entomopathogenic fungus Aspergillus sp. (BC 639) in the management of Creontiades dilutus (Stal) (Hemiptera: Miridae) and beneficial insects on transgenic cotton crops

The development and adoption of transgenic (Bt) crops that express the Bacillus thuringiensis (Bt) toxin has reduced the use of synthetic insecticide on transgenic crops to target Helicoverpa spp., the major insect pest of cotton in Australia. However, it has also increased the threat posed by sucking pests, particularly Creontiades dilutus (green mirid), which are unaffected by the Bt toxins in transgenic cotton crops. Here we report the efficacy of the entomopathogenic fungus Aspergillus sp. (BC 639) in controlling the infestation of transgenic cotton crops by C. dilutus and promoting interactions of transgenic cotton with beneficial insects. The results showed that the number of C. dilutus adults and nymphs recorded on plots treated with 1000, 750, 500, 250 ml/ha BC 639 fungus formulation were the same as on plots treated with the recommended concentration of the commercial insecticide Fipronil. The fungus was found to have minimal effect on predatory insects compared with Fipronil and was most effective against C. dilutus when applied at the rate of 500 ml/ha (equivalent to 50 g spores/ha). At this rate, the fungus was as effective as Fipronil for controlling C. dilutus populations and ensured the survival of predatory beetles, lacewings and spiders compared with Fipronil treatment. The yield from fungus-treated plots was 5.24 bales per acre compared with 5.40 and 3.88 bales per acre for Fipronil-treated and unsprayed plots, respectively. The ability of the BC 639 strain to control C. dilutus infestations of transgenic cotton crops while conserving beneficial insect populations suggests its potential for supplementing integrated pest management programs to reduce the use of synthetic insecticides for transgenic cropping systems.

Reduced fungicide use on a new Australian peanut cultivar, highly resistant to the late leaf spot and rust pathogens

Rust (caused by Puccinia arachidis) and late leaf spot (LLS, caused by Mycosphaerella berkeleyi) can cause significant yield losses in Australian peanut crops. Until recently, all commercial peanut varieties were highly susceptible to these pathogens, but the new Australian cultivar Sutherland has significantly higher levels of resistance than the older cultivars. Field trials were conducted at two sites in Queensland to (a) confirm the improved resistance of cv. Sutherland over another commercial cultivar, Menzies, (b) study the effects of timing of first spray, spray interval and cultivar on disease severity and yield, and (c) develop a suitable fungicide management program for cv. Sutherland. In the 2006 and 2007 trials, rust and LLS developed slower and had lower final disease ratings and AUDPC values on unsprayed plots of cv. Sutherland than on cv. Menzies. The timing of the first spray is critical in managing both rust and late leaf spot, with the results demonstrating that the first fungicide spray on cv. Sutherland should be applied as soon as rust and LLS are first seen on cv. Menzies. In most trials spray intervals of 14 days or 21 days were suitable to effectively control rust and LLS. In years with low disease pressure, few, if any, fungicide applications will be needed to manage the diseases, but in other years up to four sprays may be necessary.

Management of white mold in type III common bean with plant spacing and fungicide

White mold is a yield-limiting disease during the fall-winter season in southeastern Brazil when irrigated type III common beans are generally sown 0.5 m apart with 10 to 12 plants per meter. The aim of this study was to evaluate the efficacy of combining increased row width (RW) and reduced planting density (PD), with or without fungicide, for white mold management. Treatments were arranged as a 23 factorial: RW (0.50 or 0.75 m), PD (6 or 12 plants per meter), and fungicide (sprayed or unsprayed). Two trials were conducted in Vicosa, State of Minas Gerais. In 2002, the average incidence of white mold was 43.2%, the severity index, 31.1%, and the yield, 2513 kg ha-1. In 2003, the values of these variables were 48.0%, 22.6%, and 2159 kg ha-1, respectively. Interactions involving both RW and PD were not significant for either disease intensity or yield in the combined analysis across years. Increasing RW led to reduction in white mold intensity in 2002. The lower PD reduced disease incidence in 2002 and did not affect yield in the combined analysis. In fungicide sprayed plots, wide RW decreased yield in 2002, but RW did not affect yield in 2003. In unsprayed plots, RW did not affect yield in both years. We conclude that increasing RW to 0.75 m combined with low PD is a promising strategy for white mold management when fungicide is not applied. When fungicide is applied, the current row width (0.50 m) combined with low PD maximize the yield.

Forty years of spruce–fir stand development following herbicide application and precommercial thinning in central Maine, USA

We examined the development of a 33-year experiment in spruce–fir stands that received nine herbicide treatments (applied aerially in 1977), with and without precommercial thinning (PCT) (applied in 1986). We tested two commonly held assumptions about the long-term effects of herbicide and PCT in mixedwood stands managed for softwoods: (i) herbicide release produces stands dominated by softwoods and (ii) PCT promotes both softwood dominance and merchantable volume. All herbicides were effective at releasing balsam fir ( Abies balsamea (L.) Mill.) and spruce ( Picea spp.) from overtopping shrubs and hardwoods that had naturally regenerated following clearcut harvesting in 1970. Glyphosate (Roundup), triclopyr amine (Garlon 3A), 2,4,5-T, and a mixture of 2,4,5-T and 2,4-D herbicide treatments were all effective at shifting long-term species composition to softwood dominance after 30 years, irrespective of the rates applied. Herbicide-treated stands that did not receive PCT were overstocked with softwoods for at least 24 years and produced less than half of the merchantable softwood volume by 40 years than herbicide-treated stands that received PCT. Intolerant hardwoods dominated untreated stands for all 33 years of the experiment. When unsprayed plots received PCT, however, they produced stands at 40 years that were compositionally and structurally indistinguishable from those that had received both herbicide and PCT treatments. Results from this experiment clearly demonstrated that early herbicide application, regardless of type and rate of herbicide, created softwood-dominated mixedwood stands over the long term and that PCT more than doubled merchantable softwood volume within 25 years of application.

Can Imazapic Increase Native Species Abundance in Cheatgrass (Bromus tectorum) Invaded Native Plant Communities?

Native plant communities invaded by cheatgrass (Bromus tectorum L.) are at risk of unnatural high intensity fires and conversion to cheatgrass monocultures. Management strategies that reduce cheatgrass abundance may potentially allow native species to expand and minimize further cheatgrass invasion. We tested whether the selective herbicide imazapic is effective in reducing cheatgrass and “releasing” native species in a semiarid grassland and shrub steppe in north-central Oregon. The experiment consisted of a completely randomized design with two treatments (sprayed with 70 g ai · ha−1 of imazapic and unsprayed) and three replicates of each treatment applied to either 2.5 or 4 ha plots. We repeated this experiment in three different sites dominated by the following native species: 1) bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Löve ssp. spicata) and needle and thread (Hesperostipa comata [Trin. & Rupr.] Barkworth), 2) needle and thread and Sandberg bluegrass (Poa secunda J. Presl), and 3) big sagebrush (Artemisia tridentata Nutt.). Nested frequency of all plant species in 1-m2 quadrats was collected for 1 yr pretreatment and 4 yr posttreatment. In all three sites, cheatgrass frequencies were significantly lower in sprayed plots than unsprayed plots for 3–4 yr posttreatment (P < 0.1). Other annual plant species were also impacted by imazapic, but the effects were highly variable by species and site. Only two native perennial species, hoary tansyaster (Machaeranthera canescens [Pursh] Gray) and big sagebrush, increased in sprayed plots, and increases occurred only at two sites. These results suggest that a short-term reduction in cheatgrass alone is not an effective strategy for increasing the abundance of most native perennial plant species. Las comunidades de plantas nativas que están invadidas del pasto cheatgrass (Bromus tectorum L.) están en riesgo de fuego intencionales de alta intensidad y convertirse en monocultivos de éste pasto. Estrategias de manejo que reduzcan la abundancia del pasto cheatgrass podrían tener el potencial de permitir que especies nativas se expandan y minimicen la posible invasión por el cheatgrass. Probamos sí el herbicida Imazapicis que es efectivo en reducir el pasto cheatgrass y “liberar” especies nativas en pastizales semiáridos y matorral estepario en la parte centro-norte de Oregon. El experimento consistió en un diseño completamente al azar con dos tratamientos (asperjar con 70 g ia · ha−1 de Imazapic y sin asperjar) y tres repeticiones por cada tratamiento aplicado ya sea a parcelas de 2.5 o 4 ha. Repetimos el experimento en tres diferentes sitios dominados por las siguientes especies nativas: 1) (Pseudoroegneria spicata [Pursh] A. Löve ssp. spicata) y (Hesperostipa comata [Trin. &Rupr.] Barkworth), 2) (Hesperostipa comata [Trin. &Rupr.] Barkworth) y (Poa secunda J. Presl), y 3) Artemisa (Artemisia tridentata Nutt.). Se recolectaron todas las especies en un cuadrante de 1 m2 en frecuencia anidada por un año como pre-tratamiento y cuatro años como post-tratamiento. En todos los sitios las frecuencias de pasto cheatgrass fueron significativamente más bajas en las parcelas asperjadas que en las parcelas de control en tres de cuatro años de post-tratamiento (P < 0.1). Otras especies anuales fueron impactadas por Imazapic pero los efectos fueron muy variables por especie y sitio. Solo dos especies perennes nativas (Machaeranthera canescens[Pursh] Gray) y la Artemisa aumentaron en solo dos de las parcelas asperjadas. Estos resultados sugieren que en el corto plazo la reducción de pasto cheatgrass por sí solo no es una estrategia efectiva para aumentar la abundancia de muchas especies de plantas nativas.

Interaction between host plant resistance and biological activity of Bacillus thuringiensis in managing the pod borer Helicoverpa armigera in chickpea

The legume pod borer Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) has developed high levels of resistance to conventional insecticides, and therefore, efforts are being made to develop transgenic chickpea expressing toxin genes from the bacterium Bacillus thuringiensis ( Bt) for controlling this pest. However, there is an apprehension that acid exudates in chickpea might interfere with the biological activity of Bt. Therefore, we studied the biological activity of Bt (Biolep R) on four chickpea genotypes with different levels of resistance to H. armigera under field conditions, and by incorporating lyophilized leaf and pod tissue into the artificial diet with and without Bt. The pH of the acid exudates varied from 2.1 to 2.9, and malic and oxalic acids were the major components of the acid exudates in different chickpea genotypes. There was no survival of H. armigera larvae in chickpea plants sprayed with 0.1, 0.2 and 0.5% Bt. There was a significant reduction in larval survival, larval and pupal weights and fecundity, and prolongation of larval and pupal periods in chickpea plots sprayed with Bt (0.05%) as compared to the unsprayed plots. Biological activity of Bt was lower on artificial diets with leaf or pod powder of chickpea genotypes, which might be because of a low intake of Bt toxins due to the antifeedant effects of acid exudates in the chickpea or reduction in biological activity of Bt due to the interaction of biochemical constituents in chickpea with the Bt toxins. Larval survival, larval and pupal weights, pupation and adult emergence were significantly lower on diets with leaf or pod powder of the H. armigera-resistant genotypes than on the susceptible check. Chickpea genotypes with resistance to H. armigera acted in concert with Bt to cause adverse effects on the survival and development of this insect. The results suggested that development of transgenic chickpeas expressing toxin genes form Bt will be quite effective for controlling of the pod borer, H. armigera.

The impact of the locust control insecticide fipronil on termites and ants in two contrasting habitats in northern Australia

Termites and ants are key ecosystem engineers and nutrient re-cyclers. Extensive spraying of insecticides presents a hazard to these insects. Using a control-treatment paired design comprising 3 replicate pairs of plots per soil type, we quantified the impact of fipronil on termite consumption of cardboard and wood baits, termite repair of deliberately damaged mounds as well as ant community composition and abundance in two different habitats (black vertisol and red kandosol soil) when applied aerially at a dose rate recommended for locust control in Australia. The study was conducted from 2006 to 2008. The diversity of termites and ants was higher and side-effects were more pronounced on kandosol than on vertisol soil. Overall termite consumption of cardboard baits was significantly greater in unsprayed than in sprayed plots while losses were significantly greater in only one (vertisol) or two (kandosol) sprayed plots, respectively, compared to paired unsprayed plots. In the first year, termite consumption of wood baits was generally significantly greater on unsprayed than on sprayed kandosol while only marginally significant on unsprayed vertisol. Again, losses were significantly greater in one (vertisol) and two (kandosol) sprayed plots, respectively, compared to paired unsprayed plots. Overall termite consumption of wood baits was similar among plots in the second year post-spray although consumption remained significantly depressed in one pair of sprayed plots in each habitat. The capacity of the mound-building termite, Drepanotermes rubriceps (Froggatt) unique to kandosol, to repair damaged mounds was significantly reduced by fipronil. Furthermore, colonies of Iridomyrmex greensladei Shattuck, an ant species associated with mounds of D. rubriceps, were not re-located five months post-spray. Ant communities were also adversely affected. On vertisol, species richness and relative abundance were similar among treatments, but significantly fewer species were caught in one individual sprayed plot. In contrast on kandosol, species richness and relative abundance were significantly reduced by fipronil. A drop in species richness was observed in all and a drop in relative abundance in two sprayed plots. On vertisol, abundances of two species were significantly lower in sprayed than in paired unsprayed plots. Likewise on kandosol, abundances of five species were significantly reduced in sprayed plots, and one species was not caught at all. The weight of evidence of our findings indicate that fipronil, even at a low rate, will have substantial adverse impacts on the diversity and activity of termites and ants, especially if applied within one to two years of earlier applications.

Evaluation of contact fungicide spray regimes for control of late blight (Phytophthora infestans) in southern Ethiopia using potato cultivars with different levels of host resistance

Late blight is an important disease of potato in Ethiopia, causing up to 100% loss in susceptible cultivars. In this study, two putatively resistant cultivars of relatively recent introduction (Jalenie and Gudenie) and a widely grown susceptible cultivar (White Flower) were evaluated for late blight severity and yield under five different contact fungicide (mancozeb) regimes and in unsprayed plots. Trials were carried out at two major potato production locations in southern Ethiopia: Awassa and Kokate, located at 1700 and 2156 meters above sea level, respectively. Greater disease pressure occurred at Awassa but the three cultivars performed consistently based on relative disease severity levels at the two locations. While White Flower was assigned a susceptibility score of 8 (highly susceptible) in both sites, Jalenie had scores between 1 and 0, and Gudenie had scores of 3 and 4, in Awassa and Kokate, respectively. With Jalenie, one application of contact fungicide was sufficient in both trials, while for Gudenie, two sprays appeared more appropriate as little or no gain was clear with three or more sprays. With the susceptible White Flower, four sprays were beneficial but little benefit was evident with the fifth spray.

Yield loss of faba bean (Vicia faba) due to chocolate spot (Botrytis fabae) in sole and mixed cropping systems in Ethiopia

Two field experiments were conducted during 2004 and 2005 cropping seasons at Adet Agricultural Research Center, Ethiopia to assess yield losses caused by chocolate spot(Botrytis fabae) of faba bean in sole and mixed cropping systems using two cultivars. Cropping systems were sole faba bean (FB), faba bean mixed with field pea (FB: FP), barley (FB: BA) and maize (FB: MA). Mancozeb was sprayed at the rate of 2.5 kg a.i/ha at 7-, 14- and 21-day interval to generate different levels of chocolate spot disease in all the four cropping systems, and unsprayed control was also included. The treatments were arranged in a randomised complete block design (RCBD) with four replications. FB: MA mixed cropping significantly reduced disease severity and the area under disease progress curve (AUDPC) and increased faba bean grain yield. The highest faba bean grain yield among the three mixed croppings under different spray schedules was obtained from FB: MA mixed cropping in both 2004 and 2005 (2.56 and 3.74 t/ha, respectively) cropping seasons. There were highly significant yield differences (P < 0.05) among the spray intervals of mancozeb in both seasons. The highest grain yield (4.9 t/h) was recorded from the 7-day spray interval in 2005. The unsprayed faba bean had a lower grain yield (1.9 t/ha in 2004 and 2.3 t/ha in 2005) compared to the sprayed plots. The highest relative yield loss (67.5%) was calculated in 2005 from FB: FP mixed cropping in unsprayed plots. The relative yield losses in the unsprayed plots were in the range of 35.8–41.5% in 2004 and 52.6–67.5% in the 2005 cropping season. Severity and AUDPC were inversely correlated with faba bean grain yield. Significant differences were recorded in the 100-seed weight and days to maturity (DM). The unsprayed plots had shorter DM ranging from 126 to 128.5 day (except FB: MA mixed cropping) in 2004 and 122–123.9 days in 2005. In the sprayed plots DM was relatively longer than the unsprayed plots. A higher seed weight was recorded in the sole FB (56 g) and FB: MA (55 g) mixed cropping, and the lowest value of 100-seed weight was recorded from FB: FP (53 g) mixed cropping. The productivity of the mixed cropping evaluated by land equivalent ratio (LER) exceeded that of sole cropping. Faba bean grain yield was highly influenced by the severity of chocolate spot. The disease affects the DM, forcing early maturing of the plants.

Evaluation of a Warning System for Controlling Primary Infections of Grapevine Downy Mildew.

A warning system based on (i) a model that simulates the development of all cohorts of Plasmopara viticola oospores, from oospore germination to infection; (ii) short-term weather forecasts; and (iii) a mobile phone short message system was tested in Northern Italy, from 2006 to 2008. An unsprayed control was compared with a "Warning A" treatment (WA, fungicides were applied whenever the warning system predicted an infection period), a "Warning B" treatment (WB, fungicides were applied as in the WA treatment but only when the relative dimension of any oospore cohort predicted by the model exceeded a threshold), and a "grower" treatment (fungicides were applied according to a conventional schedule). Average disease incidence on leaves was reduced by up to 90% in sprayed plots compared with unsprayed plots. On bunches, efficacy was always >90% at fruit set; when most berries were touching, efficacy was higher for the WA (96%) than for grower (89%) and WB (85%) treatments. On average, 6.8 fungicide sprays were applied following the grower's schedule; use of the warning system reduced applications by about one-half (WA treatment) or two-thirds (WB treatment). The grower's schedule had an average cost of 337 €/ha; the average saving with the WA and the WB treatments was 174 and 224 €/ha, respectively.

Effect of fungicide use strategies on the control of early blight (Alternaria solani) and potato yield

A series of experiments were undertaken to evaluate fungicide use strategies for the control of early blight (Alternaria solani), the most significant foliar disease of potatoes in Australia. The protective and curative activity of fungicides was evaluated in glasshouse and field studies. Boscalid, azoxystrobin and difenoconazole were highly effective in the control of early blight when applied up to three days before or three days after inoculation. Boscalid completely inhibited disease development when applied one day before inoculation. Applied five days after inoculation boscalid did not control early blight while difenoconazole was the most effective fungicide. Spray programs of 4-6 applications of fungicides from several different mode-of-action groups were evaluated in 4 field trials on commercial properties in South Australia, Western Australia and Queensland. All spray programs inhibited the development of disease and improved marketable tuber yields compared with unsprayed plots. The most effective spray programs included those with boscalid + metiram in the first two applications, and resulted in significant increases in early blight control and tuber yields of over 20%. These findings show that products with strong protective activity such as boscalid were more effective when applied earlier in the spray program. Products with strong curative activity such as difenoconazole were more effective than protective fungicides when early blight infections were well established in the crop. This information can be used to assist potato growers to improve early blight control and increase marketable tuber yields.

Comparison of monitoring- and weather-based risk indicators of botrytis leaf blight of onion and determination of action thresholds

Botrytis leaf blight (Botrytis squamosa) is the key disease for scheduling fungicide sprays in many onion-growing areas. Various disease predictors have been developed to identify the best time to initiate fungicide spray programs or to time spray intervals. However, individual predictors have not been rigorously evaluated based on their predictive accuracy. Receiver operating characteristic (ROC) curve analysis was used to evaluate the reliability of the following seven predictors at various damage thresholds: (1) the number of lesions on the oldest leaves; (2) the number of lesions on the youngest leaves; (3) the airborne conidia concentration (ACC); (4) the sporulation index (SI); (5) the inoculum production index (IPI); (6) the infection probability (IP); and (7) a disease severity value (DSV). Data on disease intensity were concurrently collected in sprayed and unsprayed plots. The analysis was conducted with data from the entire season (185 samplings) and for data after the critical disease level of 1 lesion per leaf was reached (107 samplings). At damage thresholds of 1 or 5 lesions per leaf, predictors based on biological monitoring generally were more reliable at predicting disease risk than weather-based predictors. At a damage threshold of 10 lesions per leaf, most pairwise comparisons showed that there were no significant differences among the areas under the ROC curves (AUCs) for any predictors except for the AUC for ACC which was significantly higher and for and IP that were significantly higher and significantly lower, respectively, than AUC for most other predictors. The difference between the two types of predictors was smaller when only data after the first fungicide spray were analysed. Best action thresholds were 8.06 to 13.79 conidia per m3 of air, 2.73 to 3.56 lesions per oldest leaf, and 0.16 to 0.43 lesions per youngest leaf. The most reliable weather-based predictor was SI at an action threshold of 82.55 to 86.46 SI value. Best time to initiate the spray program could be predicted using monitoring-based predictors at the lowest action threshold. The interval between sprays could be best predicted using monitoring-based predictors at a higher threshold combined with SI values above 80. Key words: disease management, epidemiology, forecasting systems, risk management. La brûlure de la feuille de l’oignon (Botrytis squamosa) est une maladie clef dans la gestion des fongicides dans la plupart des régions de production. Plusieurs indicateurs de risque ont été développés afin de déterminer le meilleur moment pour débuter les applications de fongicides ou pour déterminer l’intervalle entre les pulvérisations. Toutefois, ces indicateurs n’on jamais été évalués pour leur fiabilité à estimer les risques de maladie. L’analyse ROC « receive operating characteristic curve » a été utilisée pour évaluer la fiabilité de sept indicateurs en fonction de différents seuils de dommages: (1) nombre moyen de lésions sur la feuille du bas (OLD); (2) nombre moyen de lésions sur la feuille du haut (YNG); (3) la concentration aérienne de conidies (ACC); (4) l’indice de sporulation (SI); (5) l’indice de production d’inoculum (IPI); (6) la probabilité d’infection (IP); et (7) la valeur de sévérité (DSV). Des données sur l’intensité de la maladie ont été recueillies simultanément dans des parcelles traitées et non traitées avec des fongicides. L’analyse a été faite sur les données comprenant toute la saison et seulement la période après l’atteinte du seuil critique de une lésion par feuille pour un total de 185 et 107 échantillonnages. Au seuil de dommage de 1 et 5 lésions par feuille, les indicateurs basés sur le dépistage se sont avérés plus fiables que ceux basés sur des prédictions. Par contre, au seuil de 10 lésions par feuille, il n’y a pas de différences significatives entre les aires sous la courbe ROC (AUC) pour chaque paire d’indicateurs à l’exception de ACC dont l’AUC était significativement plus élevée et de IP dont l’AUC était significativement plus faible. Peu de différence entre les AUCs ont été observées pour les données recueillies après l’atteinte du seuil de une lésion par feuille. Le meilleur seuil d’intervention a été établit à 8,06 à 13,79 conidies par m3 d’air, 2,73 à 3,56 lésions par feuille du bas et 0,16 à 0,43 lésions par feuille du haut. Parmi les autres indicateurs, l’indice de sporulation (SI) s’est avéré être le plus fiable à un seuil d’intervention de 82,55 à 86,46. Le meilleur moment pour débuter les applications de fongicides devraient être déterminées en fonction du dépistage et par la suite l’intervalle entre les pulvérisations devrait être déterminé en fonction du dépistage avec un seuil plus élevé combiné avec un indice de sporulation (SI) au seuil de 80. Mots-clés : gestion des maladies, épidémiologie, lutte raisonnée, systèmes prévisionnels, gestion des risques.

Invasion of a West Everglades wetland by Melaleuca quinquenervia countered by classical biological control

The population dynamics of Melaleuca quinquenervia were monitored over a 5-year period in a cypress-pine wetland while subjected to two levels of herbivory. The trees had been recruited during 1998–1999 after a destructive crown fire. Half of 26 experimental plots were sprayed every 4–6 weeks with an insecticide to reduce herbivory by the biological control agents Oxyops vitiosa and Boreioglycaspis melaleucae. After only 1-year melaleuca density increased by 26% in sprayed plots and by 7% in unsprayed plots. However, over the entire 5-year period melaleuca density increased in sprayed plots by 0.1% while decreasing by 47.9% in unsprayed plots when compared to initial densities. Annual mortality of melaleuca never exceeded 6% in any year in sprayed plots but ranged from 11% to 25% in unsprayed plots. There was a significant year by treatment interaction indicating the importance of the environment on tree mortality. Limited seed production occurred on sprayed trees but never on unsprayed trees. Mean tree height increased by 19.6% in sprayed plots while declining by 30.6% in unsprayed plots. Coverage by native vegetation did not increase with decreasing melaleuca density. This is the first study with controls that quantifies the population level regulation of melaleuca by introduced biological control agents and corroborates other correlative studies that documented significant changes in melaleuca communities after the introduction and establishment of biological control agents.

Reaction of South African soybean cultivars to rust caused by Phakopsora pachyrhizi

Soybean rust was recorded in South Africa in February 2001 in the Vryheid area of KwaZulu-Natal, from where it spread throughout production areas east of the escarpment. Management of the disease is presently, based on fungicide spray programs. Commercial soybean cultivars were evaluated for rust resistance at Cedara during 2003/04 and 2004/05. Field trials consisted of 26 entries in fungicide sprayed and unsprayed plots, replicated three times. Weekly (2003/04) or two weekly (2004/05) disease assessments were expressed as affected leaf area based on visual assessments of rust severity and the degree of defoliation. Physiological stage at first detection of disease differed significantly between cultivars with the greatest delay in LS678 during both seasons. There were significant differences in Area Under the Disease Progress Curve (AUDPC) between cultivars and AUDPC was significantly, negatively related to growth stage at first detection. Yield loss was poorly related to AUDPC values but was significantly related to days to maturity, with lower losses in shorter season varieties than in longer season varieties. This, in turn, was related to premature defoliation which was 16 to 21 days earlier in the longer maturity group, in unsprayed plots. Reductions in yield losses were, however, not sufficient to be of economic value and none of the cultivars showed significant rust tolerance.

Control of Thrips tabacci Linderman (Thysanoptera: Thripldae) on Onion (Allium cepa Linn.) in Sokoto, Nigeria

Five onion, transplants were sprayed with A.-cyhalothrin (Karate 2.5EC) at 18.67g a. i. Iha (747mllha) from Jan.-May in 2002 and Dec.-May in 2003. Spraying was spaced at fortnightly intervals starting at 3 weeks after transplanting (WAT). Results indicated that unsprayed plots significantly differed (P&lt; 0.05) from the sprayed plots in thrips population at 4, 6, 8 and 10 WAT. Two sprays significantly increased onion bulb yield by 39% in 2001/2002 whist three sprays increased it by 26% in 2002/2003.

Climatic variability alters the outcome of long‐term community assembly

Summary We used seed additions to test experimentally whether long‐term community assembly unfolds consistently (deterministic model) or whether different outcomes are possible depending on variations in climate and the presence of exotic dominants (stochastic model). The experiment was conducted in homogeneous semi‐arid grassland on the northern Great Plains of North America. Native grass seed additions in each of 3 years (1994–96) were combined factorially with repeated selective herbicide applications to reduce the cover of the extant dominant, a Eurasian C3 grass (Agropyron cristatum). We assessed whether composition converged or diverged among treatments between 1996 and 2004. The outcome of long‐term community assembly varied by establishment year, in association with significant variations in monthly growing‐season precipitation from 68‐year averages during the years when seed was added. In 2004, the added native C4 grass Bouteloua gracilis dominated plots sprayed and seeded in 1994 and 1995, resisting re‐invasion and having significantly higher plot diversity. In contrast, plots sown in 1996 – one of the driest years on record – reverted to the exotic C3 grass. Seeded but unsprayed plots maintained their pre‐treatment dominance by exotic grass. Rainfall variability was also associated with the extent of exotic grass cover and bare soil in unsprayed plots, and natural recruitment by native species from nearby prairie. Establishment success varied significantly among the added species. All seeded grasses, other than B. gracilis, were mostly absent by 2004. The failure of these native C3 species to establish appears consistent with deterministic assembly models where local abiotic conditions filter out unsuitable species. Synthesis. Our results indicate that stochastic and deterministic processes operate simultaneously to influence community assembly, depending on interactions among climate, seed availability, species identity and disturbance during the initial stages of establishment. Multiple assembly trajectories developed, but the assembled communities did not include all the added species. Conversion of degraded grassland back to native‐dominated grassland was possible, but only with the removal of the exotic dominant and seed additions during years that were suitably wet for establishment. Once formed, however, the assembled native community resisted re‐invasion. This suggests that native grassland restoration will depend on establishment measures as intense as those used during initial cultivation almost a century earlier.