Gibberellic acid decreases Melanocallis caryaefoliae (Hemiptera: Aphididae) population density and chlorotic feeding injury to foliage in pecan orchards

BACKGROUND

Melanocallis caryaefoliae (Davis), Monellia caryella (Fitch), and Monelliopsis pecanis Bissell (Hemiptera: Aphididae) attack pecan foliage (Carya illinoinensis [Wangenh.] K. Koch). Unlike M. caryella and M. pecanis, feeding by M. caryaefoliae triggers a physiological change within foliage mimicking natural leaf senescence; it can lead to defoliation. Pretreatment of pecan foliage with gibberellic acid (GA₃ ) mitigates M. caryaefoliae-elicited physiological disturbances. GA₃ application to pecan was evaluated for efficacy regarding effects on M. caryaefoliae populations and possible negative side-effects on two natural enemy species and on return bloom of pecan.

RESULTS

All GA₃ treatment rate schedules significantly reduced M. caryaefoliae nymphs but not adults or adults and nymphs of M. caryella or M. pecanis. Percentage leaf chlorosis elicited by M. caryaefoliae was significantly reduced by GA₃ (i.e., 39.5 to 197.7 g a.i./ha). No negative side-effects of GA₃ treatment were detected regarding certain key natural enemy species or on return bloom of pecan.

CONCULUSION

Application of GA₃ to the orchard canopy protects foliage from senescence-like physiological responses triggered by M. caryaefoliae. This reduces detrimental leaflet chlorosis, both senescence and abscission processes and horticulturally significant feeding injury. Additionally, the absence of apparent negative side-effects on key natural enemies and return bloom is suggestive of a practical means for efficacious non-insecticidal control of M. caryaefoliae populations in orchards. This novel protective effect of GA₃ against aphid-elicited, senescence-like physiological responses may merit investigation as an IPM tool to manage aphid species eliciting similar senescence-like damage to other crop species. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.

Evidence for Sexual Reproduction: Identification, Frequency, and Spatial Distribution of Venturia effusa (Pecan Scab) Mating Type Idiomorphs

Venturia effusa (syn. Fusicladium effusum), causal agent of pecan scab, is the most prevalent pathogen of pecan (Carya illinoinensis), causing severe yield losses in the southeastern United States. V. effusa is currently known only by its asexual (conidial) stage. However, the degree and distribution of genetic diversity observed within and among populations of V. effusa are typical of a sexually reproducing fungal pathogen, and comparable with other dothideomycetes with a known sexual stage, including the closely related apple scab pathogen, V. inaequalis. Using the mating type (MAT) idiomorphs from V. inaequalis, we identified a single MAT gene, MAT1-1-1, in a draft genome of V. effusa. The MAT1-1-1 locus is flanked by two conserved genes encoding a DNA lyase (APN2) and a hypothetical protein. The MAT locus spanning the flanking genes was amplified and sequenced from a subset of 14 isolates, of which 7 contained MAT1-1-1 and the remaining samples contained MAT1-2-1. A multiplex polymerase chain reaction screen was developed to amplify MAT1-1-1, MAT1-2-1, and a conserved reference gene encoding β-tubulin, and used to screen 784 monoconidial isolates of V. effusa collected from 11 populations of pecan across the southeastern United States. A hierarchical sampling protocol representing region, orchard, and tree allowed for analysis of MAT structure at different spatial scales. Analysis of this collection revealed the frequency of the MAT idiomorphs is in a 1:1 equilibrium of MAT1-1:MAT1-2. The apparent equilibrium of the MAT idiomorphs provides impetus for a renewed effort to search for the sexual stage of V. effusa. [Formula: see text] Copyright © 2018 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Draft genome sequence of Venturia carpophila, the causal agent of peach scab

Venturia carpophila causes peach scab, a disease that renders peach (Prunus persica) fruit unmarketable. We report a high-quality draft genome sequence (36.9 Mb) of V. carpophila from an isolate collected from a peach tree in central Georgia in the United States. The genome annotation is described and a phylogenetic analysis of the pathogen is presented. The genome sequence will be a useful resource for various studies on the pathogen, including the biology and ecology, taxonomy and phylogeny, host interaction and coevolution, isolation and characterization of genes of interest, and development of molecular markers for genotyping and mapping.

Population Genetic Structure of Venturia effusa, Cause of Pecan Scab, in the Southeastern United States

Venturia effusa is the most important pathogen of pecan in the southeastern United States. Little information exists on the population biology and genetic diversity of the pathogen. A hierarchical sampling of 784 isolates from 63 trees in 11 pecan orchards in the southeastern United States were screened against a set of 30 previously characterized microsatellite markers. Populations were collected from Georgia (n = 2), Florida (n = 1), Alabama (n = 2), Mississippi (n = 1), Louisiana (n = 1), Illinois (n = 1), Oklahoma (n = 1), Texas (n = 1), and Kansas (n = 1). Clonality was low in all orchard populations (≤10.1% of isolates), and there were consistently high levels of genotypic diversity (Shannon-Weiner's index = 3.49 to 4.59) and gene diversity (Nei's measure = 0.513 to 0.713). Analysis of molecular variance showed that, although 81% of genetic diversity occurred at the scale of the individual tree, 16% occurred between orchards and only 3% between trees within orchards. All populations could be differentiated from each other (P = 0.01), and various cluster analyses indicated that some populations were more closely related compared with other pairs of populations. This is indicative of some limited population differentiation in V. effusa in the southeastern United States. Bayesian and nearest-neighbor methods suggested eight clusters, with orchards from Georgia and Florida being grouped together. A minimum spanning tree of all 784 isolates also indicated some isolate identification with source population. Linkage disequilibrium was detected in all but one population (Kansas), although 8 of the 11 populations had <20% of loci at disequilibrium. A Mantel test demonstrated a relationship between physical and genetic distance between populations (Z = 11.9, r = 0.559, P = 0.001). None of the populations were at mutation-drift equilibrium. All but 3 of the 11 populations had a deficiency of gene diversity compared with that expected at mutation-drift equilibrium (indicating population expansion); the remaining populations had an excess of gene diversity compared with that expected at mutation-drift equilibrium (indicating a recent bottleneck). These observations are consistent with the known history of pecan and pecan scab, which is that V. effusa became an issue on cultivated pecan in the last approximately 120 years (recent population expansion). Recently reported mating type genes and the sexual stage of this fungus may help explain the observed population characteristics, which bear a strong resemblance to those of other well-characterized sexually reproducing ascomycete pathogens.

Severity of Scab and its Effects on Fruit Weight in Mechanically Hedge-Pruned and Topped Pecan Trees

Scab is the most damaging disease of pecan in the southeastern United States. Pecan trees can attain 44 m in height, so managing disease in the upper canopy is a problem. Fungicide is ordinarily applied using ground-based air-blast sprayers. Although mechanical hedge-pruning and topping of pecan is done for several reasons, improved management of scab is an important reason in the humid, wet Southeast. Resulting shoot growth on cut limbs of susceptible cultivars could lead to more severe scab. In three experiments over three years, we explored the effect of hedge-pruning trees to ∼12 to 14 m compared with non-hedge-pruned trees. All trees received fungicide treatments (air-blast sprays and ≤3 aerial applications). Hedge-pruning either had no effect, or increased or decreased scab severity only slightly on leaflets, immature, or mature fruit (a -9.95 to +14.63% difference in scab severity compared with the control). However, height in the canopy invariably had a large and significant effect on scab severity, and amounted to a 0.05 to 73.77% difference in severity between the lowest and highest sample in the canopy. Fruit weight depended on sample height, with fruit most often weighing less when collected at greater sample heights. A robust relationship between fruit weight and scab severity was found at the highest sample heights where scab was also most often severe (R² = 0.21 to 0.67, P < 0.0001). Hedge-pruning and topping pecan tree canopies to manage tree size will enable better fungicide coverage, reducing risk of a scab epidemic as more of the canopy is assured efficacious fungicide spray coverage.

Scab Susceptibility of a Provenance Collection of Pecan in Three Different Seasons in the Southeastern United States

Pecan scab (caused by Fusicladium effusum) is the most economically destructive disease of pecan in the Southeast United States. Wet, humid conditions typical of the Southeast are known to provide conditions conducive to epidemics. A provenance collection of pecan from 19 locations representing the native range of the tree is located in Byron, Georgia, and was assessed for pecan scab severity in 1998, 2013, and 2014. There were significant differences among the 19 provenances (F = 5.6 to 62.5, P < 0.0001). Provenances from wetter locations (generally north of Texas) had the greatest proportion of scab resistant trees, while provenances from the drier southern areas (Texas and Mexico) tended to be the most susceptible to scab. The association with rainfall was borne out by correlation analysis (r = -0.625 to -0.823 [P < 0.0001 to 0.004]). Other factors consistently associated with scab severity included leaflet tilt and droop angle (r = -0.533 to -0.883 [P < 0.0001 to 0.02]). Multiple regression analysis demonstrated that leaflet droop angle was a particularly good predictor of provenance susceptibility. Leaflet characteristics vary with provenance location, and whether there is a direct relationship between scab severity and leaflet characteristics is not established. Estimates of heritability were not entirely consistent among years, but different methods were used to assess scab severity in 1998 (a 1 to 5 category scale) compared with 2013 and 2014 (the percent ratio scale). Despite using different methods, there was generally good agreement among years in regard to severity of disease on individual trees. In conclusion, trees from more northern populations (in areas with greater annual rainfall) are most likely to provide valuable and diverse sources of resistance to scab. The provenance collection contains a range of scab-resistant genotypes from diverse locations that can contribute to genetic improvement regarding scab resistance.

Draft genome sequence of Fusicladium effusum, cause of pecan scab

Pecan scab, caused by the plant pathogenic fungus Fusicladium effusum, is the most destructive disease of pecan, an important specialty crop cultivated in several regions of the world. Only a few members of the family Venturiaceae (in which the pathogen resides) have been reported sequenced. We report the first draft genome sequence (40.6 Mb) of an isolate F. effusum collected from a pecan tree (cv. Desirable) in central Georgia, in the US. The genome sequence described will be a useful resource for research of the biology and ecology of the pathogen, coevolution with the pecan host, characterization of genes of interest, and development of markers for studies of genetic diversity, genotyping and phylogenetic analysis. The annotation of the genome is described and a phylogenetic analysis is presented.

The Effect of Sample Height on Spray Coverage in Mature Pecan Trees

Pecan scab (caused by Fusicladium effusum) is the most damaging disease of pecan in the southeastern United States. Large air-blast sprayers for orchards are used to apply fungicide to control the disease but little quantitative information exists on the spray coverage achieved in the canopy of these trees. A series of experiments using water-sensitive spray cards to record spray coverage (percent area) at different heights and locations up to 15 m in the canopy of pecan trees showed a significantly greater percentage of card area covered at the lowest sample height when compared with the highest sample height. At the lowest height (5 m), spray coverage on individual cards ranged from 4.7 to 73.5% and, at the highest sample height (15 m), spray coverage ranged from 0.02 to 9.5%. In general, there was little significant difference in spray card coverage up to at least 10 m but, at 12.5 and 15 m, there was significantly less spray coverage compared with the coverage at 5 m. Regression analysis indicated a consistent linear relationship between sample height in the tree and the percent area covered. When spray cards were positioned at different heights without possible interference from pecan limbs and foliage, similar effects of sample height on spray coverage were noted. Wind speed measurements showed that air movement declined rapidly with distance from the sprayer fan. Whereas, at 2 m from the fan, wind speeds were approximately 26 m s^-1, by 10 m, speeds had declined to 2 to 4 m s^-1. At distances >12 m, wind speed was approaching ambient air movement of about 1 to 3 m s^-1. Although aerial application resulted in numerically greater spray coverage at sample heights >10 m, it was not significant even though a weak linear relationship (R² = 0.21 to 0.25) suggested an effect of height. Characterizing and understanding pesticide spray coverage in pecan will allow us to discern limits imposed by existing technology, and provide the basis for improving spray application methods (or tree management) for more efficacious disease control.

Genetic Diversity and Population Structure of Fusicladium effusum on Pecan in the United States

Fusicladium effusum causes pecan scab, which is the most destructive disease of pecan orchards in the United States. Conidia of the pathogen are spread by rain splash and wind. The fungus is pathogenically diverse; yet there is no information on its genetic diversity or population genetics. Universally primed polymerase chain reaction (UP-PCR) was used to investigate the genetic diversity and population structure on a hierarchical sample of 194 isolates collected from 11 orchard locations from Florida to Texas, consisting of three to four isolates from each of five to six trees at each location. Genetic variation was high throughout the region, with all but nine of the multilocus haplotypes being unique. Nei's average gene diversity ranged from 0.083 for a population from Mississippi to 0.160 for a population from Kansas. An analysis of molecular variance of the hierarchically sampled populations found that the majority of the genetic variability (82.6%) occurred at the scale of the individual tree and only relatively small amounts among populations in trees from an orchard (5.0%) or within groups (i.e., orchard location populations) (12.5%). The results suggest little population differentiation in F. effusum in the southeastern United States, although φpt values of genetic distance for pairwise comparisons indicated some populations could be differentiated from others. There was evidence of linkage disequilibrium in certain populations, and the common occurrence of asexual reproduction in F. effusum could lead to measurable linkage disequilibrium under certain circumstances. However, the degree of genetic diversity and the scale over which diversity is distributed is evidence that F. effusum undergoes regular recombination despite no known sexual stage.

Nickel affects xylem Sap RNase a and converts RNase A to a urease

BACKGROUND

Nickel (Ni) is an essential micronutrient; however, its metabolic or physiological functions in plants and animals are largely uncharacterized. The ribonucleases (RNase, e.g., RNase A) are a large family of hydrolases found in one form or many forms facilitating nitrogen (N) cycling. It is currently unknown how either a deficiency or excess of Ni influences the functionality of ribonucleases, like RNase A. This is especially true for perennial crops possessing relatively high Ni requirements.

RESULTS

We report that the 'rising' xylem sap of pecan [Carya illinoinensis (Wangenh.) K. Koch, a long-lived tree] at bud break contains a 14 kDa RNase A (aka, RNase 1), which amount has a 33% greater in Ni-deficient as in Ni-sufficient trees when exposed to Ni ions exhibits ureolytic activity. The homologous 13.4 kDa bovine pancreatic RNase A likewise exhibits ureolytic activity upon exposure to Ni ions. Ni therefore affects enzymatic function of a typically non-metalloenzyme, such as it transforms to an enzyme capable of hydrolyzing a linear amide; thus, converting an endonuclease esterase into a urease.

CONCLUSIONS

We conclude that Ni potentially affects the level and activity of RNase A present in the spring xylem sap of pecan trees, and probably in other crops, it has the same influence. The catalytic property of RNase A appears to shift from a nuclease to a urease relying on Ni exposure. This is suggestive that RNase A might possess novel metabolic functionality regarding N-metabolism in perennial plants. The ability of Ni to convert the activity of plant and animal RNase A from that of a ribonuclease to a urease indicates a possible unrecognized beneficial metabolic function of Ni in organisms, while also identifying a potential detrimental effect of excessive Ni on N related metabolic activity if there is sufficient disruption of Ni homeostasis.

The Effect of Horsfall-Barratt Category Size on the Accuracy and Reliability of Estimates of Pecan Scab Severity

Pecan scab (Fusicladium effusum) is a destructive pecan disease. Disease assessments may be made using interval-scale-based methods or estimates of severity to the nearest percent area diseased. To explore the effects of rating method-Horsfall-Barratt (H-B) scale estimates versus nearest percent estimates (NPEs)-on the accuracy and reliability of severity estimates over different actual pecan scab severity ranges on fruit valves, raters assessed two cohorts of images with actual area (0 to 6, 6⁺ to 25%, and 25⁺ to 75%) diseased. Mean estimated disease within each actual disease severity range varied substantially. Means estimated by NPE within each actual disease severity range were not necessarily good predictors of the H-B scale estimate at <25% severity. H-B estimates by raters most often placed severity in the wrong category compared with actual disease. Measures of bias, accuracy, precision, and agreement using Lin's concordance correlation depended on the range of actual severity, with improvements increasing with actual disease severity category (from 0 to 6 through 25⁺ to 75%); however, the improvement was unaffected by the H-B assessments. Bootstrap analysis indicated that NPEs provided either equally good or more accurate and precise estimate of disease compared with the H-B scale at severities of 25⁺ to 75%. Inter-rater reliability using NPEs was greater at 25⁺ to 75% actual disease severity compared with using the H-B scale. Using NPEs compared with the H-B scale will more often result in more precise and accurate estimates of pecan scab severity, particularly when estimating actual disease severities of 25⁺ to 75%.

Vertical Distribution of Scab in Large Pecan Trees

Pecan scab (Fusicladium effusum) is a destructive disease of pecan in the southeastern United States. This study was conducted to investigate the vertical distribution of scab in tall pecan trees (14 to 16 m tall) in three experiments in 2010 and 2011. Although 2010 had average rainfall, a factor that drives scab epidemics, 2011 was a very dry year with a consequently low scab severity. A total of eight trees were included in each experiment, four were nontreated controls, and four were sprayed using a ground-based air-blast sprayer. Trees were assessed for foliar and fruit scab at 0-5.0, 5.0⁺-7.5, 7.5⁺-10.0, 10.0⁺-12.5, and 12.5⁺-15.0 m. Mixed model analysis showed main effects of height, fungicide treatment, and height*treatment interactions in all three experiments, although on foliage the effects were less consistent (P value = 0.003-0.8), perhaps due to delayed fungicide applications early in the season. However, fruit of nontreated trees had more severe scab low in the canopy compared to fungicide-treated trees, with a consistent height*treatment interaction (P value = <0.0001-0.04). Most often the severity of scab in the upper canopy was similar in trees on fungicide-treated and nontreated trees, suggesting that fungicide had less impact at heights ≥10.0 m compared to <10.0 m in the canopy. There was a consistent reduction in scab severity on foliage and on immature fruit in August due to fungicide treatment at heights ≤10.0 m. Above 10.0 m, the effect was inconsistent, but late in the season (October), the fungicide-treated trees showed lower scab severity throughout the canopy. A metallic tracer study using cerium (Ce) showed an exponential relationship between quantities of Ce recovered and sample height in the canopy, with the quantity of Ce at ≥10 m being statistically equal to background levels. The relationship between scab severity on fruit and sample height in the canopy of nontreated trees was most often described by a negative linear function, but there was no discernible relationship on fungicide-treated trees, as the severity of scab in the lower canopy was most often similar to that in the upper canopy. Gradients in fungicide coverage and scab severity have ramifications for scab management options and potentially for the development of fungicide resistance in F. effusum.

Pecan Scab Severity-Effects of Assessment Methods

Pecan scab is caused by the fungus Fusicladium effusum, and is the most destructive disease of pecan in the United States. Accurate and reliable disease assessments are needed to ensure that data provide a measure of actual disease intensity. The Horsfall-Barratt (H-B) category scale and its derivatives are commonly used to assess disease. Estimates using the H-B scale were compared with nearest percent estimate (NPEs) for rating disease severity of pecan scab on valves of fruit. Both inexperienced and experienced raters were included in the experiment. Lin's concordance correlation showed that agreement using NPEs was variable (ρ_c = 0.57 to 0.96), whereas estimates of disease severity using the H-B scale had similar agreement among most raters (ρ_c = 0.59 to 0.98). Converted values of NPEs to the H-B midpoints (NPEH-B) also provided a similar range (ρ_c = 0.61 to 0.96). Neither experienced nor inexperienced raters were consistently better using any of the three methods. Bootstrap analysis indicated that, among experienced raters, precision (r) and agreement (ρ_c) were often reduced when using the H-B scale compared with NPEs. There was no consistent effect of converting NPEs to NPEH-B midpoint values compared with actual H-B values. Inter-rater reliability using the H-B scale was never better than NPEs. Bootstrap analysis indicated no difference in the length of time needed to assess disease but regression analysis suggested that raters who were inherently fast in assessing disease with NPEs were often slower when using the H-B scale; conversely, raters who were slow assessing with NPEs were often faster when using the H-B scale. Thus, there appears to be no advantage in accuracy or reliability or reduction in time when inexperienced or experienced raters used a category rating scale to assess pecan scab.

Effects of entomopathogenic fungus species, and impact of fertilizers, on biological control of pecan weevil (Coleoptera: Curculionidae)

The pecan weevil, Curculio caryae (Horn), is a key pest of pecan, Carya illinoinensis (Wangenh.) K. Koch. Prior research indicated the potential for use of Hypocreales fungi to suppress C. caryae. We compared the efficacy of two fungal spp., Beauveria bassiana (GHA strain) and Metarhizium brunneum (F52), in their ability to cause C. caryae mortality. The fungus, B. bassiana, was applied to trunks of pecan trees (a method previously shown to be effective in C. caryae suppression) and efficacy was compared with M. brunneum applied to the ground or to the trunk with or without SoyScreen Oil as an ultraviolet protecting agent. Results indicated B. bassiana to be superior to M. brunneum regardless of application method; consequently, the potential for applying B. bassiana to control C. caryae was explored further. Specifically, the impact of different fertilizer regimes (as used by pecan growers) on the persistence of B. bassiana (GHA) in soil was determined. B. bassiana was applied to soil in a pecan orchard after one of several fertilizer treatments--i.e., ammonium nitrate, crimson clover, poultry litter, clover plus poultry litter, and a no-fertilizer control. B. bassiana persistence up to 49 d in 2009 and 2010 was assessed by plating soil onto selective media and determining the number of colony forming units, and by baiting soil with a susceptible host, Galleria mellonella (L.). Fertilizer treatments did not impact B. bassiana persistence. We conclude that standard fertilizers for nitrogen management, when applied according to recommended practices, are unlikely to negatively impact survival of B. bassiana in pecan orchards when the fungus is applied for C. caryae suppression during weevil emergence. Additional research on interactions between entomopathogenic fungi and fertilizer amendments (or other tree nutrition or soil management practices) is merited.

Control of key pecan insect pests using biorational pesticides

Key pecan insect pests include the black pecan aphid, Melanocallis caryaefoliae (Davis), pecan weevil, Curculio caryae (Horn), and stink bugs (Hemiptera: Pentatomidae). Alternative control tactics are needed for management of these pests in organic and conventional systems. Our objective was to evaluate the potential utility of several alternative insecticides including three plant extract formulations, eucalyptus extract, citrus extract-8.92%, and citrus extract-19.4%, and two microbial insecticides, Chromobacterium subtsugae (Martin et al.) and Isaria fumosorosea (Wize). In the laboratory, eucalyptus extract, citrus extract-8.92%, citrus extract-19.4%, and C. subtsugae caused M. caryaefoliae mortality (mortality was reached approximately 78, 83, and 96%, respectively). In field tests, combined applications of I. fumosorosea with eucalyptus extract were synergistic and caused up to 82% mortality in M. caryaefoliae. In laboratory assays focusing on C. caryae suppression, C. subtsugae reduced feeding and oviposition damage, eucalyptus extract and citrus extract-19.4% were ineffective, and antagonism was observed when citrus extract-19.4% was combined with the entomopathogenic nematode, Steinernema carpocapsae (Weiser). In field tests, C. subtsugae reduced C. caryae damage by 55% within the first 3d, and caused 74.5% corrected mortality within 7 d posttreatment. In the laboratory, C. subtsugae and eucalyptus extract did not cause mortality in the brown stink bug, Euschistus servus (Say). Applications of C. subtsugae for suppression of C. caryae, and eucalyptus extract plus I. fumosorosea for control of M. caryaefoliae show promise as alternative insecticides and should be evaluated further.

Cumulative impact of a clover cover crop on the persistence and efficacy of Beauveria bassiana in suppressing the pecan weevil (Coleoptera: Curculionidae)

The pecan weevil, Curculio caryae (Horn), is a key pest of pecan. Endemic levels of the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin can occur in pecan orchards and contribute to natural control of C. caryae. Commercial formulations of the fungus can also be applied for suppression of C. caryae. We hypothesized that a clover cover crop enhances B. bassiana efficacy and persistence (e.g., by protecting the fungus against abiotic environmental stresses). The hypothesis was tested by conducting field trials in a pecan orchard in Byron, GA, in 2009 and 2010. The study included four treatments arranged in a factorial with two levels of fungus (endemic fungus only, and application of a commercial B. bassiana product), and two levels of clover (white clover, Trifolium repens L., and no clover). Fungal persistence was measured by determining the number of CFUs per gram of soil over time (during 42 d postapplication of B. bassiana in 2009 and 29 d in 2010). Efficacy was measured by capturing naturally emerging C. caryae and subsequently determining mortality and mycosis (over 24 d in 2009 and 17 d in 2010). In 2009, greater prevalence of B. bassiana conidia was detected in plots receiving fungal applications compared with no fungus applications, and no clear effect of clover was observed in plots receiving B. bassiana applications in either year. In 2010, B. bassiana prevalence in the endemic fungus plus clover treatment was higher than fungus without clover, and was similar to plots receiving additional B. bassiana applications. Given that we observed enhanced persistence of endemic B. bassiana in 2010 but not 2009, the impact of clover appears to be a cumulative effect. Mortality of C. caryae (averaged over the sampling periods) ranged between 68-74% in plots receiving B. bassiana applications and 51-56% in plots with endemic fungus only. C. caryae mortality and mycosis data also provided evidence that endemic B. bassiana efficacy was enhanced by clover relative to plots without clover (with no clear clover effect on plots receiving fungus applications). Thus, we conclude that natural control of C. caryae can increase when clover is grown in pecan orchards with endemic populations of B. bassiana.

Effects of combining microbial and chemical insecticides on mortality of the Pecan Weevil (Coleoptera: Curculionidae)

The pecan weevil, Curculio caryae (Horn), is a key pest of pecan [Carya illinoinensis (Wangenh.) K. Koch]. Current control recommendations are based on chemical insecticide applications. Microbial control agents such as the entomopathogenic nematode, Steinernema carpocapsae (Weiser) and the fungus Beauveria bassiana (Balsamo) Vuillemin occur naturally in southeastern U.S. pecan orchards and have shown promise as alternative control agents for C. caryjae. Conceivably, the chemical and microbial agents occur simultaneously within pecan orchards or might be applied concurrently. The objective of this study was to determine the interactions between two chemical insecticides that are used in commercial C. caryae control (i.e., carbaryl and cypermethrin applied below field rates) and the microbial agents B. bassiana and S. carpocapsae. In laboratory experiments, pecan weevil larval or adult mortality was assessed after application of microbial or chemical treatments applied singly or in combination (microbial + chemical agent). The nature of interactions (antagonism, additivity, or synergy) in terms of weevil mortality was evaluated over 9 d (larvae) or 5 d (adults). Results for B. bassiana indicated synergistic activity with carbaryl and antagonism with cypermethrin in C. caryae larvae and adults. For S. carpocapsae, synergy was detected with both chemicals in C. caryae larvae, but only additive effects were detected in adult weevils. Our results indicate that the chemical-microbial combinations tested are compatible with the exception of B. bassiana and cypermethrin. In addition, combinations that exhibited synergistic interactions may provide enhanced C. caryae control in commercial field applications; thus, their potential merits further exploration.

Application of plant growth regulators mitigates chlorotic foliar injury by the black pecan aphid (Hemiptera: Aphididae)

BACKGROUND

Black pecan aphid, Melanocallis caryaefoliae (Davis) (Hemiptera: Aphididae), feeding elicits localized chlorotic injury to pecan foliage [Carya illinoinensis (Wangenh.) K Koch] and apparent acceleration of leaf senescence and defoliation. The ability of certain plant growth regulators (PGRs) (forchlorfenuron, gibberellic acid and aviglycine) to prevent M. caryaefoliae from triggering pecan leaf chlorosis and senescence-like processes was evaluated on two dates in both 2006 and 2007. Treatments were applied to orchard foliage and used in laboratory leaf-disc bioassays to assess possible reduction in aphid-elicited chlorosis and concomitant effects on aphid mortality and development.

RESULTS

Foliage pretreated with forchlorfenuron + gibberellic acid prior to being challenged with aphids resulted in significantly less aphid-elicited chlorosis than did control or aviglycine-treated leaf discs. No PGR affected aphid mortality; however, development time was increased by forchlorfenuron + gibberellic acid in 2006 and by aviglycine + gibberellic acid on one date in 2007.

CONCLUSION

Certain PGRs possess the potential for usage on pecan to protect foliar canopies from M. caryaefoliae via changes in the susceptibility of the host leaf to senescence-like factors being introduced by feeding aphids. This protective effect on host foliage and the associated suppressive effect on development of feeding aphids might also be relevant to pest management programs on other aphid-crop systems in which aphid-elicited chlorosis and senescence-like processes can limit profitability.

Changes of oxidase and hydrolase activities in pecan leaves elicited by black pecan aphid (Hemiptera: Aphididae) feeding

The black pecan aphid, Melanocallis caryaefoliae (Davis) (Hemiptera: Aphididae), is a foliar feeder of pecan, Carya illinoinensis (Wangenh.) K. Koch (Juglandaceae). The pest causes chlorosis of leaflet lamina, physiological damage to foliage and trees, and commonly limits the profitability of commercial pecan orchard enterprises. However, key aspects of this host-pest interaction are poorly understood. We report here the effects of M. caryaefoliae feeding on the foliar activity of oxidative (i.e., catalase, lipoxygenase [LOX]-1 and 3, and peroxidase) and hydrolytic (i.e., esterase) enzymes in relation to the degree of aphid resistance among pecan varieties. The 2-yr study showed that M. caryaefoliae-infested foliage exhibited elevated peroxidase activity only in susceptible ('Desirable', 'Sumner', and 'Schley'), but not in resistant ('Cape Fear', 'Gloria Grande', and 'Money Maker') genotypes. Susceptible genotypes also exhibited more severe leaf chlorosis in response to M. caryaefoliae feeding than the resistant genotypes; however, the aphid feeding did not influence catalase or esterase activity in all varieties, except the increase of esterase activity in Desirable and Gloria Grande. Melanocallis caryaefoliae feeding also influences activity of two lipoxygenase isozymes, with LOX3 being more frequently induced than LOX1. Foliar LOX3 activity was more frequently induced by M. caryaefoliae feeding in the moderately resistant 'Oconee' and highly resistant Money Maker and Cape Fear than in the susceptible genotypes. Therefore, the elevation of peroxidase is likely to be associated with aphid susceptibility and contributed to the severe leaf chlorosis, whereas the increase of LOX3 activity might be associated with aphid resistance in pecan. These findings contribute to our understanding of the etiology of M. caryaefoliae-elicited leaf chlorosis on pecan foliage. Such information may also be used to develop enzyme markers for identifying black pecan aphid resistance and/or susceptibility in pecan germplasm.

Chlorotic feeding injury by the black pecan aphid (hemiptera: aphididae) to pecan foliage promotes aphid settling and nymphal development

The nature of the interaction between the black pecan aphid, Melanocallis caryaefoliae (Davis) (Hemiptera: Aphididae), and the chlorosis it causes to foliage of its pecan [Carya illinoinensis (Wangenh.) K. Koch)] host is poorly understood. Laboratory experiments were conducted on the settling behavior of the black pecan aphid, when provided chlorotic pecan leaf discs resulting from previous black pecan aphid feeding and nonchlorotic leaf discs, under a normal photoperiod and constant dark. Additionally, aphid development from the first instar to the adult stage was examined when nymphs were either allowed to feed on the same leaf disc or moved daily to a new, nondamaged, same age leaf disc. After 24 h, a significantly higher percentage of black pecan aphids settled on chlorotic than on nonchlorotic leaf discs, regardless of photoperiod. When starting from the first instar, nymphs that were prevented from inducing leaf chlorosis by moving daily to new, same-age leaf discs took approximately 5 d longer to complete development, had a shorter body length, and had higher mortality than when aphids remained on the same leaf disc. These results show that black pecan aphid-induced leaf chlorosis plays an important role in the interaction of the black pecan aphid with its pecan host.

Interaction of Concurrent Populations of Meloidogyne partityla and Mesocriconema xenoplax on Pecan

The effect of the interaction between Meloidogyne partityla and Mesocriconema xenoplax on nematode reproduction and vegetative growth of Carya illinoinensis 'Desirable' pecan was studied in field microplots. Meloidogyne partityla suppressed reproduction of M. xenoplax, whereas the presence of M. xenoplax did not affect the population density of M. partityla second-stage juveniles in soil. Above-ground tree growth, as measured by trunk diameter 32 months following inoculation, was reduced in the presence of M. partityla alone or in combination with M. xenoplax as compared with the uninoculated control trees. The interaction between M. partityla and M. xenoplax was significant for dry root weight 37 months after inoculation. Results indicate that the presence of the two nematode species together caused a greater reduction in root growth than M. xenoplax alone, but not when compared to M. partityla alone. Mouse-ear symptom severity in pecan leaves was increased in the presence of M. partityla compared with M. xenoplax and the uninoculated control. Infection with M. partityla increased severity of mouse-ear symptoms expressed by foliage. The greater negative impact of M. partityla on vegetative growth of pecan seedlings in field microplots indicates that it is likely a more detrimental pathogen to pecan than is M. xenoplax and is likely an economic pest of pecan.

Comparison of application methods for suppressing the pecan weevil (Coleoptera: Curculionidae) with Beauveria bassiana under field conditions

The pecan weevil, Curculio caryae (Horn), is a key pest of pecans. The entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin is pathogenic to C. caryae. One approach to managing C. caryae may be application of B. bassiana directed toward adult weevils as they emerge from the soil to attack nuts in the tree canopy. Our objective was to compare different application methods for suppression of C. caryae adults. Treatments included direct application of B. bassiana (GHA strain) to soil under the tree canopy, soil application followed by cultivation, soil application in conjunction with a cover crop (Sudan grass), direct application to the tree trunk, and application to the trunk with an UV radiation-protecting adjuvant. The study was conducted in a pecan orchard in Byron, GA, in 2005 and 2006. Naturally emerging C. caryae adults, caught after crawling to the trunk, were transported to the laboratory to determine percentage mortality and signs of mycosis. When averaged over the 15-d sampling period, weevil mortality and signs of mycosis were greater in all treatments than in the nontreated control in 2005 and 2006; >75% average mortality was observed with the trunk application both years and in the trunk application with UV protection in 2005. Results indicated trunk applications can produce superior efficacy relative to ground application, particularly if the ground application is followed by cultivation. Efficacy in the cover crop treatment, however, did not differ from other application approaches. Future research should focus on elucidating the causes for treatment differences we observed and the extent to which B. bassiana-induced C. caryae mortality reduces crop damage.

Identification and quantitation of asparagine and citrulline using high-performance liquid chromatography (HPLC)

High-performance liquid chromatography (HPLC) analysis was used for identification of two problematic ureides, asparagine and citrulline. We report here a technique that takes advantage of the predictable delay in retention time of the co-asparagine/citrulline peak to enable both qualitative and quantitative analysis of asparagine and citrulline using the Platinum EPS reverse-phase C18 column (Alltech Associates). Asparagine alone is eluted earlier than citrulline alone, but when both of them are present in biological samples they may co-elute. HPLC retention times for asparagine and citrulline were influenced by other ureides in the mixture. We found that at various asparagines and citrulline ratios [= 3:1, 1:1, and 1:3; corresponding to 75:25, 50:50, and 25:75 (microMol ml(-1)/microMol ml(-1))], the resulting peak exhibited different retention times. Adjustment of ureide ratios as internal standards enables peak identification and quantification. Both chemicals were quantified in xylem sap samples of pecan [Carya illinoinensis (Wangenh.) K. Koch] trees. Analysis revealed that tree nickel nutrition status affects relative concentrations of Urea Cycle intermediates, asparagine and citrulline, present in sap. Consequently, we concluded that the HPLC methods are presented to enable qualitative and quantitative analysis of these metabolically important ureides.

Identification and Quantitation of Asparagine and Citrulline Using High-Performance Liquid Chromatography (HPLC)

High-performance liquid chromatography (HPLC) analysis was used for identification of two problematic ureides, asparagine and citrulline. We report here a technique that takes advantage of the predictable delay in retention time of the co-asparagine/citrulline peak to enable both qualitative and quantitative analysis of asparagine and citrulline using the Platinum EPS reverse-phase C18 column (Alltech Associates). Asparagine alone is eluted earlier than citrulline alone, but when both of them are present in biological samples they may co-elute. HPLC retention times for asparagine and citrulline were influenced by other ureides in the mixture. We found that at various asparagines and citrulline ratios [= 3:1, 1:1, and 1:3; corresponding to 75:25, 50:50, and 25:75 (μMol m−1/μMol ml−1)], the resulting peak exhibited different retention times. Adjustment of ureide ratios as internal standards enables peak identification and quantification. Both chemicals were quantified in xylem sap samples of pecan [ Carya illinoinensis (Wangenh.) K. Koch] trees. Analysis revealed that tree nickel nutrition status affects relative concentrations of Urea Cycle intermediates, asparagine and citrulline, present in sap. Consequently, we concluded that the HPLC methods are presented to enable qualitative and quantitative analysis of these metabolically important ureides.

Effect of Soil Moisture and a Surfactant on Entomopathogenic Nematode Suppression of the Pecan Weevil, Curculio caryae

Our overall goal was to investigate several aspects of pecan weevil, Curculio caryae, suppression with entomopathogenic nematodes. Specifically, our objectives were to: 1) determine optimum moisture levels for larval suppression, 2) determine suppression of adult C. caryae under field conditions, and 3) measure the effects of a surfactant on nematode efficacy. In the laboratory, virulence of Heterorhabditis megidis (UK211) and Steinernema carpocapsae (All) were tested in a loamy sand at gravimetric water contents of negative 0.01, 0.06, 0.3, 1.0, and 15 bars. Curculio caryae larval survival decreased as moisture levels increased. The nematode effect was most pronounced at -0.06 bars. At -0.01 bars, larval survival was </=5% regardless of nematode presence, thus indicating that intense irrigation alone might reduce C. caryae populations. Overall, our results indicated no effect of a surfactant (Kinetic) on C. caryae suppression with entomopathogenic nematodes. In a greenhouse test, C. caryae larval survival was lower in all nematode treatments compared with the control, yet survival was lower in S. carpocapsae (Italian) and S. riobrave (7-12) treatments than in S. carpocapsae (Agriotos), S. carpocapsae (Mexican), and S. riobrave (355) treatments (survival was reduced to approximately 20% in the S. riobrave [7-12] treatment). A mixture of S. riobrave strains resulted in intermediate larval survival. In field experiments conducted over two consecutive years, S. riobrave (7-12) applications resulted in no observable control, and, although S. carpocapsae (Italian) provided some suppression, treatment effects were generally only detectable one day after treatment. Nematode strains possessing both high levels of virulence and a greater ability to withstand environmental conditions in the field need to be developed and tested.

Taxonomic and Biological Characterization of Steinernema rarum Found in the Southeastern United States

Two Steinernema isolates found in Louisiana and Mississippi were later identified as isolates of S. rarum. DNA sequences of ITS regions of the United States isolates are identical with sequences of Argentinean S. rarum strains Samiento and Noetinger and differ by two bases from the Arroyo Cabral isolate from Córdoba, Argentina. SEM observations revealed several new structures in the isolates from the US: female face views have a hexagonal-star perioral disc and eye-shaped lips; some females do not have cephalic papillae; lateral fields of infective juveniles are variable; there are two openings observed close to the posterior edge of the cloaca. Virulence of the US isolates to Anthonomus grandis, Diaprepes abbreviatus, Solenopsis invicta, Coptotermes formosanus, Agrotis ipsilon, Spodoptera frugiperda, and Trichoplusia ni and reproductive potential were evaluated in comparison with other heterorhabditid and steinernematid nematodes. Results such as particularly high virulence to S. frugiperda indicate that the biocontrol potential of the new S. rarum strains merits further study.

Nickel deficiency disrupts metabolism of ureides, amino acids, and organic acids of young pecan foliage

The existence of nickel (Ni) deficiency is becoming increasingly apparent in crops, especially for ureide-transporting woody perennials, but its physiological role is poorly understood. We evaluated the concentrations of ureides, amino acids, and organic acids in photosynthetic foliar tissue from Ni-sufficient (Ni-S) versus Ni-deficient (Ni-D) pecan (Carya illinoinensis [Wangenh.] K. Koch). Foliage of Ni-D pecan seedlings exhibited metabolic disruption of nitrogen metabolism via ureide catabolism, amino acid metabolism, and ornithine cycle intermediates. Disruption of ureide catabolism in Ni-D foliage resulted in accumulation of xanthine, allantoic acid, ureidoglycolate, and citrulline, but total ureides, urea concentration, and urease activity were reduced. Disruption of amino acid metabolism in Ni-D foliage resulted in accumulation of glycine, valine, isoleucine, tyrosine, tryptophan, arginine, and total free amino acids, and lower concentrations of histidine and glutamic acid. Ni deficiency also disrupted the citric acid cycle, the second stage of respiration, where Ni-D foliage contained very low levels of citrate compared to Ni-S foliage. Disruption of carbon metabolism was also via accumulation of lactic and oxalic acids. The results indicate that mouse-ear, a key morphological symptom, is likely linked to the toxic accumulation of oxalic and lactic acids in the rapidly growing tips and margins of leaflets. Our results support the role of Ni as an essential plant nutrient element. The magnitude of metabolic disruption exhibited in Ni-D pecan is evidence of the existence of unidentified physiological roles for Ni in pecan.

A first linkage map of pecan cultivars based on RAPD and AFLP markers

We report here the first genetic linkage maps of pecan [Carya illinoinensis (Wangenh.) K. Koch], using random amplified polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP) markers. Independent maps were constructed for the cultivars 'Pawnee' and 'Elliot' using the double pseudo-testcross mapping strategy and 120 F1 seedlings from a full-sib family. A total of 477 markers, including 217 RAPD, 258 AFLP, and two morphological markers were used in linkage analysis. The 'Pawnee' linkage map has 218 markers, comprising 176 testcross and 42 intercross markers placed in 16 major and 13 minor (doublets and triplets) linkage groups. The 'Pawnee' linkage map covered 2,227 cM with an average map distance of 12.7 cM between adjacent markers. The 'Elliot' linkage map has 174 markers comprising 150 testcross and 22 intercross markers placed in 17 major and nine minor linkage groups. The 'Elliot' map covered 1,698 cM with an average map distance of 11.2 cM between adjacent markers. Segregation ratios for dichogamy type and stigma color were not significantly different from 1:1, suggesting that both traits are controlled by single loci with protogyny and green stigmas dominant to protandry and red stigmas. These loci were tightly linked (1.9 cM) and were placed in 'Elliot' linkage group 16. These linkage maps are an important first step towards the detection of genes controlling horticulturally important traits such as nut size, nut maturity date, kernel quality, and disease resistance.

The potential for enhanced fungicide resistance in Beauveria bassiana through strain discovery and artificial selection

Our objectives were to determine the (1) natural variation in fungicide resistance among Beauveria bassiana strains, (2) potential to increase fungicide resistance in B. bassiana through artificial selection, and (3) stability of virulence in selected B. bassiana strains. Fungicides included dodine, fenbuconazole, and triphenyltin hydroxide, which are commonly used in pecan and other horticultural crops. Comparison of seven B. bassiana strains indicated some are substantially more resistant to fungicides than others; a commercial strain (GHA) was less resistant than all wild strains isolated from pecan orchards. Artificial selection resulted in enhanced fungicide resistance in the GHA strain but not in a mixed wild strain. Removal of selection pressure for three passages did not reduce the enhanced fungicide resistance. Sub-culturing with exposure to fungicides did not affect the GHA strain's virulence to pecan weevil, Curculio caryae, larvae, whereas fungicide exposure increased virulence in a mixed wild population of B. bassiana.

Particle film affects black pecan aphid (Homoptera: Aphididae) on pecan

Three species of aphids attack pecan foliage, Carya illinoensis (Wang.) K. Koch, and cause economic damage. We tested a kaolin-based particle film against one of these aphid species, black pecan aphid, Melanocallis caryaefoliae (Davis). Effect of particle film on host selection, adult mortality, and production of nymphs by M. caryaefoliae was tested on seedling pecans in the laboratory. Fewer M. caryaefoliae adults selected treated foliage compared with untreated foliage. A higher percentage of adults that did select treated foliage were recovered from upper leaf surfaces compared with the percentage of adults recovered from upper leaf surfaces of untreated leaves. Observations with a microscope revealed an accumulation of particle film on aphid body parts, especially on tarsi, and strongly suggests that aphid mobility was restricted. Adult mortality was higher on treated foliage and led to an overall decrease in production of nymphs on those seedlings. In addition, we measured spectral properties of treated seedling pecan foliage. Light reflectance by treated foliage was increased and absorptance decreased compared with control foliage whereas transmittance of light through control and particle film-treated leaves was similar. We did not detect any phytotoxic effect on pecan due to application of particle film.

First Record of Meloidogyne partityla on Pecan in Georgia

In July 2000, tree decline was observed in a commercial pecan (Carya illinoensis (Wang.) K. Koch) orchard in Crisp County, GA. Most affected trees exhibited dead branches in the upper canopy, stunted growth, and feeder roots with small galls and associated egg masses typical of root-knot nematode infection. All declining trees that were examined had root systems infected with a Meloidogyne sp. Efforts to culture the nematode on tomato (Lycopersicon esculentum Mill. 'Rutgers') were unsuccessful. Identification of the nematode was determined by two laboratory procedures in March 2001. Female nematodes were teased from fresh pecan root galls of declining trees in Georgia, and identified by determining the esterase phenotype from replicate samples of single females compared with standard root-knot nematode species, including a population of M. partityla (3). Galled roots also were sent to New Mexico State University in Las Cruces, where mitochondrial DNA from specimens was extracted and compared with that from standard root-knot nematode species and known populations of M. partityla (2). Specimens had esterase phenotypes and DNA patterns consistent with M. partityla. Esterase phentoypes were inconsistent with M. incognita and M. arenaria, and DNA patterns were inconsistent with M. incognita, M. javanica, and M. hapla. Specimens at both locations were identified as M. partityla Kleynhans (1). To our knowledge, this is the first report of M. partityla from Georgia and the third report of this nematode outside of South Africa. The first and second report of M. partityla from pecan in the United States occurred in Texas and New Mexico in 1996 and 2001, respectively (3,4). Our inability to culture the M. partityla-GA isolate on tomato substantiates previous experience with this nematode in the United States (3) and is not surprising, since this species has a host range limited to the Juglandaceae. Furthermore, M. partityla may be endemic to North America and not South Africa. It is believed this nematode entered South Africa on pecan seedling roots imported from the United States between 1912 and 1940 (1). The unusually narrow host range may explain why M. partityla has gone unrecognized for so long in the United States compared with the more common Meloidogyne spp. (i.e., M. incognita and M. arenaria) with wider host ranges found in pecan orchards. Determining the distribution of M. partityla within the major pecan-growing regions of Georgia and throughout North America is warranted. References: (1) K. P. N. Kleynhans. Phytophylactica 18:103, 1986. (2) T. O. Powers and T. S. Harris. J. Nematol. 25:1, 1993. (3) J. L. Starr et al. J. Nematol. 28:565, 1996. (4) S. H. Thomas et al. Plant Dis. 85:1030, 2001.

Outcrossing rates and relatedness estimates in pecan (Carya illinoinensis) populations

Estimates of single and multilocus outcrossing rates as well as relatedness among progeny of individual seed trees were obtained for 14 populations of pecan [Carya illinoinensis (Wangenh.) K. Koch]. Mean outcrossing estimates were not significantly different from 1.0 and relatedness values indicate that most progeny within families are half sibs. Biparental inbreeding was insignificant in all study sites, and inbreeding coefficients indicated that populations were close to inbreeding equilibrium.

Impact of Meloidogyne incognita on the Incidence of Peach Tree Short Life in the Presence of Criconemella xenoplax

The relationship between Cricenemella xenoplax alone and in combination with Meloidogyne incognitaon the incidence of peach tree short life disease was studied in field microplots during 1989-96. The presence of M. incognita suppressed the population density of C. xenoplax on Lovell peach. Tree trunk diameter was significantly reduced in the presence of both nematode species prior to 1993. Soil pH was lowest in the co-infection treatment as compared with the uninoculated control on three of the four sampling dates. In 1994, 80% of the trees growing in soil infested with C. xenoplax alone developed typical disease symptoms and died. The remaining tree died in 1995. No trees died in the M. incognita alone, C. xenoplax + M. incognita, or uninoculated control treatments. Parasitism by C. xenoplax, but not by M. incognita, made Lovell peach trees more susceptible to the disease. These findings were confirmed in an orchard site naturally infested with both C. xenoplax and M. incognita where Redhaven trees budded to Lovell rootstock exhibited a reduction of 1.6 years in average tree life for every centimeter increase in trunk diameter.

Peach Leaf Senescence Delayed by Criconemella xenoplax

Fall annual leaf senescence of peach was delayed in the field and in microplots in the presence of Criconemella xenoplax. Soil from the rhizosphere of orchard trees with greener leaves had ca. 2.5 x more nematodes than soil around trees in a more advanced state of fall senescence. In microplots, monoclonal antibody enzyme immunoassay (EIA) of leaf cytokinins indicated that concentration of zeatin riboside-like substances and chlorophyll content were greater in leaves of trees growing in nematode-infested soil than in trees in uninfested soil. EIA also indicated the presence of substances resembling trans-zeatin, zeatin riboside, dihydrozeatin, and dihydrozeatin riboside-like substances in whole body homogenates of C. xenoplax. Levels of zeatin-like substances were present in the nematode in greater levels than the other related substances.