Intermittent wetting of soils at high temperature reduces survival of the take-all fungus

Two pot experiments using naturally infested soil and a range of watering regimes were conducted to study the possible effect of level and frequency of wetting of hot soil (to simulate the period between growing seasons in Western Australia) on inoculum of the take-all fungus (Gaeumannomyces graminis var.tritici). In combination with the high soil temperatures, all watering regimes reduced infectivity and propagule number of the take-all fungus, this reduction being absent in dry soils.     

Natural suppression of take-all disease of wheat in Montana soils

This research was initiated to determine whether soils suppressive to take-all of wheat caused by Gaeumannomyces graminis var. tritici (Ggt) occur in Montana, and to identify the organisms most likely involved in this suppression. From an initial screening of eight soils collected from different wheat growing areas of Montana, two were highly suppressive to take-all. Microbial characterization of these soils indicated that different mechanisms were involved in the suppression. In Larslan soil, mycoparasitism appeared to be the main mechanism. Two different fungi with exceptional ability to reduce the severity of take-all were isolated from this soil. One of these fungi could parasitize the hyphae of Ggt. Field tests with these fungi in Ggt infested soil showed increases of over 100% in both harvestble tillers and grain yield as compared to treatments without these two fungi. In tests with 48 different bacteria and 10 actinomycetes from Larslan soil, none were able to consistently reduce severity of take-all alone, or in mixtures. In Toston soil, antibiosis by actinomycetes and perhaps the involvement of Pseudomonas spp. in production of antibiotics and/or siderophores appeared to be the most likely mechanisms involved in take-all suppression. Increases in shoot dry weight over that in the Ggt infested control using mixtures of pseudomonads and actinomycetes ranged from 25% to 87%. Actinomycetes added individually or in mixtures to soil infested with Ggt consistently reduced the severity of the disease to a greater extent than did mixtures of Pseudomonas spp.     

The copepod Centropages abdominalis as a carrier of the stalked ciliate Zoothamnium

Adult specimens of the calanoid copepod Centropages abdominalis collected from February through April 1986 in Shinhamako, a saline lake connected to Tokyo Bay, were heavily infested with the stalked ciliate Zoothamnium. The number of copepods and those infested varied during the infestation period; in February when the copepod population was large, the incidence of infestation was low, whereas in March when the copepod population was small, copepods infested with Zoothamnium were proportionally more abundant. Numerical variations of infested copepods may correspond to changes in the Zoothamnium population. No specimens of Acartia omorii, which coexists with C. abdominalis, were infested in 1986, although a small number of A. omorii were infested with peritrich ciliates in 1985. Some shrimp larvae were also covered with the same peritrichs as copepods. Nevertheless, as a whole, the relationship between the phoront and its carrier seems to be specific. The highest incidence of infested copepods was approximately 30% similar to that in 1985. The sex ratio of C. abdominalis changed on each sampling date but in general females were more numerous than males. The percentage of infested males was higher than that of infested females, suggesting that the former can be attached to more easily than the latter.     

Evaluation of entomopathogenic nematodes against the olive fruit fly, Bactrocera oleae (Diptera: Tephritidae)

Infectivity of six entomopathogenic nematode (EPNs) species against Bactrocera oleae was compared. Similar infection levels were observed when third-instar larvae were exposed to infective juveniles (IJs) on a sand-potting soil substrate. When IJs were sprayed over naturally infested fallen olives, many larvae died within treated olives as well as in the soil; Steinernema feltiae caused the highest overall mortality of 67.9%. In addition, three laboratory experiments were conducted to optimize a time period for S. feltiae field application. (1) Abundance of fly larvae inside fallen olives was estimated over the 2006–2007 season with the highest number of susceptible larvae (3 mm and larger) per 100 olives being observed during December, 2006. (2) S. feltiae efficacy against fly larvae dropped to the soil post-IJ-application was determined. B. oleae added to the substrate before and after nematode application were infected at similar levels. (3) Effect of three temperature regimes (min–max: 10–27, 6–18, and 3–12 °C) corresponding to October through December in Davis, California on S. feltiae survival and infectivity was determined. After 8 weeks, the IJs at the 3–12 °C treatment showed the highest survival rate. However, the cold temperature significantly limited S. feltiae infectivity. Our results demonstrate that B. oleae mature larvae are susceptible to EPN infection both in the soil and within infested olives. Being the most effective species, S. feltiae may have the potential to suppress overwintering populations of B. oleae. We suggest that November is the optimal time for S. feltiae field application in Northern California.     

A New Method to Estimate the Population Size of Coconut Mite, Aceria Guerreronis, on a Coconut

Studies of the coconut mite, Aceria guerreronis, often require accurate assessment of the population density on individual infested coconuts. Here an efficient and accurate method was developed to estimate its population density on an infested coconut. The coconut mites were removed by washing the bracts and surface of an infested coconut with 30 ml of a detergent solution. Shaking the wash for 5 s allowed the mites to distribute uniformly. The number of mites in the first 1 ml of the first wash (X) yielded a very accurate predictor of the total number of mites on a coconut (Y): Y = 30.1X (R² = 0.99; p <0.0001), also confirming that the wash was indeed homogeneous. The advantages and disadvantages of this method are discussed.     

Chemical,physical and biological control of carrot seedling diseases

In naturally infested soil containingPythium ultimum, P. acanthicum andPhytophthora megasperma, onlyP. ultimum was associated with root rot and damped-off seedlings. Damping-off was promoted by low soil temperatures and by flooding. Seedling stands were markedly reduced when seed was pre-incubated in soil at 12°C but not at 25°C or 35°C. Dusting carrot seed with metalaxyl significantly increased seedling stands in the field at rates from 1.5–6 g kg⁻¹ seed and in both flooded and unflooded, naturally infested soil at 3.15 g kg⁻¹. In greenhouse experiments using artifically infested soil,P. ultimum andP. paroecandrum caused damping-off of carrot seedlings andRhizoctonia solani reduced root and shoot weights.R. solani caused damping-off in nutrient-enriched soil.P. acanthicum andP. megasperma were not pathogenic to seedlings, although both fungi colonized roots. Soil populations of allPythium spp., particularlyP. ultimum, increased during growth of seedlings and population growth ofP. megasperma was promoted by periodic flooding. Infestation of soil withP. acanthicum did not reduce damping-off of carrot seedlings byP. ultimum orP. paroecandrum, but significantly increased root and shoot weights and decreased root colonization byR. solani P. acanthicum has potential as a biocontrol agent againstR. solani.     

Effect of solarization of soil within plastic bags on root rot of gerbera (Gerbera jamesonii L.)

Solarization of soil, (potting mix = coarse sand:Eucalyptus marginata fines = 1∶1) infested with 3 fungi pathogenic to gerbera (Phytophthora cryptogea, Fusarium oxysporum andRhizoctonia solani), for 3 to 4 weeks within transparent polyethylene bags controlled root rot of gerbera. Solarization for 2 weeks however, was less effective. All plants grown in the infested potting mix which had been kept in shade for 2, 3 or 4 weeks were severely attacked. Solarization of soil within plastic bags for 4 weeks also increased availability of nutrients such as NH₄ ⁺-N, PO₄ ⁻ and K⁺ in comparison to bagged soil kept in the shade for the same period.     

褐梗天牛幼虫和成虫空间分布的地统计学研究

褐梗天牛(Arhopalus rusticus Linnaeus)是一种对松、杉、柏等林木危害非常严重的蛀干害虫,主要危害针叶树的衰弱木以及火灾后的枯立木,是继松褐天牛之后携带拟松材线虫能力最强的蛀干害虫。为了更好地控制其危害,深入地研究其种群空间格局,运用地统计学方法分析了3块受害程度不同的油松林内褐梗天牛幼虫和成虫的空间分布特性。结果表明:3种受害程度不同的林分内,褐梗天牛的危害具有显著差异,轻度受害林分内有虫株率为30.8%,中度受害林分内有虫株率为44.3%,而重度受害林分内高达78.3%。根据变异函数曲线图分析得知:轻度受害林、中度受害林和重度受害林内褐梗天牛幼虫种群空间分布最优拟合模型分别为高斯模型、高斯模型和指数模型,成虫的种群空间分布最优拟合模型均为线性模型。在3种林分中褐梗天牛幼虫数量具有明显的空间依赖性,轻度受害林、中度受害林和重度受害林内幼虫数量的空间依赖范围分别是19.10、11.97、61.98m,其空间连续性强度分别是0.646、0.784和0.500;成虫的空间依赖范围分别是43.08、43.23、44.17m,其空间连续性强度分别是0.044、0.021和0.171,但其成虫的数量在空间呈随机分布,没有表现出空间依赖性。根据垂直分布图分析得出:褐梗天牛幼虫和成虫在油松上主要集中聚集在某个高,然后随着高度增加密度降低,随着高度接近地面密度也降低。用Kriging插值法生成的空间分布图显示幼虫在空间分布上具有明显的聚集性,其聚集中心主要集中在林地中心,由林地中心株向整个林地扩散,而成虫则表现为随机分布。     

Ecology and impact of Allorhogas sp. (Hymenoptera: Braconidae) and Apion sp. (Coleoptera: Curculionoidea) on fruits of Miconia calvescens DC (Melastomataceae) in Brazil

Two fruit-feeding insects, a gall wasp, Allorhogas sp. (Hymenoptera: Braconidae), and a beetle, Apion sp. (Coleoptera: Curculionoidea), were evaluated in their native habitat in Brazil as potential biological control agents of Miconia calvescens DC (Melastomataceae). Allorhogas sp. occurred at two out of three field sites with native populations of M. calvescens, and Apion sp. occurred at all three sites. Both species exhibited aggregated distributions among M. calvescens trees sampled at each site. Allorhogas sp. infested 9.0% and 3.8% of fruits at each of two sites. The number of larvae and pupae of Allorhogas sp. and/or an unidentified parasitoid (Hymenopetera: Eulophidae: Tetrastichinae) ranged from one to five per infested fruit. Fruits infested with Allorhogas sp. were 20% larger and had 79% fewer seeds than healthy fruits. Although adults of Apion sp. were found on leaves and inflorescences of M. calvescens at all three sites, larvae and pupae were found in fruits at only one site, where a maximum of 1.4% of fruits were infested. Fruits infested by Apion sp. contained only one larva or pupa, and were 15% smaller and had 62% fewer seeds than healthy fruits. While a variety of apionids have been used for biological control in the past, this is the first time a braconid wasp has been considered for biological control of a weed.     

Evaluation of endophytic actinobacteria as antagonists of Gaeumannomyces graminis var. tritici in wheat

Endophytic actinobacteria isolated from healthy cereal plants were assessed for their ability to control fungal root pathogens of cereal crops both in vitro and in planta. Thirty eight strains belonging to the genera Streptomyces, Microbispora, Micromonospora, and Nocardioidies were assayed for their ability to produce antifungal compounds in vitro against Gaeumannomyces graminis var. tritici (Ggt), the causal agent of take-all disease in wheat, Rhizoctonia solani and Pythium spp. Spores of these strains were applied as coatings to wheat seed, with five replicates (25 plants), and assayed for the control of take-all disease in planta in steamed soil. The biocontrol activity of the 17 most active actinobacterial strains was tested further in a field soil naturally infested with take-all and Rhizoctonia. Sixty-four percent of this group of microorganisms exhibited antifungal activity in vitro, which is not unexpected as actinobacteria are recognized as prolific producers of bioactive secondary metabolites. Seventeen of the actinobacteria displayed statistically significant activity in planta against Ggt in the steamed soil bioassay. The active endophytes included a number of Streptomyces, as well as Microbispora and Nocardioides spp. and were also able to control the development of disease symptoms in treated plants exposed to Ggt and Rhizoctonia in the field soil. The results of this study indicate that endophytic actinobacteria may provide an advantage as biological control agents for use in the field, where others have failed, due to their ability to colonize the internal tissues of the host plant.     

Parasitism by Pteromalus cerealellae (Hymenoptera: Pteromalidae) on the Cowpea weevil, Callosbruchus maculatus (Coleoptera: Bruchidae): Host density, temperature effects, and host finding ability

Studies on interactions between a larval parasitoid, Pteromalus cerealellae (Boucek) and one of its hosts, Callosobruchus maculatus (F.) were carried out in the laboratory. The number of host larvae parasitized by P. cerealellae increased with host larvae at low densities and tended to a plateau at a density of 25 larvae per female parasitoid. Each parasitoid was able to parasitize more hosts and produced more offspring at 20 and 25 °C than at 30 °C. The number of non-infested seeds mixed with seeds infested with the last instar of C. maculatus did not preclude P. cerealellae from identifying infested seeds and attacking larvae inside them. When infested seeds were tightly packed, several host larvae escaped parasitism. P. cerealellae may be a useful biological control agent in newly harvested cowpea with low C. maculatus infestation, and lowering the temperature of the storage system may enhance the effectiveness of this parasitoid.     

Field ecology of the ochratoxin A-producing Penicillium verrucosum: Survival and resource colonisation in soil

A field experiment was conducted to elucidate the survival of P. verrucosum in infested bulk soil(T1) and infested soil with waste grain (T2). The infested soil and reference soil (T3) was filled into steel cylinders, which were buried and sampled 13 times during a period from October 1994 to March 1996.The abundance of P. verrucosum and indigenous soil fungi were assessed by dilution plating on a selective and diagnostic medium (DYSG). Kernel infection was examined in T2. According to our results, P. verrucosum seems well adapted to survival in arable soil and little affected by indigenous fungi. During the first autumn and winter the grain caused a proliferation of P. verrucosum while its abundance in bulk soil was more constant except for a decrease in February 95, which is ascribed to frost/thaw alternations. In T2, P. verrucosum initially infected more than 50% of the kernels but during the first few months it was ousted by other fungi. A hypothesis regarding waste grain as the natural niche for the fungus in the field was therefore partly rejected. A gradual decrease in the abundance of P. verrucosum in soil during spring, a die-off in the dry summer and aproliferation during the second winter were found in both T1 and T2. Our results cannot provide the reason for the increase during the second winter. On an overall basis, however, they show that P. verrucosum can survive in the field, proliferate on soil organic matter and probably become an integral part of the soil ecosystem. This may constitute a risk of grain contamination when given appropriate environmental conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

Pre- and post-landing response of the parasitoid Encarsia formosa to whitefly hosts on Gerbera jamesonii

One of the factors that may complicate biological control of the greenhouse whitefly on Gerbera jamesonii by Encarsia formosa is the rosette shape of this ornamental, which differs from the vertical shape of most vegetable plants (cucumber, egg plant, tomato, etc.). Therefore, host-habitat location and the behaviour prior to landing on uninfested and infested leaves was studied. Attraction of E. formosa from a short distance by infested leaves could not be detected: the parasitoid females landed at random on uninfested and infested leaves. After the first landing, a redistribution of the wasps occurred on the leaves. After 24 h three times as many wasps were found on the infested leaves than on uninfested ones. In a dispersal experiment with four plants, E. formosa appeared to have no preference for landing on leaves of the medium age class, which is the age class on which most of the whiteflies in a suitable stage for parasitism occur. Twenty percent of the parasitoids were found on the plants 20 min after releasing them. These results were independent of the plant cultivar and the host density on the plants. In the course of 8 h, the number of E. formosa females recovered from plants increased linearly, and this increase was greater on plants where hosts were present and also greater on the plant cultivar with the lowest trichome density. After 24 h, the percentage of females was highest (56%) on plants with the highest host density. E. formosa females were arrested on leaves where hosts were present. Contrary to our expectation, the results from the two G. jamesonii cultivars that differed strongly in leaf hairiness were not significantly different in most experiments. Only at the high host density was parasitism found to be lower on the cultivar with the higher hair density. Parasitoids may walk on top of the `hair coverlet' of cultivars with high trichome density and, therefore, be hampered less than expected.     

Nature of interference potential of leaf debris of Ageratum conyzoides

The present study investigated the allelopathic interference of leaf debris of Ageratum conyzoides (billy goat weed; Asteraceae)—a weed of cultivated land—against rice (Oryza sativa). Seedling length and dry weight of rice were significantly reduced (16–20%) in soil from A. conyzoides infested fields compared to the soil from an area devoid of the weed. It indicated the presence of certain phytotoxins in the A. conyzoides infested soil. To explore the possible contribution of the weed in releasing these phytotoxins, growth studies involving leaf debris extracts and amended soils (prepared by incorporating leaf debris—5, 10, 20 g kg⁻¹ soil, w/w, or its extracts—0.5%, 1.0% and 2.0%, v/v) were conducted. The growth of rice was severely inhibited in A. conyzoides leaf debris- and debris extract-amended soils compared to unamended control soil. A significant amount of water-soluble phenolics, the potent phytotoxins, was found in the A. conyzoides infested soil, leaf debris, and debris-amended soils. These phenolics were identified as gallic acid, coumalic acid, protocatechuic acid, catechin and p-hydroxybenzoic acid. Among these, protocatechuic acid was in the maximum amount (35.72%) followed by coumalic acid (33.49%) and these two accounted for >69% of total phenolic compounds. Further, there was a significant increase in the available nutrient content in soil amended with A. conyzoides leaf debris thus ruling out the possibility of any resource depletion upon residue incorporation and their negative role in causing growth reduction. Based on the observations, the present study concludes that leaf debris of A. conyzoides deleteriously affects the early growth of rice by releasing water-soluble phenolic acids into the soil environment and not through soil nutrient depletion.     

降香黄檀土壤微生物数量的时空动态特征

【目的】研究降香黄檀土壤微生物数量在时间和空间上的动态特征,可为降香黄檀栽培技术的完善提供数据支持。【方法】采用稀释平板涂布法,比较分析降香黄檀根际和非根际土壤微生物三大类群(细菌、放线菌、真菌)数量的季节及垂直分布情况。【结果】(1)降香黄檀根际和非根际土壤均表现为春季细菌、真菌数量较多,冬季细菌数量较少,秋季真菌数量较少;非根际土壤放线菌冬季数量较多,秋季数量较少。(2)细菌的根际效应最为明显,其R/S为0.12~62.96;真菌次之,其R/S为0.22~2.46;放线菌的根际效应较小,其R/S为0.32~1.01。(3)降香黄檀上层土(0~20 cm)的微生物数量普遍高于其他土层;随着土层加深,微生物数量表现为逐渐下降或先下降后升高2种变化趋势。【结论】降香黄檀土壤微生物数量的分布受季节和土层的影响。     

Effect of Soil Texture and Moisture on the Activity of Entomopathogenic Nematodes Against Female Boophilus annulatus Ticks

Soil texture, chemistry and moisture have a profound effect upon the activity and persistence of entomopathogenic nematodes (EPNs). Whereas nematodes’ natural habitat is within the soil, ticks and other arthropod pests prefer to stay on the soil surface and under stones or leaf litter; they spend much of their life cycle in the humid environment of the soil upper layer, therefore consideration of the effect of the soil environment on nematode activity is a pre-requisite for the sucessful use of EPNs against arthropod pests. In the present study we investigated the effects of soil type, and humidity on various nematode strains and on their effectiveness against ticks. Many infective juveniles (IJs) of Steinernema carpocapsae and S. riobrave were found in the uppermost soil layer whereas the heterorhabditid strains were almost absent from the upper 6 cm of the soil profile. The IJs of S. feltiae, and the S. carpocapsae strain S-20, exhibited an intermediate behavior. It was found that the activity of IJs of S. carpocapsae in the soil upper layer (1 cm depth) was strongly affected by soil type: the greatest number of IJs were recorded from sandy loam soil; less were found in the lighter soils – ‘Marine sand’ and ‘Calcareous sandstone’ – and only very few were recovered from heavy soils. Strikingly, even when the soil moisture was low and the number of nematodes found in the upper layer correspondingly low, tick mortality remained high. The results demonstrate: (a) the possible use of the nematodes as an anti-tick agent; (b) the importance of knowing the exact interaction of nematodes with the immediate environment of the pest, in order to optimize the pest-control activity of the nematode.     

Enhancement of sporulation in species of Bipolaris, Curvularia, Drechslera, and Exserohilum by growth on cellulose-containing substrates

Nine species of Bipolaris, Curvularia, Drechslera, and Exserohilum were compared for sporulation on agar media and for enhancement of sporulation by growth on four cellulose-containing substrates (index card, filter paper, cheesecloth, cotton fabric). On two natural and one synthetic agar media, sporulation varied from profuse to nonexistent among three isolates of each species. Growth of all species on cellulose substrates resulted in large and significant increases in sporulation. Growth on index card pieces often provided the greatest increases, but no single substrate was superior for all species, and significant substrate × isolate interactions were observed within species. Overlay of filter paper onto whole colonies in agar plates resulted in 2 to 18-fold increases in sporulation for eight of nine species and production of spores in sufficient quantity for most experimental purposes. Overlay of soil dilution plates with filter paper to promote sporulation of colonies enabled detection of B. spicifera, B. hawaiiensis, C. lunata, and E. rostratum at relatively low population levels (≤1.3 × 10³ colony-forming units per gram of soil) in samples of a naturally infested soil. Results indicate that enhancement of sporulation by growth of species of Bipolaris, Curvularia, Drechslera, and Exserohilum on cellulose substrates may facilitate (i) their identification in culture, (ii) production of spores at relatively high concentrations, and (iii) detection and enumeration of these fungi in soil.     

Survival ofXanthomonas campestris pv.vesictoria in pepper seeds and roots in symptomless and dry leaves in non-host plants and in the soil

Summary A population ofXanthomonas campestris pv.vesicatoria developed as endophytes in the leaves and rhizosphere of apparently symptomless plants grown under mist but not under dry conditions. The pathogen survired for long periods on, and could be isolated from, the surface of infested dried seeds, inoculated sandy loam soil, dried leaves, and the rhizosphere of pepper and of other non-host plants. In addition, small numbers of the pathogen survived for 18 months in a field previously cropped with pepper diseased with bacterial scab. Healthy nursery or mature plants developed symptoms while growing in soil containing infested leaves, either buried or placed on the soil surface.     

Searching and Oviposition Behavior of Anagrus atomus L. (Hymenoptera: Mymaridae) on Four Host Plants of Its Host, the Green Leafhopper Empoasca decipiens Paoli (Homoptera: Cicadellidae)

Anagrus atomus L. is an important egg parasitoid of the green leafhopper Empoasca decipiens Paoli. In this study the ability of the parasitoid to locate and parasitize its host was investigated on four host plants, i.e., broad beans (Vicia faba L.), sweet pepper (Capsicum annuum L.), cucumber (Cucumis sativus L.), and French beans (Phaseolus vulgaris L.). For each plant species, the behavior of the parasitoid was observed on E. decipiens infested and noninfested plants. Searching and oviposition behavior were characterized by drumming, probing, and resting. Parasitoids spent significantly less time on non-infested than infested plants, 274.5 and 875.7 s, respectively, and no probing behavior was observed on non-infested plants. Frequency of resting behavior was significantly greater on non-infested than on infested plants. Total foraging time was significantly longer on infested than on non-infested plants, indicating that A. atomus females can efficiently discriminate between leaves with and without infestation. Parasitism of A. atomus was influenced by parasitoid density, with the highest parasitism rate (64.0%) obtained at a density of 10 A. atomus females/0.1356 m² but the number of parasitized eggs per female and the searching efficiency decreased with increasing parasitoid density.     

Survival and proliferation in soil of Trichoderma koningii in alginate prills

Trichoderma koningii may act as an antagonist of several phytopathogenic fungi. The incorporation of its biomass as alginate prills with or without wheat bran and the storage temperature effects on viability were evaluated. The prills were produced by adding T. koningii (Tkp) biomass to a sodium alginate suspension and dripping into calcium chloride solution. In one of the treatments wheat bran was added. The prills were dried and stored for 1 month, 2 months, 1 year and 2 years at 5 and 25 °C. To assess viability, plastic trays filled with sterile soil were infested. The number of colony-forming units (c.f.u.) of T. koningii was evaluated after 4, 8 and 12 weeks. Wheat bran prills remained biologically active for more than 2 years when kept at 5 °C. After 12 weeks the c.f.u. number per gram was 3.7 × 10⁴. This value indicates that T. koningii might be applied to the soil as alginate prills.