Control of Tylenchulus semipenetrans on Citrus With Aldicarb,Oxamyl, and DBCP

Soil application of DBCP (l,2-dibromo-3-chloropropane) and foliar applications of oxamyl (methyl N'',N''-dimethyl-N-[(methylcarbamoyl)oxy]-l-thiooxamimidate) were compared for control of Tylenchulus semipenetrans in a grapefruit (Citrus paradisi) orchard, DBCP reduced nematode populations and increased fruit growth rate, fruit size at harvest, and yield compared to the untreated controls in the 2 years following treatments. Foliar applications of oxamyl reduced nematode populations and increased fruit growth rate slightly the first year, but not in the second. Foliar applications of oxamyl did not improve control attained by DBCP alone. Soil application of aldicarb [2-methyl-2-(methylthio)propionaldehyde-O-(methylcarbamoyl)oxime] or DBCP to an orange (C. sinensis) orchard reduced T. semipenetrans populations in the 3 years tested and increased yield in 1 of 3 years. Aldicarb treatment reduced fruit damage caused by the citrus rust mite, Phyllocoptruta oleivora. Aldicarb, applied at 5.7 or 11.4 kg/ha, by disk incorporation or chisel injection, was equally effective in controlling nematodes, improving yields, fruit size, and external quality. In a grapefruit orchard, chisel-applied aldicarb reduced nematode populations and rust mite damage and increased yields in both years and increased fruit size in one year. The 11.4-kg/ha rate was slightly more effective than the 5.7-kg/ha rate. Aldicarb appears to be an adequate substitute for DBCP for nematode control in Texas citrus orchards and well-suited to an overall pest management system for Texas citrus.     

Aggressiveness and Damage Potential of Central American and Caribbean Populations of Radopholus spp. in Banana

Monoxenic cultures of burrowing nematode populations extracted from banana roots from Belize, Guatemala, Honduras, and Costa Rica were established on carrot discs. Cultures of Radopholus spp. were also obtained from Florida, Puerto Rico, Dominican Republic, and Ivory Coast. The aggressiveness (defined as reproductive fitness and root necrosis) of these populations was evaluated by inoculating banana plants (Musa AAA, cv. Grande Naine) with 200 nematodes/plant. Banana plants produced by tissue culture were grown in 0.4-liter styrofoam cups, containing a 1:1 mix of a coarse and a fine sand, at ca. 27 °C and 80% RH. Banana plants were acclimated and allowed to grow for 4 weeks prior to inoculation. Plant height, fresh shoot and root weights, root necrosis, and nematode population densities were determined 8 weeks after inoculation. Burrowing-nematode populations varied in aggressiveness, and their reproductive fitness was generally related to damage reported in the field. Plant height and fresh shoot and root weight did not reflect damage caused by nematodes under our experimental conditions. Necrosis of primary roots was closely related to the reproductive fitness of the nematode populations. Variation in aggressiveness among nematode populations followed a similar trend in the two susceptible hosts tested, Grande Naine and Pisang mas. All nematode populations had a low reproductive factor (Rf ≤2.5) in the resistant host except for the Ivory Coast population which had a moderate reproductive factor (Rf ≤ 5) on Pisang Jari Buaya. This is the first report of a burrowing nematode population parasitizing this important source of resistance to R. similis.     

Effects of Culture Method and Formulation on the Virulence of Steinernema riobrave (Rhabditida: Steinernematidae) to Diaprepes abbreviatus (Coleoptera: Curculionidae)

The Diaprepes root weevil, Diaprepes abbreviatus, is a pest of vegetables, ornamental plants, sugarcane, and citrus in Florida and the Caribbean. The entomopathogenic nematode, Steinernema riobrave, can reduce larval populations of D. abbreviatus substantially. Efficacy of entomopathogenic nematodes, however, may be affected by culture method and formulation. Using D. abbreviatus as the host, we compared the efficacy of two commercial S. riobrave formulations, a liquid and a waterdispersible granule (WDG), with each other and with in vivo produced S. riobrave. Nematodes in the commercial formulations were produced in vitro through liquid fermentation; the in vivo nematodes were cultured in Galleria mellonella and applied in aqueous suspension. Laboratory experiments measured nematode virulence in plastic cups containing soil and seventh-eighth instar D. abbreviatus. One laboratory experiment was conducted using only fresh nematodes (less than 5 days old); another experiment included WDG nematodes that were stored for 25 days at 10 °C. Two field experiments were conducted in which nematodes were applied either to potted citrus (containing D. abbreviatus larvae) placed beneath mature citrus trees or to soil directly beneath the tree. In the latter experiment, efficacy was determined by measuring mortality of caged D. abbreviatus larvae that were buried beneath the soil surface prior to application. Mortality of D. abbreviatus treated with nematodes ranged from 80-98% and 50-75% in laboratory and field experiments, respectively. In all experiments, we did not detect any significant effects of culture method or formulation.     

Nematode–citrus plant interactions: host preference,damage rate and molecular characterization of Citrus root nematode Tylenchulus semipenetrans

• Citrus plants are host to several plant parasitic nematodes (PPNs), which are microscopic organisms. Among PPNs, the citrus root nematode, T. semipenetrans (Cobb 1913) (Tylenchida: Tylenchulidae), causes significant damage to citrus plantations worldwide. Understanding citrus nematode populations, precise identification, host preference among citrus species, and damage threshold are crucial to control T. semipenetrans. The minutiae of citrus plant–nematode interactions, nematode density and molecular nematode identification are not well understood. In this study, nematode species and density in citrus orchards, host specialization, molecular and morphological characteristics of nematodes were assessed.
• Molecular and morphological methods, host–nematode interactions, host (citrus species) preference, damage economic threshold (ET), and economic injury level (EIL) were determined using laboratory methods and field sampling. Citrus plantations in different provinces in the Mediterranean region of Turkey were investigated.
• Nematode species were identified molecularly and morphologically. ITS sequences revealed that samples were infected by citrus root nematode T. semipenetrans. The lowest nematode density was in C. reticulata in Mersin (53 2nd stage juveniles (J2s) 100 g⁻¹ soil), while the highest density was from Hatay in C. sinensis (12173 J2s 100 g⁻¹ soil). Highest citrus nematode population density was on roots of C. reticulata, followed by C. sinensis, C. limon, and C. paradisi.
• The citrus nematode is more common than was thought and population fluctuations change according to specific citrus species. Environmental conditions, host and ecological factors, such as temperature, soil pH, and soil nutrients, might influence nematode populations in citrus orchards. Investigating nematode density in diverse soil ecologies and the responses of different resistant/tolerant citrus species and cultivars to nematode populations is essential in future studies.

     

Responses of Tylenchulus semipenetrans to Citrus Fruit Removal: Implications for Carbohydrate Competition

Sixteen mature Valencia orange trees on rough lemon rootstock were selected on the basis of approximately equal, naturally occurring populations of Tylenchulus semipenetrans in soil. In March, fruit 1 cm in diameter or less were removed from eight of the trees, which were kept free of fruit for 15 months. In July, 4 months after fruit removal, fibrous root (<2 mm d) mass density of defruited trees was 51% greater and insoluble starch in fibrous roots was 24% less than on control trees with fruit. Female T. semipenetrans per gram of root were 64% more numerous on roots of control trees than on defruited trees at this time. Numbers of female nematodes per tree and of juveniles and males in soil did not differ between treatments 4 months after fruit removal. Root mass density remained higher on defruited than control trees for the remaining 13 months that the trees were studied, while nematode density in soil beneath defruited trees rapidly increased to levels proportionate to the additional root mass density. Nine months after fruit removal (December), starch concentration was 84% higher in roots of defruited trees compared to controls and remained 28% higher than in controls 15 months (May) following fruit removal. Between months 9 and 15 following fruit removal, nematode density in soil beneath defruited trees increased at a rate five times that of nematode density beneath control trees. In May, female fecundity (eggs/female) on defruited trees was 41% greater than on control trees. The data were consistent with the hypothesis that carbohydrate competition between developing citrus fruit and T. semipenetrans influences seasonal fluctuations in nematode population densities.     

Influence of the Sting Nematode, Belonolaimus longicaudalus,on Young Citrus Trees

The sting nematode, Belonolaimus longicaudatus, was associated with poor growth of citrus in a central Florida nursery. Foliage of trees was sparse and chlorotic. Affected rootstocks included Changsha and Cleopatra mandarin orange; Flying Dragon, Rubidoux, and Jacobsen trifoliate orange; Macrophylla and Milam lemon; Palestine sweet lime; sour orange; and the hybrids - Carrizo, Morton, and Rusk citrange and Swingle citrumelo. Root symptoms included apical swelling, development of swollen terminals containing 3-5 apical meristems and hyperplastic tissue, coarse roots, and a reduction in the number of fibrous roots. Population densities as high as 392 sting nematodes per liter soil were detected, with 80% of the population occurring in the top 30 cm of soil; however, nematodes were detected to 107 cm deep. Although an ectoparasite, the nematode was closely associated with citrus root systems and was transported with bare root nursery stock. Disinfestation was accomplished by hot water treatment (49 C for 5 minutes).     

Genome Similarity Implies that Citrus-Parasitic Burrowing Nematodes do not Represent a Unique Species

Burrowing nematodes from Central America, Dominican Republic, Florida, Guadeloupe, Hawaii, and Puerto Rico were characterized for their ability to parasitize citrus, but citrus parasites were found only in Florida. Sequence tag sites originally amplified from a citrus-parasitic burrowing nematode were polymorphic among 37 burrowing nematode isolates and were not correlated with citrus parasitism, nematode isolate collection site, or amplification of a 2.4-kb sequence tag site (DK#1). Results of a RAPD analysis and characterization of the isozymes phosphoglucose isomerase, lactate dehydrogenase, and malate dehydrogenase indicated that the burrowing nematode isolates were highly similar. Citrus parasitism in Florida appears to be associated with limited changes in the burrowing nematode genome. Findings did not substantiate a previous report that R. citrophilus was present in Hawaii. Overall, these data do not support assignment of sibling species status to burrowing nematodes that differ with respect to citrus parasitism.     

Antagonists of Plant-parasitic Nematodes in Florida Citrus

In a survey of antagonists of nematodes in 27 citrus groves, each with a history of Tylenchulus semipenetrans infestation, and 17 noncitrus habitats in Florida, approximately 24 species of microbial antagonists capable of attacking vermiform stages of Radopholus citrophilus were recovered. Eleven of these microbes and a species of Pasteuria also were observed attacking vermiform stages of T. semipenetrans. Verticillium chlamydosporium, Paecilomyces lilacinus, P. marquandii, Streptomyces sp., Arthrobotrys oligospora, and Dactylella ellipsospora were found infecting T. semipenetrans egg masses. Two species of nematophagous amoebae, five species of predatory nematodes, and 29 species of nematophagous arthropods also were detected. Nematode-trapping fungi and nematophagous arthropods were common inhabitants of citrus groves with a history of citrus nematode infestation; however, obligate parasites of nematodes were rare.     

A Classification of Tylenchulus semipenetrans Biotypes

The presence of two biotypes of the citrus nematode (Tylenchulus semipenetrans) in Italian citrus and olive orchards has been confirmed by comparing host specificity. Host reaction to California biotypes C1 and C3 and to three populations from Arizona, Texas, and Florida indicates that of these five United States biotypes, all except C3 consistently fit biotype C1. These findings, and the results of host-range studies in other countries, show that four biotypes of T. semipenetrans are distributed worldwide: the "Poncirus biotype," the "Citrus biotype," the "Mediterranean biotype," and the "Grass biotype."     

A Bioassay to Estimate Root Penetration by Nematodes

An in vitro bioassay with a 96-well microtiter plate was used to study the effect of lectins on burrowing nematode penetration of citrus roots. In each well, one 4-mm root segment, excised from the zone of elongation of rough lemon roots, was buried in 0.88 g dry sand. Addition of a Radopholus citrophilus suspension containing ca. 300 nematodes in 50 μ1 test solution completely moistened the sand in each well. The technique assured uniform treatment concentration throughout the medium. Within 16-24 hours, burrowing nematodes penetrated citrus root pieces, primarily through the cut ends. The lectins (100 μg/ml) Concanavalin A (Con A), soybean agglutinin (SBA), wheat germ agglutinin (WGA), and Lotus tetragonolobus agglutinin (LOT) stimulated an increase in penetration of citrus root segments by Radopholus citrophilus. Concentrations as low as 12.5 μg/ml Con A, LOT, and WGA stimulated burrowing nematode penetration of citrus roots. Heat denaturation of the lectins reversed their effect on penetration; however, incubation of nematodes in lectin (25 μg/ml) with 25 mM competitive sugars did not. The reason for enhanced penetration associated with lectins is unclear.     

Dynamics of Belonolaimus longicaudatus Parasitism on a Susceptible St. Augustinegrass Host

St. Augustinegrass (Stenotaphrum secundatum) cv FX-313 was used as a model laboratory host for monitoring population growth of the sting nematode, Belonolaimus longicaudatus, and for quantifying the effects of sting nematode parasitism on host performance in two samples of autoclaved native Margate fine sand with contrasting amounts of organic matter (OM = 7.9% and 3.8%). Following inoculation with 50 Belonolaimus longicaudatus per pot, nematodes peaked at a mean of 2,139 nematodes per pot 84 days after inoculation, remained stable through 168 days at 2,064 nematodes per pot, and declined at 210 days. The relative numbers of juveniles and adults demonstrated senescence after 84 days. Root dry weight of nematode-inoculated plants increased briefly to an apparent equilibrium 84 days after inoculation, whereas root weights of uninoculated controls continued to increase, exceeding those of inoculated plants from 84 to 210 days (P < 0.01). At 210 days, uninoculated plants had 227% the root dry weight of inoculated plants. Transpiration of FX-313 was reduced by nematodes (P < 0.0001) at 84 and 126 days after inoculation; reduction was first observed at 42 days and last observed 168 days after inoculation (P < 0.05). OM content affected all plant performance variables at multiple dates, and generally there were no inoculation x OM content interactions. OM content had no effect on nematode numbers per pot, although there was a slight (P < 0.05) increase in the number of nematodes per gram root dry weight in the low-OM soil compared with the high-OM soil.     

Distribution of Tylenchulus semipenetrans in a Texas Grapefruit Orchard

Distribution of the citrus nematode (Tylenchulus semipenetrans) was studied over 18 months in a 6-year-old orchard of grapefruit (Citrus paradisi cv. Ruby Red) on sour orange (C. aurantium) rootstock. The 1.8-ha orchard was under chemical weed control, no tillage, and flood irrigation. Highest numbers of nematodes were found in the top 15 cm of the soil profile. The nematode population peaked in April and declined to lowest levels in August and September. Numbers of nematodes were negatively correlated (r = -0.95) with soil temperatures above 29 C. Soil populations of nematodes were not correlated with soil moisture. The distribution of the nematode in the field was highly skewed and was described by a negative binomial. In this 1.8-ha block, five soil samples of 12 cores each would provide an estimate within 20% of the true nematode population mean with 95% confidence.     

Grower Acceptance of Entomopathogenic Nematodes: Case Studies on Three Continents

Projects to manage arthropod pests using entomopathogenic nematodes (EPNs) in Brazil, Korea and USA are reviewed to identify conditions and practices that affected the use of EPNs for pest management. A proliferation of covered agriculture in Korea, the growth in demand for high value, pesticide-free produce in Korea and Brazil, and the cost-effectiveness of EPNs created favorable conditions for the widespread adoption of EPN products in Brazilian guava orchards and Korean vegetable greenhouses. In Florida, EPNs imported from South America function successfully as classical biocontrol agents against invasive mole crickets attacking pasture and turf. However, the low value of pasture and the availability of cost-effective chemical insecticides in turf have depressed the demand for EPN products to control mole crickets. In Florida citrus orchards, a recent, dramatic increase in the use of chemical insecticides to control an arthropod vector of a devastating bacterial disease of citrus (huanglongbing) reduced the demand for EPN products to control Diaprepes root weevils. Nevertheless, a rich and diverse EPN fauna in the Florida peninsula provides significant control of subterranean stages of root weevils in some habitats, and is the focus of research to develop cultural practices that exploit the potential for increased pest management through EPN conservation.     

From Augmentation to Conservation of Entomopathogenic Nematodes: Trophic Cascades, Habitat Manipulation and Enhanced Biological Control of Diaprepes abbreviatus Root Weevils in Florida Citrus Groves

The use of entomopathogenic nematodes (EPN) for management of the root weevil, Diaprepes abbreviatus, in Florida citrus groves is considered a biological control success story and typically involves augmentation in which EPN are applied inundatively as biopesticides to quickly kill the pest. However, recent evidence indicates that efficacy of EPN applications in Florida citrus depends on soil type. They are very effective in the well drained coarse sands of the Central Ridge but often less so in poorly drained fine-textured soils of the Flatwoods. Moreover, groves on the Central Ridge can harbor rich communities of endemic EPN that might often suppress weevil populations below economic thresholds, whereas Flatwoods groves tend to have few endemic EPN and frequent weevil problems. Current research is examining the ecological dynamics of EPN in Florida citrus groves, the potential impact of EPN augmentation on soil food webs, especially endemic EPN, and whether habitat manipulation and inoculation strategies might be effective for conserving and enhancing EPN communities to achieve long-term control in problem areas. Conservation biological control could extend the usefulness of EPN in Florida citrus and be especially appropriate for groves with persistent weevil problems.     

Comparative Response of Alfalfa to Pratylenchus penetrans Populations

Four populations of Pratylenchus penetrans did not differ (P > 0.05) in their virulence or reproductive capability on Lahontan alfalfa. There was a negative relationship (r = -0 .7 9 ) between plant survival and nematode inocula densities at 26 ± 3 C in the greenhouse. All plants survived at an inoculum level (Pi) of 1 nematode/cm³ soil, whereas survival rates were 50 to 55% at 20 nematodes/cm³ soil. Alfalfa shoot and root weights were negatively correlated (r = - 0.87; P < 0.05) with nematode inoculum densities. Plant shoot weight reductions ranged from 13 % at Pi 1 nematode/cm³ soil to 69% for Pi 20 nematodes/cm³ soil, whereas root weight reductions ranged from 17% for Pi 1 nematode/cm³ soil to 75% for Pi 20 nematodes/cm³ soil. Maximum and minimum nematode reproduction (Pf/Pi) for the P. penetrans populations were 26.7 and 6.2 for Pi 1 and 20 nematodes/cm³ soil, respectively. There were negative correlations between nematode inoculum densities and plant survival (r = 0.84), and soil temperature and plant survival (r = -0 .7 8 ). Nematode reproduction was positively correlated to root weight (r = 0.89).     

Response of Peach Seedlings to Infection by the Root Lesion Nematode Pratylenchus penetrans under Controlled Conditions

Twenty-one open pollinated populations of peach rootstock seedlings were evaluated for their response to infection by the root lesion nematode, Pratylenchus penetrans, over a period of 98 days. Nematode-infected peach seedling populations were shorter in plant height and had less shoot weight but more dry root weight than nematode-free controls. Rootstock differences were demonstrated for nematode increase over the 98-day period, and average total numbers of nematodes in soil and roots. Rootstocks were classified into three groups differing in total nematode population levels, ratio of nematode increase, and the number of nematodes per root. The heritable nature of rootstock response to nematodes was evident. Rootstocks showing the lowest response to nematode infection included Tzim Pee Tao, Rutgers Red Leaf, and two progenies of a cross of these two rootstocks.     

Lectin Binding to Radopholus citrophilus and R. similis Proteins

Lectin-binding glycoproteins in seven populations of two burrowing nematode sibling species were probed with five different biotinylated lectins on Western blots, and differences were correlated with nematode ability to parasitize citrus and to overcome citrus rootstock resistance. Banding patterns of molecular weight standards were fit best by an exponential decay function, and a predictive equation was used to estimate molecular weights (r² = 0.999). A band (131 kDa) that labeled with the lectin Concanavalin A (Con A) occurred in extracts from cuticles and egg shells of populations of Radopholus citrophilus that parasitize citrus. Wheat germ agglutin labeled a band (58 kDa) in aqueous homogenates of populations that reproduce in roots of citrus rootstock normally resistant to burrowing nematodes. The two sibling species R. citrophilus and R. similis were distinguished by a high molecular weight Con A-labeled band (608 kDa) from cuticle and egg shells. Probing blots with the lectin Limulus polyphemus agglutinin indicated that each population contained a band (12-16 kDa) specifically inhibited by the addition of 25 mM neuraminic acid, suggesting that glycoproteins with sialic acid moieties are present in burrowing nematodes.     

环渤海湾苹果产区老果园与连作果园土壤线虫群落特征

应用类群多样性、类群丰富度、个体密度和功能类群指数等群落参数,于2009年5-10月分3次对环渤海湾苹果栽植区老果园不同位置和连作果园取样研究了土壤线虫群落特征。结果表明:苹果连作对果园土壤生物环境具有明显的恶化作用,连作果园土壤环境条件恶劣;对功能类群指数的统计表明,苹果连作明显改变了果园土壤中植物寄生性线虫r-选择和k-选择的比例,与自由生活线虫比较,连作对植物寄生线虫影响更明显;连作提高了果园的土壤线虫PPI/MI,说明连作对果园土壤健康的扰动最大;土壤不同食性线虫数量统计结果显示,环渤海湾苹果栽植区植食性线虫数量未达线虫伤害阈值,线虫不是引起环渤海湾地区苹果连作障碍的主要原因。     

In Vitro Culture and Feeding Behavior of Belonolaimus longicaudatus on Excised Zea mays Roots

A greenhouse population of the sting nematode, Belonolaimus longicaudatus, obtained from an infested golf course in California''s Coachella Valley, was surface-decontaminated and cuhured on excised roots of Zea mays supported by Gamborg''s B5 medium. At 26-27 °C the females laid eggs, and newly emerged juveniles of the second generation completed three molts within 29 days after egg deposition. Sixty days after inoculation with 60 females and 40 males, an average of 529 nematodes and 83 eggs were recovered from the culture. The feeding process consisted of probing, stylet penetration, ingestion, and stylet retraction. Feeding seemed to be necessary before egg deposition or molting occurred. The sting nematode was observed feeding exclusively as an ectoparasite and preferably at the region of cell division and elongation. Vigorous feeding by many nematodes usually caused discoloration of root tips and termination of growth.