Effects of benzyl isothiocyanate on the reproduction of Meloidogyne incognita on Glycine max and Capsicum annuum

Abstract

Reproduction of Meloidogyne incognita on either Capsicum annuum or Glycine max was suppressed when infective second-stage juveniles (J2) were exposed to 0.03 mM benzyl isothiocyanate (BITC) for 2 h prior to inoculation of the host. Infectivity as rated by a gall index was significantly reduced on both G. max and C. annuum. In C. annuum, nematode egg masses were recovered from 48% of the plants inoculated with BITC-treated J2 compared with 98% for the controls, and egg mass scores were reduced. Egg mass production was reduced in C. annuum by 69% and mean total eggs/plant was reduced by 97% in G. max. When plants were inoculated with treated J2, two measures of plant health, root weight and shoot weight, were reduced in C. annuum but not in G. max. However, root and shoot weights were not correlated with egg production for either host plant, irrespective of treatment. There were strong interactions between egg production, as measured by mean total eggs/plant (G. max) or mean total eggs/egg mass (C. annuum), and hatching of J2 from eggs obtained from all combinations of plant host and J2 treatment. Hatch of J2 from eggs obtained from G. max was significantly lower when plants had been inoculated with BITC-treated J2 than when plants had been inoculated with control J2. Such effects were not observed with C. annuum. BITC may have important residual consequences on the progeny of M. incognita not directly exposed to the chemical.

Managing nematodes without methyl bromide

Methyl bromide is an effective pre-plant soil fumigant used to control nematodes in many high-input, high-value crops in the United States, including vegetables, nursery plants, ornamentals, tree fruits, strawberries, and grapes. Because methyl bromide has provided a reliable return on investment for nematode control, many of these commodities have standardized their production practices based on the use of this chemical and will be negatively impacted if effective and economical alternatives are not identified. Alternative control measures based on other chemicals, genetic resistance, and cultural practices require a greater knowledge of nematode biology to achieve satisfactory results. Here, we provide an overview of nematode management practices that we believe will be relied upon heavily in U.S. high-value crop production systems in a world without methyl bromide. Included are case studies of U.S. high-value crop production systems to demonstrate how nematode management practices other than methyl bromide may be incorporated.

Toxicity of 2,4-diacetylphloroglucinol (DAPG) to plant-parasitic and bacterial-feeding nematodes

The antibiotic 2,4-diacetylphloroglucinol (DAPG) is produced by some isolates of the beneficial bacterium Pseudomonas fluorescens. DAPG is toxic to many organisms, and crop yield increases have been reported after application of DAPG-producing P. fluorescens. This study was conducted to determine whether DAPG is toxic to selected nematodes. The plant-parasitic nematodes Heterodera glycines, Meloidogyne incognita, Pratylenchus scribneri and Xiphinema americanum, and the bacterial-feeding nematodes Caenorhabditis elegans, Pristionchus pacificus, and Rhabditis rainai, were immersed in concentrations ranging from 0 to 100 μg/ml DAPG. Egg hatch and viability of juveniles and adults were determined. DAPG was toxic to X. americanum adults, with an LD₅₀ of 8.3 μg/ml DAPG. DAPG decreased M. incognita egg hatch, but stimulated C. elegans hatch during the first hours of incubation. Viability of M. incognita J2 and of C. elegans J1 and adults was not affected. There were no observed effects on the other nematodes. The study indicated that DAPG is not toxic to all nematodes, and did not affect the tested species of beneficial bacterial-feeding nematodes. Augmentation of DAPG-producing P. fluorescens populations for nematode biocontrol could be targeted to specific nematode species known to be affected by this compound and by other antibiotics produced by the bacteria, or these bacteria could be used for other possible effects, such as induced plant resistance.

Behavioural response of Meloidogyne incognita to benzyl isothiocyanate

Abstract

The breakdown of brassicaceous plant material produces nematotoxic isothiocyanates (ITCs). However, after the incorporation of brassicaceous plant material into soil, many nematodes are likely to be exposed only to sublethal concentrations of these compounds. Although unknown, the effect of these low ITC concentrations on the behaviour of plant-parasitic nematodes could play a role in nematode suppression. To address this question, the behaviour of infective second-stage juveniles (J2) of Meloidogyne incognita was evaluated following in vitro exposure to sublethal concentrations of benzyl isothiocyanate (BITC). Behaviour was qualitatively and quantitatively affected. Overall nematode activity in treatment groups scored visually, and individual nematode movement frequencies, quantified by video assay, were each significantly reduced within 2 h of exposure to 0.01 mM BITC. All responses were dose dependent. Infectivity of BITC-treated J2 of M. incognita on soybean (Glycine max) was measured directly by root staining and gall rating, and indirectly by egg production. All experiments showed significantly reduced infectivity after treatment of J2 with 0.01 mM BITC. In addition, egg production was almost completely eliminated (<5% of control) by 0.03 mM BITC. The correlation between the effect of BITC concentrations on J2 activity and infectivity was positive, with decreased J2 activity and infectivity resulting in decreased egg production. BITC concentrations that do not cause mortality significantly affect the behaviour of M. incognita J2, indicating that sublethal concentrations contribute to the overall nematode suppression by brassicaceous green manures.

Activity of Hydroxamic Acids from Secale cereale Against the Plant-Parasitic Nematodes Meloidogyne incognita and Xiphinema americanum

ABSTRACT Cyclic hydroxamic acids are secondary metabolites found in the family Poaceae and have been implicated in the allelopathy of rye (Secale cereale). The toxicity of these compounds against plant-parasitic nematodes is unknown. DIBOA (2,4-dihydroxy-(2H)-1,4-benzoxazin-3(4H)-one), DIMBOA (2,4-hydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one), and their degradation products BOA (benzoxazolin-2(3H)-one) and MBOA (6-methoxy-benzoxazolin-2(3H)-one) were screened in vitro against Meloidogyne incognita second-stage juveniles (J2) and eggs and mixed-stages of Xiphinema americanum. Xiphinema americanum was more sensitive to DIBOA and DIMBOA than M. incognita J2, with a maximum apparent mortality of 96 and 92% compared to 73 and 72% at 90 mug/ml. Eggs of M. incognita were less sensitive to the hydroxamic acids than J2; only DIBOA resulted in a 50% reduction in egg hatch, with a lethal concentration (LC(50)) of 74 mug/ml compared to 21 mug/ml for J2. When M. incognita J2 were exposed to DIBOA for 48 h and the compound was removed and replaced with water, the LC(50) value increased from 21.0 to 40.7 mug/ml. MBOA was not toxic to X. americanum or M. incognita eggs, but was toxic to M. incognita J2, with LC(50) values of 44 and 20 mug/ml before and after the compound was removed and replaced with water. BOA was the least toxic hydroxamic acid tested; it did not reduce M. incognita egg hatch after 1 week of exposure or increase X. americanum mortality after 24 h of exposure. While in vitro studies provide a valuable starting point in determining the toxicity of the chemical component of rye, the relevance of the data to soil remains to be determined.

Effect of Rice Root-Knot Nematode on Growth and Yield of Yellow Granex Onion

Rice root-knot nematode, Meloidogyne graminicola, infects all commercially grown onion cultivars in rice-onion cropping systems in the Philippines, but its economic importance has not been established. The effects of different preplant population levels (Pi) of M. graminicolaon Yellow Granex onion in pots and of different percentages of galled roots in a naturally infested field were evaluated. Leaf weight and root length of 'Yellow Granex 429' seedlings decreased with increasing Pi, while low Pi mildly stimulated plant height at the vegetative growth stage. Age of transplant and Pi influenced growth and yield of onion at maturity. Onion yield and root and leaf weights decreased as the age of the transplants increased. Growth and yield decreased with increased Pi. Bulb weight was reduced by 7 to 82% and diameter by 10 to 62% when plants were inoculated with 50 to 10,000 second-stage juveniles. Onion bulbs from the field were reduced by 16, 32, and 35% in weight and by 6, 17, and 18% in diameter when the percentage of roots galled was 10, 50, and 100%, respectively. Management of M. graminicolapopulations at the start of the growing season must be part of onion culture in rice-onion systems to reduce onion yield losses and to prevent economic loss.

Allelopathy in the management of plant-parasitic nematodes

There are numerous reports of nematicidal chemicals in crude plant homogenates, leachates, and decomposing residues. These compounds are usually assumed to be secondary metabolites, which serve as chemical defenses against disease and parasites. When such compounds are released into the rhizosphere, they are known as allelochemicals. The possibility exists to exploit allelochemicals for nematode control, and there have been many attempts to use this approach either by rotation, intercropping, or green manure treatments. Results have met with mixed success. Proof of allelochemical activity in field situations is difficult to obtain, but it is evident that some rotation crops are significantly better at reducing nematode populations than others. Rotations with non-host plants may simply deny the nematode population an adequate food source for reproduction (passive suppression), whereas allelopathic crops kill nematodes by the production of toxic compounds (active suppression). Progress toward sustainable agriculture should benefit from studies on allelopathic nematode control. However, grower acceptance of new plant-rotation strategies are based on economic and logistical considerations as well as efficacy. A potential practical application of allelopathic nematode control that involves using rapeseed as a green manure crop to reduce populations of Xiphinema americanum sensu lato in temperate orchards is presented.

Resistance Genes in a 'Williams 82' x 'Hartwig' Soybean Cross to an Inbred Line of Heterodera glycines

The number of resistance genes in soybean to soybean cyst nematode (SCN) Heterodera glycines was estimated using progeny from a cross of 'Williams 82' x 'Hartwig' (derived from 'Forrest'(3) x PI 437.654) screened with a fourth-generation inbred nematode line derived from a race 3 field population of SCN. Numbers of females developing on roots of inoculated seedlings were assigned to phenotype cells (resistant, susceptible, or segregating) using Ward's minimum variance cluster analysis. The ratio obtained from screening 220 F soybean families was not significantly different from a 1:8:7 (resistant:segregating:susceptible) ratio, suggesting a two-gene system for resistance. The ratio obtained from screening 183 F plants was not significantly different from a 3:13 (resistant:susceptible) ratio, indicating both a dominant (Rhg) and a recessive (rhg) resistance gene.

Inter- and Intraspecific Variation in Wild-type and Single Female-derived Populations of Xiphinema americanum-group Nematodes

Ten populations of Xiphinema americanum-group nematodes were reared from individual females to evaluate inter- and intraspecific variation under identical host and environmental conditions. Data indicated that morphometric variability of X. americanum was the result of genetic variation rather than phenotypic plasticity and that genetic heterogeneity was greater than previously thought. Morphometrics of single female derived (SFD) populations identified different genotypes present in the field populations. Stylet length was the least variable morphometric character of SFD populations, but collectively stylet measurements of all individuals formed an uninterrupted continuum ranging from 107-148 mum. Range and frequency of stylet measurements of field populations could be accounted for by the relative proportion of different genotypes in the population. Nine SFD populations were identified as X. americanum sensu stricto, and one SFD population was similar to X. californicum.

Plant-parasitic Nematodes on Soybean in South Carolina

Fields in a concentrated area of soybean production in South Carolina were chosen for soil sampling to determine the distribution of plant-parasitic nematodes. Five hundred sampling sites were distributed over 19 counties according to county soybean acreage. Helicotylenchus and Scutellonema were identified most frequently from soil samples; together, these genera occurred in over 70% of the samples. Pratylenchus and Paratrichodorus were each observed in more than 60% of fields. Meloidogyne spp. were found in 27% of the fields and Hoplolaimus columbus in 14%. Rotylenchulus reniformis and Belonolaimus sp. each occurred in less than 10% of the fields. Tylenchorhynchus and Mesocriconema (Criconemella) were each present in over 40% of the fields, but numbers from each field were low. Of the fields sampled, 14% contained Heterodera glycines. Of these, 47% were race 14 and 32% were race 3. Races 9, 6, and 10 were also observed.

Transmission of Nepoviruses by Xiphinema americanum-group Nematodes

The transmission of North American nepoviruses by putative species belonging to the Xiphinema americanum-group is reviewed. Xiphinema americanum sensu stricto, X. californicum, and X. rivesi each transmit cherry rasp leaf (CRLV), tobacco ringspot (TobRSV), and tomato ringspot nepovirus (TomRSV), and X. bricolensis is a vector of TomRSV. The apparent lack of specificity in the transmission of North American nepoviruses by X. americanum-group species markedly contrasts with the specific associations between European nepoviruses and their vector nematode species. Two complementary projects are described examining the taxonomic identity of putative species in the X. americanum-group, their morphological and genetic relationships, their ontogeny, and their ability to transmit viruses.

Aspects of Biology and Development of Xiphinema americanum and Related Species

Identification of Xiphinema americanum-group nematodes is based on relatively subtle morphological and morphometric differences, many of which overlap. The significance and importance of these separations cannot be assessed without a basic understanding of the biological differences between species. Currently, information is accumulating on Xiphinema biology, development, and genetics that will help to confirm or refute the current systematics of species in this group. Recently, it was demonstrated that Xiphinema species pass through either three or four juvenile stages before becoming adults. This new and fundamental information divides the genus and the X. americanum group into subgroups based on their developmental evolution and provides new insight into the taxonomy and systematic positions of the species.

Effect of Soybean Tip Removal on Penetration and Development of Heterodera glycines

On a few occasions, soybeans with broken root tips were included in tests to evaluate resistance to Heterodera glycines. Although females developed on these plants, the numbers tended to be lower than on similarly treated intact roots. To test the possibility that removal of the root meristem affected nematode development, a culture system using pruned soybeans was devised that permitted access to the roots without disturbing the plants. Treatments included removal of 2 mm of root tip at various times ranging from 24 hours before to 10 days after inoculation, or roots left intact. In each experiment, all roots were inoculated at the same time with equal numbers of freshly hatched second-stage juveniles of Heterodera glycines. No differences in nematode development were detected in plants with root tips removed after inoculation compared to the control. When tips were removed at or before inoculation, fewer juveniles entered roots and relatively fewer nematodes developed. Penetration levels and development correlated with root tip removal such that progressively fewer nematodes entered roots and relatively greater numbers of nematodes remained undeveloped as the time interval between root tip removal and inoculation was increased.

Morphometric Evidence for Three Juvenile Stages in Some Species of Xiphinema americanum sensu lato

One to two hundred nematodes from each of seven Xiphinema americanum-group populations were measured to determine the range of stylet and body lengths for juveniles and adults. First-stage juveniles were identified by the position of the replacement odontostyle (i.e., the tip of the replacement odontostyle overlapped the base of the odontophore). Nematodes were identified as second stage if the functional odontostyle was the same length as the replacement odontostyle of the first stage. Subsequent stages were similarly identified by establishing the range of corresponding replacement and functional odontostyle lengths. In all populations examined, this procedure created natural divisions that clearly grouped nematodes by stylet and body length. Presumably these groups identified all juvenile and adult stages. Populations of X. americanum, X. rivesi, and X. californicum from the United States had three juvenile stages, but a population of X. pachtaicum from Bulgaria had four juvenile stages.

A Technique for Evaluating Heterodera glycines Development in Susceptible and Resistant Soybean

A technique was developed to evaluate Heterodera glycines development in susceptible and resistant soybean. Roots of 3-day-old soybean were exposed to infective juveniles of H. glyci.nes in sand for 8 hours followed by washing and transfer to hydroponic culture. The cotyledons and apical meristem were removed and plants were maintained under constant light, which resulted in a dwarfed plant system. After 15 or 20 days at 27 C, nematodes were rated for development. Emerged males were sieved from the culture water and females were counted directly from the roots. Nematodes remaining in the roots were rated for development after staining and clearing the tissues. The proportion of nematodes at each stage of development and the frequency of completed molts for each stage were calculated from these data. This technique showed that resistance to H. glycines was stage related and did not affect males and females equally in all resistant hosts. The resistance of plant introduction PI 209332 primarily affected development of third and fourth-stage juveniles; 'Pickett' mainly affected second and third-stage juveniles, whereas PI 89772 affected all stages. Male development was markedly affected in PI 89772 and 'Pickett' but not in PI 209332.

A Modified Screening Test for Determining Heterodera glycines Resistance in Soybean

A modified version of a standard greenhouse bioassay for determining soybean cyst resistance in soybean plants is described. White plastic laundry tubs served as microplots for rearing large numbers of nematodes in a confined space; up to 3 million eggs of each generation were collected per tub. Before screening, SCN populations were evaluated on susceptible and resistant soybean to characterize female development; these were periodically retested. Screening tests took place in Todd planter flats (120 plants per flat). Test plants were inoculated with 1,200 eggs per plant and evaluated for resistance 33-37 days after inoculation. The plants were pruned at the cotyledonary node which resulted in a greatly reduced root system. Staining the roots in Toluidine Blue created contrast with the white females and facilitated counting. Greenhouse space was conserved, and the labor to set up and maintain the screening test was reduced.

Heterodera glycines-Soybean Association: A Rapid Assay Using Prun Seedlings

A 16-day bioassay to determine compatible and incompatible associations of soybeans and cyst nematodes is described. This permits large-scale experiments in the laboratory. Seedlings are placed between two sheets of moist paper towelling so that the root tips are even with the bottom edge. The towelling is then rolled and set on the surface of cyst-infested soil. Radicles are permitted to grow through the soil for 24 hours after which cotyledons and apical portions are trimmed to retard root growth. The plants are assembled in bundles and maintained in aerated water in test tubes for 15 days before counts of emerged nematodes are taken. This system is economical in both space and labor.

Inbreeding and hybridizing cyst nematodes on pruned soybeans in petri plates

Inbred nematodes propagated on a selecting host are likely to have homozygous genes of interest for investigating the genetics of host-parasite associations. A technique is presented to inbreed soybean cyst nematodes, by sibling matings at each generation, and to cross inbred lines. Soybean seedlings with severely trimmed cotyledons survive well on 0.8% agar. Eggs from a single female are incubated in water in a microtiter well. Virgin as well as mated females result from inoculation of two juveniles per root. Sibling males from the same source are produced by mass inoculations of eggs. Males are added individually to unmated females. Overall success for fertile females was 14% in 1,368 isolations. Three generations of inbreeding by siblings were achieved using nematodes from two populations that differ in their ability to reproduce on differential soybeans. Hybrids from crosses of the two inbred lines tested on differential hosts showed that the influence of Population 1 (selected and inbred on PI 209332) is greater than that of Population 2 (selected and inbred on PI 89772). Reciprocal crosses suggest that the influence of males is stronger than that of females in determining host specificity of F offspring in these crosses. Our technique is simple and effective for inbreeding and crossing soybean cyst nematodes.