Accelerated Degradation of Aldicarb and Its Metabolites in Cotton Field Soils

The degradation of aldicarb, and the metabolites aldicarb sulfoxide and aldicarb sulfone, was evaluated in cotton field soils previously exposed to aldicarb. A loss of efficacy had been observed in two (LM and MS) of the three (CL) field soils as measured by R. reniformis population development and a lack of cotton yield response. Two soils were compared for the first test—one where aldicarb had been effective (CL) and the second where aldicarb had lost its efficacy (LM). The second test included all three soils: autoclaved, non-autoclaved and treated with aldicarb at 0.59 kg a.i./ha, or not treated with aldicarb. The degradation of aldicarb to aldicarb sulfoxide and then to aldicarb sulfone was measured using high-performance liquid chromatography (HPLC) in both tests. In test one, total degradation of aldicarb and its metabolites occurred within 12 days in the LM soil. Aldicarb sulfoxide and aldicarb sulfone were both present in the CL soil at the conclusion of the test at 42 days after aldicarb application. Autoclaving the LM and MS soils extended the persistence of the aldicarb metabolites as compared to the same soils not autoclaved. The rate of degradation was not changed when the CL natural soil was autoclaved. The accelerated degradation was due to more rapid degradation of aldicarb sulfoxide and appears to be biologically mediated.     

Rotylenchulus reniformis Management in Cotton with Crop Rotation

One-year crop rotations with corn or highly resistant soybean were evaluated at four locations for their effect on Rotylenchulus reniformis population levels and yield of a subsequent cotton crop. Four nematicide (aldicarb) regimes were included at two of the locations, and rotation with reniform-susceptible soybean was included at the other two locations. One-year rotations to corn or resistant soybean resulted in lower R. reniformis population levels (P ≤ 0.05) than those found in cotton at three test sites. However, the effect of rotation on nematode populations was undetectable by mid-season when cotton was grown the following year. Cotton yield following a one-year rotation to resistant soybean increased at all test locations compared to continuous cotton, and yield following corn increased at three locations. The optimum application rate for aldicarb in this study was 0.84 kg a.i./ha in furrow. Side-dress applications of aldicarb resulted in yield increases that were insufficient to cover the cost of application in 3 of the 4 years.     

Vertical Distribution of Rotylenchulus reniformis in Cotton Fields

The possible impact of Rotylenchulus reniformis below plow depth was evaluated by measuring the vertical distribution of R. reniformis and soil texture in 20 symptomatic fields on 17 farms across six states. The mean nematode population density per field, 0 to 122 cm deep, ranged from 0.4 to 63 nematodes/g soil, and in 15 fields more than half of the R. reniformis present were below 30.5 cm, which is the greatest depth usually plowed by farmers or sampled by consultants. In 11 fields measured, root density was greatest in the top 15 cm of soil; however, roots consistently penetrated 92 to 122 cm deep by midseason, and in five fields in Texas and Louisiana the ratio of nematodes to root-length density within soil increased with depth. Repeated sampling during the year in Texas indicated that up to 20% of the nematodes in soil below 60 cm in the fall survived the winter. Differences between Baermann funnel and sugar flotation extraction methods were not important when compared with field-to-field differences in nematode populations and field-specific vertical distribution patterns. The results support the interpretation that R. reniformis below plow depth can significantly impact diagnosis and treatment of cotton fields infested with R. reniformis.     

Comparisons of Female and Egg Assays to Identify Rotylenchulus reniformis Resistance in Cotton

More plants can be screened for reniform nematode resistance each year if the time involved can be shortened. In this study, the hypothesis that female counts are as efficient as egg counts in identifying resistant genotypes was tested. In two greenhouse experiments Gossypium genotypes which varied from resistant to susceptible to reniform nematode (Rotylenchulus reniformis) were compared to a susceptible control cultivar. Infested field soil served as the inoculum source for the first experiment, and vermiform stages extracted from greenhouse cultures were used to infest soil in the second experiment. Six replicates of each genotype were harvested 25 d after planting and swollen females were counted. The remaining plants were harvested 35 d after planting and eggs extracted from the roots were counted. Processing and counting times recorded in the first experiment were similar for both assessment methods, but 10 additional days were required for egg-based assessment. Contrast analyses showed that assessments based on females per gram of root were equivalent to assessments based on eggs per gram of root for the five genotypes tested in the first experiment and for an expanded set of 13 genotypes tested in the second experiment. The results indicated that either life stage can be used to screen for resistance.     

Interrelationships of Rotylenchulus reniformis with Rhizoctonia solani on Cotton

The interrelationships between reniform nematode (Rotylenchulus reniformis) and the cotton (Gossypium hirsutum) seedling blight fungus (Rhizoctonia solani) were studied using three isolates of R. solani, two populations of R. reniformis at multiple inoculum levels, and the cotton cultivars Dehapine 90 (DP 90) and Dehapine 41 (DP 41). Colonization of cotton hypocotyl tissue by R. solani resulted in increases (P ≤ 0.05) in nematode population densities in soil and in eggs recovered from the root systems in both 40- and 90-day-duration experiments. Increases in soil population densities resulted mainly from increases in juveniles. Enhanced reproduction of R. reniformis in the presence of R. solani was consistent across isolates (1, 2, and 3) of R. solani and populations (1 and 2) and inoculum levels (0.5, 2, 4, and 8 individuals/g of soil) of R. reniformis, regardless of cotton cultivar (DP 90 or DP 41). Severity of seedling blight was not influenced by the nematode. Rhizoctonia solani caused reductions (P ≤ 0.05) in cotton growth in 40- and 90-day periods. Rotylenchulus reniformis reduced cotton growth at 90 days. The relationship between nematode inoculum levels and plant growth reductions was linear. At 90 days, the combined effects of these pathogens were antagonistic to plant growth.     

Identification of Rotylenchulus reniformis Resistant Glycine Lines

Identification of resistance to reniform nematode (Rotylenchulus reniformis) is the first step in developing resistant soybean (Glycine max) cultivars that will benefit growers in the mid-South region of the United States. This study was conducted to identify soybean (G. max and G. soja) lines with resistance to this pathogen. Sixty-one wild and domestic soybean lines were evaluated in replicated growth chamber tests. Six previously untested soybean lines with useful levels of resistance to reniform nematode were identified in both initial screening and subsequent confirmation tests: released germplasm lines DS4-SCN05 (PI 656647) and DS-880 (PI 659348); accession PI 567516 C; and breeding lines DS97-84-1, 02011-126-1-1-2-1 and 02011-126-1-1-5-1. Eleven previously untested moderately susceptible or susceptible lines were also identified: released germplasm lines D68-0099 (PI 573285) and LG01-5087-5; accessions PI 200538, PI 416937, PI 423941, PI 437697, PI 467312, PI 468916, PI 594692, and PI 603751 A; and cultivar Stafford (PI 508269). Results of previously tested lines evaluated in the current study agreed with published reports 69.6% of the time for resistant lines and 87.5% of the time for susceptible lines. Soybean breeders may benefit from incorporating the newly identified resistant lines into their breeding programs.     

Histological Observations of Rotylenchulus reniformis on Gossypium longicalyx and Interspecific Cotton Hybrids

Observations on the development of reniform nematode (Rotylenchulus reniformis) on roots of Gossypium longicalyx, G. hirsutum, and two interspecific hybrids derived from them were made by light microscopy. Gossypium longicalyx is reported to be immune to reniform nematode, but the mechanism(s) for resistance are unknown. Penetration of G. longicalyx roots by female nematodes was confirmed, and incipient swelling of the females, indicating initiation of maturation of the reproductive system, was observed. Female maturation occurred up to the formation of a single embryo inside the female body but not beyond this point. In both hybrids, development was inhibited but progressed further than in the immune parent. Reactions ranged from highly compatible, with the formation of active syncytia and full development of females, to incompatible with little or no development of the female. Compatible plants showed characteristic hypertrophied cells, enlarged nuclei, dense cytoplasm, and partial dissolution of cell walls, whereas incompatible plant reactions included lignification of the cells adjacent to the nematode head, or the complete collapse and necrosis of the cells involved. The need to characterize reactions and to carefully select among the plants descended from the hybrids during the introgression process, as well as the importance of combining the results of reproduction tests with histological observation of the plant-nematode interactions, is discussed.     

Suppression of Rotylenchulus reniformis on Cotton by the Nematophagous Fungus ARF

The reniform nematode, Rotylenchulus reniformis Linford &Oliveira, has become a serious threat to cotton (Gossypium hirsutum L.) production in the United States during the past decade. The objective of this study is to isolate fungi from eggs of R. reniformis and select potential biological control agents for R. reniformis on cotton. Soil samples were collected from cotton fields located in Jefferson County, Arkansas. Eight genera of fungi were included in the 128 fungal isolates obtained, and among them were five strains of the nematophagous fungus ARF. The mtDNA RFLP pattern, colony growth characteristics, and pathogenicity indicate the five ARF isolates represent one described strain and one new strain. Light and electron microscopic observations suggest ARF is an active parasite of R. reniformis, with parasitism ranging from 48% to 79% in in vitro tests. Three greenhouse experiments demonstrated ARF successfully suppressed the number of reniform nematodes during the first and second generation of the nematode. Reductions in numbers of R. reniformis on the roots for the seven application rates of 0.01%, 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% ARF were 87%, 92%, 94%, 96%, 97%, 98%, and and 98%, respectively.     

Rotylenchulus reniformis below Plow Depth Suppresses Cotton Yield and Root Growth

Damage to cotton by Rotylenchulus reniformis below plow depth was evaluated in a sandy clay loam soil at Weslaco, Texas. In December 1999, 14 holes on 51-cm centers were dug 91 cm deep along the planting bed and adjacent furrow and 2 ml of 1,3-dichloropropene was placed 91, 61, and 30 cm deep as each hole was refilled and packed. This technique eliminated 96%, 81%, and 74% of R. reniformis down to 107 cm at distances 0, 25, and 51 cm laterally from the point of application (P ≤ 0.05), whereas chisel fumigation at 168 liters/ha 43 cm deep reduced nematode numbers only in the top 61 cm (P ≤ 0.001). Manual placement of fumigant increased yield 92%; chisel fumigation increased yield 88% (P ≤ 0.005). A second experiment in February 2001 placed fumigant 43 or 81 cm deep, or at both 43 and 81 cm. Holes alone had no significant effect on nematode density at planting, midseason or harvest, on root length density at midseason, or on cotton lint yield. Fumigant at 43 cm reduced nematode numbers above fumigant application depth at planting 94% (P ≤ 0.02), at midseason 37% (P ≤ 0.09), and at harvest 0%, increasing yield 57% (P ≤ 0.002). Fumigant at 81 cm reduced nematode numbers above fumigant application depth at planting 86% (P ≤ 0.02), at midseason 74% (P ≤ 0.02), and at harvest 48% (P ≤ 0.01), increasing yield 53% (P ≤ 0.002). Fumigating at both 43 and 81 cm reduced nematode numbers above 90 cm 94% at planting and 79% at midseason, increased midseason root-length density 14-fold below 76 cm, and doubled yield (P ≤ 0.02 in all cases).     

Natural Migration of Rotylenchulus reniformis In a No-Till Cotton System

Rotylenchulus reniformis is the most damaging nematode pathogen of cotton in Alabama. It is easily introduced into cotton fields via contaminated equipment and, when present, is difficult and costly to control. A trial to monitor the natural migration of R. reniformis from an initial point of origin was established in 2007 and studied over two growing seasons in both irrigated and non-irrigated no-till cotton production systems. Vermiform females, juveniles and males reached a horizontal distance of 200 cm from the initial inoculation point, and a depth of 91 cm in the first season in both systems. Irrigation had no effect on the migration of vermiform females and juveniles, but males migrated faster in the irrigated trial than in the non-irrigated trial. Population density increased steadily in the irrigated trial during both years, exceeding the economic threshold of 1,000 per 150 cm(3), but was highly correlated with rainfall in the non-irrigated trial. The average speed of migration ranged from 0- to 3.3-cm per day over 150 days. R. reniformis was able to establish in both the irrigated and non-irrigated trials in one season and to increase population density significantly.     

Effect of Temperature on the Embryogenesis of Geographic Populations of Rotylenchulus reniformis

The effect of temperature on the embryonic development of three populations of reniform nematode (Rotylenchulus reniformis) from the southeastern United States was studied. The development of eggs from single-cell stage to eclosion of second-stage juvenile was monitored at 20, 25, 30, and 35°C. All populations completed embryogenesis in 7 days at 25°C. The greatest differences among populations in time to completion of embryogenesis were observed at 20 and 35°C. Results at the intermediate temperatures (25 and 30°C) were similar for the three populations. The optimal temperature for embryogenesis was calculated to be 31.4°C for the population from Alabama, 28.4°C for the one from Mississippi, and 37.5°C for the one from South Carolina.     

Postinfection Development of Rotylenchulus reniformis on Resistant Gossypium barbadense Accessions

The reniform nematode (Rotylenchulus reniformis) causes significant cotton (Gossypium hirsutum) losses in the southeastern United States. The research objective was to describe the effects of two resistant G. barbadense lines (cultivar TX 110 and accession GB 713) on development and fecundity of reniform nematode. Nematode development and fecundity were evaluated on the resistant lines and susceptible G. hirsutum cultivar Deltapine 16 in three repeated growth chamber experiments. Nematode development on roots early and late in the infection cycle was measured at set intervals from 1 to 25 d after inoculation (DAI) and genotypes were compared based on the number of nematodes in four developmental stages (vermiform, swelling, reniform, and gravid). At 15, 20, and 25 DAI, egg production by individual females parasitizing each genotype was measured. Unique reniform nematode developmental patterns were noted on each of the cotton genotypes. During the early stages of infection, infection and development occurred 1 d faster on susceptible cotton than on the resistant genotypes. Later, progression to the reniform and gravid stages of development occurred first on the susceptible genotype, followed by G. barbadense cultivar TX 110, and finally G. barbadense accession GB 713. Egg production by individual nematodes infecting the three genotypes was similar. This study corroborates delayed development previously reported on G. barbadense cultivar TX 110 and is the first report of delayed infection and development associated with G. barbadense accession GB 713. The different developmental patterns in the resistant genotypes suggest that unique or additional loci may confer resistance in these two lines.     

Classification of Rotylenchulus reniformis Numbers in Cotton Using Remotely Sensed Hyperspectral Data on Self-Organizing Maps

Rotylenchulus reniformis is one of the major nematode pests capable of reducing cotton yields by more than 60%, causing estimated losses that may exceed millions of dollars U.S. Therefore, early detection of nematode numbers is necessary to reduce these losses. This study investigates the feasibility of using remotely sensed hyperspectral data (reflectances) of cotton plants affected with different nematode population numbers with self-organizing maps (SOM) in correlating and classifying nematode population numbers extant in a plant's rhizosphere. The hyperspectral reflectances were classified into three classes based on R. renifomis population numbers present in plant's rhizosphere. Hyperspectral data (350-2500 nm) were also sub-divided into Visible, Red Edge + Near Infrared (NIR) and Mid-IR region to determine the sub-region most effective in spectrally classifying the nematode population numbers. Various combinations of different feature extraction and dimensionality reduction methods were applied in different regions to extract reduced sets of features. These features were then classified using a supervised-SOM classification method. Our results suggest that the overall classification accuracies, in general, for most methods in most regions (except visible region) varied from 60% to 80%, thereby, indicating a positive correlation between the nematode numbers present in plant's rhizosphere and the corresponding plant's hyperspectral signatures. Results showed that classification accuracies in the Mid-IR region were comparable to the accuracies obtained in other sub-regions. Finally, based on our findings, the use of remotely-sensed hyperspectral data with SOM could prove to be extremely time efficient in detecting nematode numbers present in the soil.     

Effect of Controlled Cold Storage on Recovery of Rotylenchulus reniformis from Naturally Infested Soil

Rotylenchulus reniformis is rapidly becoming the most economically important pest associated with cotton in the southeastern United States. Incentive programs have been implemented to support sampling of production fields to determine the presence and abundance of R. reniformis. These sampling programs have dramatically increased the number of soils samples submitted to nematology laboratories during autumn. The large numbers of samples overwhelm most labs and require placement in cold storage until extraction. Therefore, the objective of this study was to examine the length of time soils infested with R. reniformis can be stored before nematode extraction without compromising the accuracy of estimates of population densities. A sandy loam and a silty loam were the two cotton production soils used in this study. Rotylenchulus reniformis numbers decreased 61%during the first 180 days of storage in both soils. Rotylenchulus reniformis numbers from the initial sampling through 180 days decreased as a linear function. The decline of R. reniformis numbers during storage was estimated as 0.28% of the population lost daily from the maximum population through 180 days. The diminution of nematode numbers from 180 through 1,080 days in storage continued, but at a slower rate. Numbers of R. reniformis declined to less than 89%, 93%, and 99% of the initial population within 360, 720, and 1,080 days, respectively, of storage. The reduction of R. reniformis numbers over 180 days can be adjusted, allowing a more accurate estimation of R. reniformis levels in soil samples stored at 4 °C.     

Interaction of Rotylenchulus reniformis with Seedling Disease Pathogens of Cotton

The impact of 10 Fusarium species in concomitant association with Rotylenchulus reniformis on cotton seedling disease was examined under greenhouse conditions. In experiment 1, fungal treatments consisted of Fusarium chlamydosporum, F. equiseti, F. lateritium, F. moniliforme, F. oxysporum, F. oxysporum f.sp. vasinfectum, F. proliferatum, F. semitectum, F. solani, and F. sporotrichioides; Rhizoctonia solani; and Thielaviopsis basicola. The experimental design was a 2 × 14 factorial consisting of the presence or absence of R. reniformis and the 12 fungal treatments plus two controls in autoclaved field soil. In experiment 2, the same fungal and nematode treatments were examined in autoclaved or non-autoclaved soil. This experimental design was a 2 × 2 × 14 factorial consisting of field or autoclaved soil, presence or absence of R. reniformis, and the 12 fungal treatments plus two controls. In both tests, Fusarium oxysporum f. sp. vasinfectum, F. solani, R. solani, and T. basicola consistently displayed extensive root and hypocotyl necrosis that was more severe (P ≤ 0.05) in the presence of R. reniformis. Soil treatment (autoclaved vs. non-autoclaved) influenced the impact of the Fusarium species on cotton seedling disease, with disease being more severe in the autoclaved soil. Rotylenchulus reniformis reproduction on cotton seedlings was greater in field soil compared to autoclaved soil (P ≤ 0.05). This study suggests the importance of Fusarium species and R. reniformis in cotton seedling disease.     

Using Geostatistical Analysis to Evaluate the Presence of Rotylenchulus reniformis in Cotton Crops in Brazil: Economic Implications

In recent years, the productivity of cotton in Brazil has been progressively decreasing, often the result of the reniform nematode Rotylenchulus reniformis. This species can reduce crop productivity by up to 40%. Nematodes can be controlled by nematicides but, because of expense and toxicity, application of nematicides to large crop areas may be undesirable. In this work, a methodology using geostatistics for quantifying the risk of nematicide application to small crop areas is proposed. This risk, in economic terms, can be compared to nematicide cost to develop an optimal strategy for Precision Farming. Soil (300 cm³) was sampled in a regular network from a R. reniformis-infested area that was a cotton monoculture for 20 years. The number of nematodes in each sample was counted. The nematode number per volume of soil was characterized using geostatistics, and 100 conditional simulations were conducted. Based on the simulations, risk maps were plotted showing the areas where nematicide should be applied in a Precision Farming context. The methodology developed can be applied to farming in countries that are highly dependent on agriculture, with useful economic implications.     

Detection of Suppressiveness against Rotylenchulus reniformis in Soil from Cotton (Gossypium hirsutum) Fields in Texas and Louisiana

Rotylenchulus reniformis is a major problem confronting cotton production in the central part of the cotton belt of the United States of America. In this study, the hypothesis that natural antagonists in some cases are responsible for unusually low densities of the nematode in certain fields was tested by assaying soils from 22 selected fields for the presence of transferable agents in pots containing cotton plants. In one field, soil from four different depth ranges was tested. In the first of two types of assays, 1 part nematode infested soil was added to 9 parts test soil that was left untreated or autoclaved before mixing; this mixture was used to fill pots. In the second type of assay, 1 part test soil was added to 9 or 19 parts pasteurized fine sand, and nematodes were introduced in aqueous suspension. In three experiments representing both types of assay, transferable or autoclavable agent(s) from four fields in South Texas suppressed nematode populations by 48, 78, 90 and 95%. In one experiment, transferable agents in five fields in Louisiana suppressed populations from 37 to 66%. Identification and evaluation of these agents for biological control of R. reniformis merits further study.     

Extraction of Root-associated Meloidogyne incognita and Rotylenchulus reniformis

A technique based on physical maceration of root tissue was developed to extract vermiform and swollen stages of Meloidogyne incognita and Rotylenchulus reniformis. Experiments conducted on soybean and tomato evaluated the efficiency of method (stir, grind), NaOC1 concentration (0%, 0.5%), and duration (lx, 2x) on extraction of nematodes and eggs from 60-day-old populations. Root-associated populations of R. reniformis were considerably lower than those of M. incognita, so development of the method focused on the latter. Grinding liberated more nematodes than stirring, but the reverse was true for egg extraction. Among grinding treatments, a duration of 10 seconds in 0.5% NaOCl provided the most efficient extraction of nematodes and eggs. Among stirring treatments, a duration of 10 minutes in 0.5% NaOCl provided the most efficient extraction of eggs. These techniques were compared on soybean roots 30 days older than those on which the procedures were first evaluated, with consistent results.     

Sensitivity of Meloidogyne incognita and Rotylenchulus reniformis to Abamectin

Avermectins are macrocyclic lactones produced by Streptomyces avermitilis. Abamectin is a blend of B_1a and B_1b avermectins that is being used as a seed treatment to control plant-parasitic nematodes on cotton and some vegetable crops. No LD₅₀ values, data on nematode recovery following brief exposure, or effects of sublethal concentrations on infectivity of the plant-parasitic nematodes Meloidogyne incognita or Rotylenchulus reniformis are available. Using an assay of nematode mobility, LD₅₀ values of 1.56 μg/ml and 32.9 μg/ml were calculated based on 2 hr exposure for M. incognita and R. reniformis, respectively. There was no recovery of either nematode after exposure for 1 hr. Mortality of M. incognita continued to increase following a 1 hr exposure, whereas R. reniformis mortality remained unchanged at 24 hr after the nematodes were removed from the abamectin solution. Sublethal concentrations of 1.56 to 0.39 μg/ml for M. incognita and 32.9 to 8.2 μg/ml for R. reniformis reduced infectivity of each nematode on tomato roots. The toxicity of abamectin to these nematodes was comparable to that of aldicarb.     

Effects of Acibenzolar-S-Methyl Application to Rotylenchulus reniformis and Meloidogyne javanica

Effects of acibenzolar-s-methyl, an inducer of systemic acquired resistance in plants, on Rotylenchulus reniformis and Meloidogyne javanica in vitro and in vivo were determined. A single foliar application of acibenzolar at 50 mg/liter (5 ml of solution per plant) to 7-day-old cowpea or soybean seedlings decreased R. reniformis and M. javanica egg production by 50% 30 days after inoculation. The mechanism of acibenzolar on plant-parasitic nematodes was then investigated. Acibenzolar at 50 to 200 mg/liter did not affect movement of R. reniformis and M. javanica or penetration of second-stage juveniles (J2) of M. javanica on cowpea. However, M. javanica development was slowed and fecundity was reduced in plants treated with acibenzolar. On average, 50% of J2 that penetrated acibenzolar-treated cowpeas developed into mature females with eggs, whereas the other 50% exhibited arrested development. The number of eggs per egg mass was 450 in water-treated cowpeas, whereas the number declined to 250 in acibenzolar-treated plants. Acibenzolar may be responsible for stimulating the plants to express some resistance to the nematodes.