Effects Of Host Plant Resistance Research Articles

English

Oilseed cultivation is the major source of energy that fulfills the cooking oil requirements of large masses across the globe. Mustard is one of the important members of the brassica family, which is attacked by an aphid, Lipaphis erysimi. This research investigated the field population fluctuations of L. erysimi on different mustard varieties for two growing years. L. erysimi population reached to peak (171.3 nymphs and 141.6 adults) in the Mustard-early variety in early March. Based on Mustard Resistance Index (MRI), the Mardan-Local variety is highly resistant having an Aphid Population Score (APS) and Leaf Damage (LD) of 1 with a pooled annual mean of 1.85 aphids. The varietal preference test results indicated that the highest L. erysimi (8.0 adults) were attracted to the Mustard-early variety while the least (2.80 adults) were attracted to Mardan-local. In the In-vitro bioassay, Lefenuron and Pyriproxyfen caused 100 and 70.4% mortality, respectively. In the field, synchronized experiment, Imidochloprid (control positive) and Lefenuron showed the highest mortalities (100%), respectively. The Pyriproxefen and Runner (Methoxyfenoxid) treated plots had caused 77 and 70.3% mortalities, respectively. The present research is the first attempt to investigate the synchronized effects of Host Plant Resistance and Insect Growth Regulators for the L. erysimi management in Mustard. It is concluded that Mardan-Local variety alongside lefenuron application provides an effective IPM for L. erysimi. These outcomes will help the farmers better understand and manage aphids; it will help the researcher to explore further management tools of Aphids in oilseed crops.

Host plant resistance, foliar insecticide application and natural enemies play a role in the management of Melanaphis sorghi (Hemiptera: Aphididae) in grain sorghum.

The invasive Melanaphis sorghi (Theobald; =Melanaphis sacchari Zehntner) is a serious pest of sorghum production in the southern USA. Demonstration of technologies that provide effective control is key to management of this pest. Here, we investigated the effect of host plant resistance (resistant cultivar: DKS37-07 and susceptible cultivar: DKS53-53) and a single foliar insecticide (flupyradifurone: Sivanto Prime) application on M. sorghi infestations and the role of natural enemy populations in grain sorghum production across five locations in four states in southeastern USA. Foliar insecticide application significantly suppressed M. sorghi infestations on both the resistant and susceptible sorghum cultivars across all locations. Planting the host plant resistant cultivar (DKS37-07) significantly reduced aphid infestation across all locations. Plant damage ratings did not vary widely, but there was generally a positive association between aphid counts and observed plant damage, suggesting that increasing aphid numbers resulted in corresponding increase in plant damage. Planting a host plant resistant cultivar and foliar insecticide application generally preserved grain yield. Both sorghum hybrids supported an array of different life stages of natural enemies (predators [lady beetle larvae and adults; hoverfly larvae and lacewing larvae] and parasitoids [a braconid and aphelinid]) for both the sprayed and non-sprayed treatments. We found a strong and significant positive relationship between the natural enemies and the M. sorghi infestation. Results suggest that planting a host plant resistant cultivar and the integration of natural enemies with insecticide control methods in the management of M. sorghi is central to the development of an effective pest management strategy against this invasive pest.

Effects of Host Plant Resistance and Fungicide Applications on Ascochyta Blight Symptomology and Yield of Chickpea.

Ascochyta blight (AB), caused by the pathogen Ascochyta rabiei, is a major threat to chickpea production worldwide, causing major yield losses and decreasing quality. Control of AB requires integrating pest management options including resistant cultivars and fungicide applications. To address this, fungicides with different modes of action were evaluated on three chickpea cultivars with differing levels of susceptibility to AB under irrigated and dryland conditions in 2015 to 2017. The fungicides were applied once or twice and compared with a no-fungicide application control on AB score and yield. The mean grain yields across locations and years were 1,753, 1,283, and 981 kg/ha, with a corresponding AB mean score of 2.6, 3.2, and 3.3 on 0 to 7 scale (where 0 is no disease and 7 is completely dead) for the moderately resistant, moderately susceptible, and susceptible chickpea cultivars, respectively. Fungicide application was not enough to control disease throughout the season. The use of AB-resistant cultivars had the most significant impact on minimizing the disease and maximizing yield, irrespective of year and location. This study supports previous research indicating that planting AB-resistant chickpea cultivars is essential for disease control, regardless of the fungicides applied.

Influence of Trichomes on the Incidence of Two Spotted Spider Mite Tetranychus urticae on Brinjal

The present study evaluates the effect of host plant resistance by antixenosis (trichomes) in 40 brinjal entries to the two spotted spider mite Tetranychus urticae Koch, an important pest of brinjal. The maximum trichome density was observed (719.15/ 0.5cm2 leaf area) with the variety Namakkal local and minimum (276.82/ 0.5cm2 leaf area) was with Tiruppur local. Stellate type of trichomes were observed on all the entries, with maximum length on the upper surface of the leaves being 200.18 μm on Namakkal local, and on lower surface of leaves (269.36 μm) on Chinthapalli local-2. Manapparai local measured 247.40 μm long on midrib and 206.84 μm on veins. The correlation coefficients between the trichome density, length and T. urticae incidence revealed a significant negative correlation. Arka Nidhi was found to be resistant (2.07/cm2 leaf area) with low trichome density (427.50 μm).

Effects of host plant resistance and fungicide application on phoma stem canker, growth parameters and yield of winter oilseed rape

Multifactorial field trials were conducted over three cropping seasons (2012–2015) to assess the effects of selection of triazole fungicides when applied either alone or in combination on growth of winter oilseed rape, development of phoma stem canker, crop wintering and crop yield. The plots consisted of four winter oilseed rape cultivars differing in their levels of resistance to phoma stem canker and subplots of five fungicides with growth regulatory activity (prochloraz/tebuconazole, mepiquat/metconazole, tebuconazole, tebuconazole/prothioconazole and a mixture of fluxapyroxad/tebuconazole) which were applied twice; in autumn (BBCH 14–18) and in spring (BBCH 30–39). Visual scoring such as growth parameters like plants per m2 and plant height, yield parameters like number of side branches, pods and seeds per plant, thousand grain weights and seed yield as well as evaluation of phoma leaf spots and basal stem canker were performed using standard procedures. Plant height was significantly affected by cultivars and fungicide treatments. In contrast, winter killing (%) was not reduced significantly. Application of mepiquat/metconazole reduced the winter killing below 10% compared to the untreated control. Development of phoma stem canker was affected by fungicides and cultivars. At the final rating date (BBCH 81), the moderately resistant cultivars Genie and Vitara exhibited less disease severity than the susceptible cultivars Elektra and PR 46W20 in untreated plots. Fungicide treatments were more effective in reducing the disease in moderately resistant cultivars than in susceptible ones. The lowest percentage of disease severity was achieved when fluxapyroxad/tebuconazole combination was applied to plants. To our knowledge, this study is the first to report multi-year efficacy of fluxapyroxad/tebuconazole against phoma stem canker in oilseed rape cultivations. Cultivars showed differences in all yield parameters. However, with the exception of seed yield the application of fungicides did not significantly impact the different yield parameters. Application of fluxapyroxad/tebuconazole was the most effective treatment in reducing yield losses.

Interactions of effects of host plant resistance and seed treatments on soybean aphid (Aphis glycines Matsumura) and soybean cyst nematode (Heterodera glycines Ichinohe).

The soybean cyst nematode, Heterodera glycines, and soybean aphid, Aphis glycines, are invasive, widespread and economically important pests of soybean, Glycine max, in North America. Management of these pests relies primarily on use of pesticides and soybean germplasm with genetic resistance. A 3-year field study and complementary greenhouse experiment were conducted to determine the benefits of host plant resistance (HPR) and pesticidal seed treatments for managing pest populations and preserving soybean yield. Host plant resistance significantly decreased the abundance of A. glycines and, in most study sites, suppressed H. glycines. Neonicotinoid seed treatment reduced A. glycines abundance on the cultivar that was susceptible to both aphids and nematodes, but abamectin nematicide seed treatment had no effect on H. glycines populations in the field or greenhouse. These results suggest that the seed treatments included in our experiments may suppress pests, but not consistently for all soybean cultivars or study sites. Ultimately, HPR more consistently reduced pest numbers compared with the use of pesticidal seed treatments. The planting of HPR cultivars should be a primary tool for integrated pest management of both soybean pests. © 2017 Society of Chemical Industry.

Effect of Host Plant Resistance and Seed Treatments on Soybean Aphids (Hemiptera: Aphididae) and Their Natural Enemies

Abstract The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is a serious pest of soybean in the North Central region of the United States. Management tools include resistant varieties and insecticidal seed treatments, used alone or in combination, which may have variable effects on different pest or natural enemy species. In this 1-yr field study we examined the response of soybean aphids and natural enemy abundance to aphid-resistant Rag1 soybeans, with and without thiamethoxam seed treatment. Rag1 resistance, thiamethoxam seed treatment, and both together, significantly reduced cumulative aphid days and increased yield compared to the untreated susceptible control, though there was no interaction of these factors. Peak aphid density in the control was 799.4 ± 174.8 aphids/plant. In Rag1 resistant soybeans, peak aphid density (312.2 ± 121.3 aphids/plant) was above the economic threshold but below the economic injury level, whereas in susceptible soybeans with seed treatment, the peak ap...

Crop hosts and genotypic resistance influence the biological activity of Bacillus thuringiensis towards Helicoverpa armigera

Studies on the influence of genotypic resistance on biological activity of a commercial formulation of Bacillus thuringiensis (Bt) and pure Bt toxin Cry1Ac were carried out to develop appropriate strategies for pod borer, Helicoverpa armigera management in chickpea, sorghum, pigeonpea and cotton. The interaction effects of host plant resistance and biological activity of commercial Bt/Cry1Ac were studied by incorporating the lyophilized tissues of chickpea leaves, milk stage sorghum grain, pigeonpea pods and cotton squares into the artificial diet with and without Bt formulation or Cry1Ac. The H. armigera larval weights were significantly lower in insects reared on diets with square powder of the insect - resistant Bt-cotton RCH 2 + Bt/Cry1Ac and pod powder of insect - resistant pigeonpea genotype, ICPL 332WR + Bt/Cry1Ac as compared to the larvae reared on diets with leaf powder of H. armigera susceptible chickpea genotype, ICCC 37 and the standard artificial diet. Pupation and adult emergence were significantly lower in insects reared on diets with tissues of pod borer-resistant genotypes + Bt/Cry1Ac as compared to insects reared on diets with tissues of the insect susceptible genotypes + Bt/Cry1Ac. Insects reared on diets containing insect-resistant and -susceptible genotypes of sorghum, pigeonpea and cotton and pod borer-resistant genotype of chickpea (ICC 506EB) + Bt/Cry1Ac did not lay any eggs. However, eggs were laid by the insects reared on diets containing pod borer-susceptible genotype of chickpea, ICCC 37 and on the standard artificial diet + Bt/Cry1Ac. The insects reared on diets with sorghum genotype, ICSV 745, and Bt-cotton, RCH 2 without Bt/Cry1Ac also did not lay eggs. The results suggested that Bt/Cry1Ac is more effective for management of H. armigera when deployed in combination with insect-resistant genotypes of cotton, chickpea, pigeonpea and sorghum.

Effects of Host Plant Resistance and Fungicides on Severity of Cucumber Downy Mildew

Cucurbit downy mildew caused by the oomycete Pseudoperonospora cubensis (Berk. And Curt) Rostov is a major disease of cucumber (Cucumis sativus L.) (Palti and Cohen, 1980) globally. Chemical control of downy mildew is necessary to achieve high yields in the absence of adequate host plant resistance. Most of the currently grown cultivars have some resistance to downy mildew. Before the resurgence of the disease in 2004, host plant resistance was sufficient to control the disease without fungicide use, and downy mildew was only a minor problem on cucumber. There are currently no cultivars that show resistance at a level equal to that observed before 2004. However, differences in resistance exist among cultivars, ranging from moderately resistant to highly susceptible. In this study, we evaluated the disease severity and yield of four cucumber cultivars that differed in disease resistance and were treated with fungicide programs representing a range of efficacy levels. The experiment was a split plot design with six replications and four years. Disease was evaluated as chlorosis, necrosis, and reduction in plant size on a 0 to 9 scale. Cultigen had a large effect in all four years. Fungicide has a smaller effect on resistance component traits and a larger effect on yield traits. The effects of cultivar resistance and fungicides appear to be additive until a threshold where maximum yield is reached. Highly resistant cultigens such as PI 197088 required only the least effective fungicides to achieve highest yields, whereas moderately resistant cultigens required a more effective fungicide to reach a similar level of yield. Susceptible cultigens did not achieve high yield even with the most effective fungicide treatments. It is likely that, even as highly resistant cultivars are released, growers will need to continue a minimal fungicide program to achieve maximum yield.

Plant Resistance Affects the Olfactory Response and Parasitism Success of Cotesia vestalis

Understanding the factors influencing host-selection behavior of parasi- toids is essential in studies on host-parasitoid ecology and evolution, and in combin- ing sustainable strategies of pest management, such as host-plant resistance and biological control. The effects of host-plant resistance on the olfactory response and parasitism success by Cotesia vestalis, a parasitoid of diamondback moth (Plutella xylostella) larvae were examined. Here, it was demonstrated that host-plant resistance can strongly influence foraging behavior and parasitism success of the parasitoid. In olfactometer experiments, C. vestalis did not differentiate between crucifer plant types with similar levels of susceptibility or resistance to P. xylostella but showed a strong preference for susceptible compared with partially-resistant host plants. The influence of previous oviposition activity varied with the host-plant type experienced by the parasitoid. In cage experiments, C. vestalis preferred to parasitize P. xylostella larvae on a susceptible plant compared with larvae on a partially resistant host plant when exposed to hosts for 24 h. However, this preference appeared to be transitory, and was not found after 96 h exposure. The present study suggests that combining partial host-plant resistance with biological control by C. vestalis for the control of P. xylostella may in some circumstances be antagonistic and negatively affect para- sitism success.

Evaluating the effect of host plant resistance and planting dates on the incidence of legume pod borer ( Maruca vitrata Geyer) on African yam bean in Nigeria.

Legume pod borer ( Maruca vitrata (Geyer)) is one of the major biotic constraints to increasing the productivity of most grain legumes in the tropics. In this study, field experiments were conducted to assess the effect of host plant resistance and planting dates on the incidence of African yam bean pod borer Maruca vitrata (Geyer) in Nigeria. Planting of African yam bean ( Sphenostylis stenocarpa ) varieties in May led to less infested by M. vitrata and differed significantly (P < 0.05) from those planted in June and July. Of all the varieties assessed, TSs9 was the most resistance and differed significantly (P < 0.05) from the rest of the varieties; while TSs84 was the most susceptible with poorest grain yields. All the varieties when planted in May had less post-flowering insect infestations and produced higher grain yields than those planted later. Flowering and podding formation stages of African yam bean (AYB) planted in July coincided with the peak population densities of M. vitrata , resulting in a significant reduction in yields. The highest grain yields were recorded under a combination of early planting with resistant varieties. Key Words: Maruca vitrata, Sphenostylis stenocarpa , susceptibility

Field‐level effects of preventative management tactics on soybean aphids (Aphis glycinesMatsumura) and their predators

AbstractA 2‐year field experiment was conducted in northern Illinois to evaluate the effects of host plant resistance and an insecticidal seed treatment (thiamethoxam) on soybean aphids,Aphis glycinesMatsumura and their predators. Densities of soybean aphids varied between the 2 years of the experiment. During both years, resistant plants experienced fewer cumulative aphid days than susceptible plants. Populations of soybean aphids on resistant plants rarely exceeded the economic injury level of 250 soybean aphids per plant. The use of thiamethoxam reduced cumulative aphid days in 2007, but not in 2008. Although soybean aphids reached densities that were sufficient to cause yield‐loss for untreated and susceptible plants, no yield‐benefit was associated with using the two management tactics in either year. This latter finding suggests that densities of soybean aphids need to be greater and sustained for a longer period of time than what we observed if the two management tactics are expected to provide a yield‐benefit. Monitoring natural enemies revealed that densities of key aphidophagous predators were relatively unaffected by host plant resistance or thiamethoxam; the effect of these management tactics on densities of predators, as well as the effectiveness of the method used to sample predators, is discussed.

Effects of host plant resistance on insect pests and its parasitoid: A case study of wheat-aphid–parasitoid system

Host plant resistance can effectively reduce pest insect populations, but a concern is whether plant resistance could also negatively affect the natural enemies of the insect pests. In this paper the effect of three wheat cultivars on the population of an aphid species, Sitobion avenae (F.) and it’s parasitoids, Aphidius spp., were investigated in the field experiments in 2004 and 2005. Percentage of parasitized aphids at peak sampling date was also recorded. Both the aphid and parasitoid populations varied in the three wheat cultivars at the sampling dates. The density peak of parasitoid population in all the three wheat cultivars was 9–12 days behind the density peak of the aphid population. At the population peaks both the aphid and parasitoid densities were significantly higher in the susceptible than in the resistant cultivars. Furthermore, the accumulative parasitism was significantly higher in the resistant than in the susceptible cultivar, and level of resistance of the wheat cultivars had no effect on aphid mummy weight parasitized by A. spp. These results suggested that the host plant resistance against aphids enhanced the parasitism of aphid species S. avenae (F.) by its parasitoid A. spp. in wheat field.

Effect of Host Plant Resistance on Haustorium Formation in Cereal Rust Fungi

AbstractHaustoria of Puccinia triticina (wheat leaf rust fungus) and P. hordei (barley leaf rust fungus) were isolated from susceptible and partially resistant wheat lines, and susceptible, hypersensitive and partially resistant barley lines. Haustoria were counted and measured. The size of haustoria was similar in the partially resistant and susceptible genotypes but haustoria were smaller in the hypersensitive barley line L94+Pa7. The number of haustoria was reduced in both partially and hypersensitive lines when compared with susceptible ones. Therefore it seems that the reduction in the number of haustoria is a consequence of the resistance that can be attributable either to early abortion of infection units or reduced colony growth. The reduction of the number of haustoria was more pronounced in the adult plant stage.

Effect of host plant resistance in some cowpea ( Vigna unguiculata {L.} Walp.) cultivars on growth and developmental parameters of the flower bud thrips, Megalurothrips sjostedti (Trybom)

The effect of host plant resistance on growth and developmental parameters of the bean flower thrips Megalurothrips sjostedti Trybom was determined on nine cowpea ( Vigna unguiculata {L.}Walp.) cultivars under laboratory conditions. Newly emerged adults M. sjostedti were confined on peduncles with floral buds of the different cowpea cultivars. M. sjostedti suffered 100% pre-imaginal mortality on cowpea lines Moussa local, TVu1509, TVx3236, Sewe and Sanzibanili. Antibiosis is believed to be the mechanism of resistance operating in these cultivars. On KVx404-8-1, M. sjostedti had a prolonged developmental period though not significantly different from Vita 7, the susceptible check at P<0.01. IT91K-180 supported the development and survival of M. sjostedti with the suitability index (growth index) not significantly different from the susceptible check (Vita 7) at P<0.01. Hence the mechanism of its resistance is tolerance.

Effect of Host Plant Resistance to Tomato yellow leaf curl virus (TYLCV) on Virus Acquisition and Transmission by Its Whitefly Vector

ABSTRACT The effect that Tomato yellow leaf curl virus (TYLCV)-infected resistant tomato plants may have on virus epidemiology was studied. Four tomato genotypes that exhibit different levels of viral resistance, ranging from fully susceptible to highly resistant, served as TYLCV-infected source plants. Viral acquisition and transmission rates by white-flies following feeding on the different source plants were evaluated. TYLCV transmission rate by whiteflies that had fed on infected source plants 21 days postinoculation (DPI), shortly after the appearance of TYLCV symptoms, was negatively correlated with the level of resistance displayed by the source plant. Therefore, the higher the resistance, the lower the transmission rate. In addition, TYLCV DNA accumulation was shown to be lower in the resistant source plants compared with the susceptible plants. Whitefly survival rate, following feeding on source plants 21 DPI, was similar for all the cultivars tested. Significant differences in whitefly survival were found, however, following feeding on the infected source plants at 35 DPI; here, whitefly survival rate increased with higher levels of resistance displayed by the source plant. At 35 DPI, the susceptible plants had developed severe TYLCV disease symptoms, and transmission rates from these plants were the lowest, presumably due to the poor condition of these plants. Transmission rates from source plants displaying a medium level of resistance level were highest, with rates declining following feeding on source plants displaying higher levels of TYLCV resistance. TYLCV DNA accumulation in whiteflies following feeding on infected source plants at both 21 and 35 DPI was directly correlated with viral DNA accumulation in source plants. Results show that, in essence, the higher the resistance expressed, the less suitable the plant was as a viral source. Consequently, following acquisition from a highly resistant plant, TYLCV transmission by whiteflies will be less efficient.

Effect of Host Plant Resistance and Reduced Rates and Frequencies of Fungicide Application to Control Potato Late Blight.

Field experiments were conducted during 1998 to 2000 to determine the response of commercial potato cultivars and advanced breeding lines (ABL) differing in susceptibility to foliar late blight (caused by Phytophthora infestans) to reduced rates and frequencies of residual, contact fungicide applications. When environmental conditions were most favorable for the development of late blight, the lowest application rate of the fungicides chlorothalonil or fluazinam (33% of the manufacturers' recommended application rate [MRAR]) gave unsatisfactory control of potato late blight. Under conditions moderately conducive for late blight development, effective control was achieved with 33 to 66% MRAR with either fungicide. The Michigan State University advanced selection, MSG274-3, was the least susceptible ABL tested and, during 1998 to 2000, late blight was effectively managed using reduced rates of fungicides. Application rates of chlorothalonil (33 to 100% MRAR) significantly reduced late blight in the cultivar Snowden (5-day application interval) compared with the nontreated control; whereas, late blight was not effectively controlled in Snowden even at 100% MRAR of chlorothalonil at either 10- or 15-day application intervals in 1999 or 2000. The ABL MSG274-3 was the least susceptible of all cultivars and ABL used in this study, and required minimal chemical protection against late blight. The study demonstrates that ABL with reduced susceptibility to late blight can be managed with reduced fungicide rates and longer application intervals, thus offering more economical control of this disease.

Occurrence of phytophagous insects on wild Vigna sp. and cultivated cowpea: comparing the relative importance of host-plant resistance and millet intercropping

The effects of host-plant resistance on cowpea phytophagous insects and their natural enemies under pure and mixed crop conditions was evaluated at Minjibir, Kano State, Nigeria, in 1992–1994 crop seasons. Cowpea Vigna unguiculata cv `IT86D-715' (susceptible to insect pests) and a wild Vigna line Vigna vexillata `TVnu 72' (resistant to most insect species) were planted alone and in mixtures with millet ( Pennisetum glaucum) in plots of 25×25 m. Mixed cropping had limited effect on major insects and natural enemies. Colonies of Aphis craccivora were significantly smaller and there were more adults of Maruca vitrata in crop mixtures than in monocultures. However, flower and raceme infestation by larval M. vitrata, Megalurothrips sjostedti, and Sericothrips sp. were similar in crop mixtures and monocultures. Empoasca sp. populations and seedling infestation by beanfly Ophiomyia phaseoli were also similar in mixtures and monocultures as well as pod damage by M. vitrata and populations of Clavigralla tomentosicollis. Parasitization rates of M. vitrata, C. tomentosicollis and O. phaseoli and predator–prey ratios of spiders and Orius sp. were similar across cropping systems. Host-plant resistance in TVnu 72 drastically reduced insect populations and damage. Grain yield per hill was high in cowpea IT86D-715 and was not affected by intercropping with millet. Grain yield of TVnu 72 was poor and reflected the low yield potential of this accession. Host-plant resistance is an effective means of controlling insect pest damage in cowpea and there is no evidence that high levels of resistance reduced natural biological control.

Effect of host plant resistance and insecticide on brown planthopper Nilaparvata lugens (Stål) and predator population development in the Mekong Delta, Vietnam

Crop yield and populations of brown planthopper Nilaparvata lugens (Stål), whitebacked planthopper Sogatella furcifera (Horvath) (Homoptera: Delphacidae), green leafhopper Nephotettix spp. (Homoptera: Cicadellidae), and hemipteran and spider predators were monitored on rice varieties susceptible, moderately resistant, and highly resistant to N. lugens, under insecticide-treated and insecticide-free conditions. Nilaparvata lugens outbreaks and hopperburned plants were observed only in plots of the insecticide-treated susceptible varieties. In insecticide-free plots, the yield of the susceptible variety was lower than those of resistant varieties in only one of four seasons. In plots of a moderately resistant and a highly resistant variety, populations of N. lugens, S. furcifera, Nephotettix spp., and predators were generally similar, and yields did not differ, under both insecticide-treated and insecticide-free conditions. In insecticide-free plots, predator- N. lugens ratios were generally higher on resistant varieties than susceptible varieties. Two conclusions can'be drawn from our results relevant to the question of what levels of N. lugens host plant resistance are appropriate for farmers who do not overuse insecticides. First, susceptible varieties will not necessarily be damaged by N. lugens, even when N. lugens outbreaks occur in adjacent plots. Second, moderate and high levels of N. lugens resistance do not appear to be incompatible with biological control of N. lugens or other homopteran pests.

Integrated Effect of Host Plant Resistance and Fungicidal Seed Treatment on Rice Blast Control in Brazil.

A field experiment was conducted in two successive years at Goiânia, GO, Brazil, to study the effect of seed treatment with pyroquilon fungicide (4.0 g a.i. per kg of seed) on rice blast in cultivars that differed in their level of resistance. Leaf and panicle blast progress was monitored in the newly released, early maturing upland rice cultivar Guarani and the advanced line CNA 4136, and compared with disease progress in the susceptible commercial cultivars IAC 165 and IAC 25 in plots with treated and untreated seed. Seed treatment suppressed leaf blast until 62 and 47 days after seeding in 1987 and 1988, respectively, when averaged across cultivars. The slow blasting nature of Guarani and CNA 4136 increased the level of disease control when environmental conditions favored blast development in 1988. Under high disease pressure the correlation between leaf blast and panicle blast severity in the field plots was negative (r = -0.5, P = 0.01). Leaf blast severity at 47 days after seeding was negatively correlated to grain yield (r = -0.709, P = 0.001). Guarani exhibited an increase in grain yield over the untreated control of 28%, corresponding to 826 kg/ha.