Microplot studies with Xiphinema diversicaudatum under annual and perennial cropping regimes

Annual and perennial crops were grown in monoculture over three- or four-year experiments. Strawberry, raspberry and ryegrass maintained high population densities of Xiphinema diversicaudatum. Hop maintained relatively low population densities as did all annual crops tested, including winter wheat, winter beans, spring barley, potatoes, sugar beet and cabbage. Differences in nematode population density were not due to differences in soil porosity.     

The long-term effects of four monocultural regimes on two field populations of the nematodes Xiphinema diversicaudatum and Longidorus spp.

Population density changes of Xiphinema diversicaudatum and several Longidorus spp. were observed under four cultural regimes in long-term field experiments. Numbers of X. diversicaudatum increased under strawberry and, to a lesser extent, ryegrass, but remained more or less unchanged in hop and fallow plots. A mixture of four Longidorus spp. increased in numbers under grass but stayed static, or declined slowly under the other cultural regimes. All changes in population density were slow to occur, taking 3 or 4 yr for differences to become statistically significant. Fluctuations in larval numbers were mainly responsible for overall population changes; numbers of adults showed little difference over 5 yr.     

Winter grass cover crop effects on nematodes and yields of double cropped soybean

Rye (Secale cereale L.), wheat (Triticum aestivum L.), and annual ryegrass (Lolium multiflorum Lam.) are commonly double cropped with soybean (Glycine max L.). Recent greenhouse studies have shown variability in plant-parasitic nematode response to cool season grass species and cultivars. However, subsequent soybean performance was not affected by previous annual ryegrass cultivar in the green-house. The objective of this research was to determine whether winter cover crop species or cultivars affected nematode populations and subsequent performance of soybean in teh field. Four cultivars of annual ryegrass, wheat, and rye, and a fallow control were seeded on a Suffolk sandy loam (fine-loamy, siliceous, thermic Typic Hapuldult) soil in each of three years. Nematode-susceptible soybeans were seeded following forage removal. Soil samples for nematode counts were taken immediately before soybean harvest each year. In another experiment, one cultivar each of annual ryegrass, wheat, and rye, and a fallow control were followed by three soybean cultivars selected for differing nematode susceptibility. Grass cultivars did not affect nematode populations under succedding soybean. The only nematodes affected by grass species in either experiment were Pratylenchus spp., Heterodera glycines Ichinohe, and Tylenchorhynchus claytoni (Kofoid and White) Chitwood. Nematode population means were usually low following ryegrass and high following the fallow control. High soybean yields followed the fallow control, and low soybean yields followed annual ryegrass.     

Nuclear changes induced by the nematodesXiphinema diversicaudatum andLongidorus elongatus in root-tips of perennial ryegrass,Lolium perenne

Summary The DNA content and size of individual nuclei from galls of perennial ryegrass root-tips induced byX. diversicaudatum andL. elongatus were measured. Feeding byX.diversicaudatum increased the DNA content of the nuclei by varying amounts. No regular doubling pattern of the DNA content was discernible. The DNA values varied up to between 32–64C. Generally the size of the nuclei was not increased, although some were larger than control nuclei. The modified nuclei probably have an altered metabolic function, which increases the food value of the gall to the nematode. Some bi-nucleate cells were also observed, which probably result from mitosis without cytokinesis. A preliminary examination of nuclei from galls induced byL. elongatus revealed similar nuclear changes, but no bi-nucleate cells were found. Editor's note: Awarded the Viviane Maggi prize for the best paper presented by a beginner at the Annual Meeting of the Histochemistry and Cytochemistry Section of the Royal Microscopical Society in April 1981. This paper is a full report of that presentation.     

Observations on the occurrence, pathogenicity and control of Pratylenchus vulnus, P. thornei and Xiphinema diversicaudatum associated with glasshouse roses

About half the nurseries and glasshouses in the Lea Valley of Hertfordshire were found to be infested with either or both Pratylenchus vulnus and Xiphinema diversicaudatum. The latter nematode probably occurred naturally in the soil on which the glasshouses were built but P. vulnus is thought to have been imported with rose rootstocks and is not known to occur outdoors in Britain. Both nematode species can cause decline of roses and even small numbers seem harmful. Numbers of P. vulnus can increase greatly, especially on rootstocks of Rosa canina, although numbers were often smaller when roses were severely damaged than on healthier crops. Numbers of X. diversicaudatum increased more slowly and R. canina was a better host for it than R. chinensis. X. diversicaudatum seemed to respond less quickly than P. vulnus to reduced host-plant vigour. The two rootstocks exhibited different host-status for the two species of nematode and cuttings of various rose cultivars showed different host-status to P. thornei, another species of lesion nematode which is not known to be pathogenic to roses. Two applications of dibromochloropropane liquid at the rate of 70 l/ha in a large volume of water maintained nematode densities at an acceptably low level, and growers who adopted this treatment as a supplement to pre-planting steam sterilisation and/or DD (dichloro-propane: dichloropropene) prolonged the productive life of their crops for several years.     

Host status of some plants for Xiphinema diversicaudatum (Micol.) and their susceptibility to viruses transmitted by this species

Forty plant species were grown in pots containing viruliferous Xiphinema diversicaudatum (Micol.) for 15 wk to assess the host range of the nematode in relation to infection with arabis mosaic (AMV) and strawberry latent ring-spot (SLRV) viruses. Host status for the nematode was determined mainly from changes in total populations, but the presence of eggs in the uteri of females and changes in the numbers of adults provided additional criteria. The nematode multiplied on relatively more woody perennials than on herbaceous crop plants or weeds. Chrysanthemum coronarium was the only plant on which numbers declined significantly below those on the controls. Most plant species became infected with either AMV or SLRV. Neither virus was detected in eight out of thirteen species of trees and shrubs although four were good hosts for the nematode. Galling or distortion of the terminal region of fine feeder roots, associated with X. diversicaudatum feeding, was seen on many of the experimental plants.     

The association of Xiphinema diversicaudatum (Micoletsky) with strawberry latent ringspot and arabis mosaic viruses in a raspberry plantation

An outbreak of strawberry latent ringspot virus (SLRV) in a plantation of Mailing Jewel raspberry coincided with the greatest abundance of the nematode vector, Xiphinema diversicaudatum. Arabis mosaic virus (AMV) was not detected in the crop but was, together with SLRV, in many weed species present. AMV was transmitted through the seed of Poa annua, Capsella bursa-pastoris and Senecio vulgaris and SLRV through the seed of Mentha arvensis. X. diversicaudatum were more numerous within the rows than between them and vertical sampling showed that most occurred between 4 and 12 in depth in both locations. Monthly sampling showed that egg laying occurred from April to July; populations increased to a peak in late autumn but declined during the winter, resulting in about a twofold annual increase in numbers. Females, males and juveniles transmitted AMV and SLRV to cucumber seedlings, and in the absence of plants the nematode retained AMV for 112 days and SLRV for 84 days.     

The transmission of strawberry latent ringspot virus by Xiphinema diversicaudatutn (Nematoda)

Strawberry latent ringspot virus (SLRV) was found in diseased rose bushes growing in a glasshouse where the soil contained Xiphinema diversicaudatum (Micol). Adult female, adult male and juvenile X. diversicaudatum all transmitted the virus to cucumber seedlings, and nematodes kept without plants for 32 days after acquiring SLRV later transmitted it. When transferred to fresh plants every 2–4 days for 3 weeks, single nematodes transmitted up to three times; one nematode did not transmit until 19 days after the transfers began. One adult X. diversicaudatum, out of 141 tested, transmitted both SLRV and arabis mosaic virus. In all respects X. diversicaudatum behaved as a vector of SLRV as it does as a vector of arabis mosaic virus.     

Isolates of Microdochium nivale and M. majus Differentiated by Pathogenicity on Perennial Ryegrass (Lolium perenne L.) and in vitro Growth at Low Temperature

Pink snow mould is a serious disease on grasses and winter cereals in cold and temperate zones during winter. To better understand the basis for the variation in pathogenicity between different isolates of Microdochium nivale and M. majus and to simplify selection of highly pathogenic isolates to use when screening for resistance to pink snow mould in perennial ryegrass, we sought traits correlated with pathogenicity. Isolates of M. nivale were more pathogenic on perennial ryegrass than isolates of M. majus, as measured by survival and regrowth of perennial ryegrass after infection and incubation under simulated snow cover. Pathogenicity as measured by relative regrowth was highly correlated with fungal growth rate on potato dextrose agar (PDA) at 2°C. Measuring fungal growth on PDA therefore seems to be a relatively simple method of screening for potentially highly pathogenic isolates. In a study of a limited number of isolates, highly pathogenic isolates showed an earlier increase and a higher total specific activity of β‐glucosidase, a cell wall‐degrading enzyme, compared with less pathogenic isolates. None of the M. majus isolates was highly pathogenic on perennial ryegrass. Our results indicate biological differences between M. nivale and M. majus and thus strengthen the recently published sequence‐based evidence for the elevation of these former varieties to species status.     

The geographical distribution of Xiphinema and Longidorus nematodes in the British Isles and Ireland

The distribution of Xiphinema and Longidorus species in the British Isles and Ireland were mapped by means of a computer programme from 5527 records. Longidorus caespiticola, L. elongatus, L. goodeyi and L. leptocephalus were found to be widespread among a wide range of vegetation types, particularly in grassland and arable crops; L. macrosoma and L. profundorum were recorded frequently in southern and central England, and L. attenuatus in East Anglia; L. vineacola was recorded once only in Eire. Paralongidorus maximus was recorded from two localities in eastern Scotland and one in southern England, on each occasion from intensively cultivated ground. Four species of Xiphinema were recorded but only X. diversicaudatum is widespread although apparently with a northerly limit in central Scotland; X. diversicaudatum was equally prevalent in arable crops, grassland and deciduous woodland; X. coxi and X. vuittenezi were recorded from a few scattered localities in England, and the few records for X. mediterraneum were all from Kent, south east England. All species were found in a wide range of soil types but mostly in light soils with a large sand fraction. Many of the species occurred together in mixed populations; L. caespiticola was most frequently associated with X. diversicaudatum and with L. leptocephalus and L. elongatus. X. diversicaudatum also was commonly found with L. goodeyi. Viruses were transmitted in laboratory bait tests in only eighteen of 325 soil samples containing X. diversicaudatum and four of 265 containing L. elongatus.     

Effects of Temperature on Rate of Feeding of the Plant Parasitic Nematodes Rotylenchus robustus,Xiphinema diversicaudatum,and Hemicycliophora conida

Rotylenchus robustus, Xiphinema diversicaudatum, and Hemicycgiophora conida were observed feeding over a range of temperatures on perennial rye-grass (Lolium perenne) seedlings grown on agar plates. R. robustus fed between 0.5 and 42.5 C, X. diversicaudatum between 5.0 and 37.0 C and H. conida between 5.0 and 34.0 C. Between 10 and 25 C there was a direct relationship between temperature and rate of esophageal bulb contractions. Above 25 C the number of esophageal contractions/min did not increase at the same rate and eventually decreased. At the extremes of temperature range, abnormal feeding behaviour was observed. Rates of esophageal bulb contraction did not differ in the different nematode life stages and sexes, or at different feeding sites on the roots.     

Candidate perennial bioenergy grasses have a higher albedo than annual row crops

The production of perennial cellulosic feedstocks for bioenergy presents the potential to diversify regional economies and the national energy supply, while also serving as climate ‘regulators’ due to a number of biogeochemical and biogeophysical differences relative to row crops. Numerous observational and model‐based approaches have investigated biogeochemical trade‐offs, such as increased carbon sequestration and increased water use, associated with growing cellulosic feedstocks. A less understood aspect is the biogeophysical changes associated with the difference in albedo (α), which could alter the local energy balance and cause local to regional cooling several times larger than that associated with offsetting carbon. Here, we established paired fields of Miscanthus × giganteus (miscanthus) and Panicum virgatum (switchgrass), two of the leading perennial cellulosic feedstock candidates, and traditional annual row crops in the highly productive ‘Corn‐belt’. Our results show that miscanthus did and switchgrass did not have an overall higher α than current row crops, but a strong seasonal pattern existed. Both perennials had consistently higher growing season α than row crops and winter α did not differ. The lack of observed differences in winter α, however, masked an interaction between snow cover and species differences, with the perennial species, compared with the row crops, having a higher α when snow was absent and a much lower α when snow was present. Overall, these changes resulted in an average net reduction in annual absorbed energy of about 5 W m^?2 for switchgrass and about 8 W m^?2 for miscanthus relative to annual crops. Therefore, the conversion from annual row to perennial crops alters the radiative balance of the surface via changes in α and could lead to regional cooling.     

Sites of virus retention in the alimentary tract of the nematode vectors, Xiphinema diversicaudatum (Micol.) and X. index (Thorne and Allen)

Electron microscopy of thin sections of Xiphinema diversicaudatum and X. index fed on plants infected respectively with arabis mosaic and grapevine fanleaf viruses showed that the viruses are retained as a monolayer of particles adsorbed on to the cuticle lining the lumina of the odontophore (stylet extension), anterior oesophagus and oesophageal bulb. During the moult of the nematode the cuticular lining is shed and together with the detached virus particles is ingested into the intestine through the oesophago-intestinal valve; this supports the limited experimental evidence that viruses transmitted by X. diversicaudatum and X. index are not retained through the moult.     

Some effects of oxamyl on the virus-vector nematodes Longidorus elongatus and Xiphinema diversicaudatutn

Oxamyl applied to field soil prevented Longidorus elongatus from acquiring and transmitting tomato black ring virus for at least 6 wk, although numbers of nematodes were not greatly decreased compared with the untreated control. Glasshouse and laboratory tests examined the effects of oxamyl on viruliferous L. elongatus and Xiphinema diversicaudatum. In these tests oxamyl (1.0 ppm) in the soil water largely prevented L. elongatus transmitting virus to bait plants over a period of 1 month. X. diversicaudatum was equally affected by smaller concentrations, 0.1 ppm being sufficient to inhibit virus transmission in one test. Inhibition of virus transmission was associated with a decrease in the number of root tip galls produced by nematode feeding, especially that of X. diversicaudatum. Few nematodes were seriously affected by oxamyl, except at the greatest concentration tested (100 ppm), when numbers of L. elongatus, but not of X. diversicaudatum, were decreased. In vitro-treated viruliferous nematodes protracted their stylets, but their subsequent ability to transmit virus was unimpaired.     

Effect of genetic background on trisomy in ryegrass

Six primary trisomics of ryegrass, Lolium perenne L., were studied in perennial and perennial x annual hybrid backgrounds. Chromosome association at meiotic metaphase I and chiasma number per cell of the individual trisomes did not differ in the two genetic backgrounds. Hybrid trisomies showed wider variation in morphology, and had higher pollen fertility than the perennial trisomics and disomics. — It is concluded that the transfer of perennial ryegrass chromosomes and segments into annual ryegrass can be accomplished without any serious consequence on the cytological stability of the reconstituted genome.     

A novel DNA-based diagnostic test for the detection of annual and intermediate ryegrass contamination in perennial ryegrass

Perennial ryegrass (Lolium perenne L.) is a preferred choice for the turf grass industry due to its ability to provide a durable turf cover. Genetic or physical contamination of annual (L. multiflorum Lam.) or intermediate (L. hybridum) ryegrass species in perennial ryegrass is one of the major problems affecting the grass seed industry. At present, seedling root fluorescence (SRF), a biochemical marker, is used for the detection of annual ryegrass contamination. Due to the unreliability of the SRF test, the seed industry is seeking an alternative, more reliable and accurate detection method. Currently, there are no DNA tests available in ryegrass for detecting contamination with annual and intermediate ryegrass types. We developed a novel quantitative polymerase chain reaction (Q-PCR)-based DNA test for the detection of annual and/or intermediate ryegrass types in perennial ryegrass. This DNA test was designed using an insertion/deletion (InDel) site in the LpVRN2_2 (Vernalization 2) gene, which is one of the several genes controlling vernalization in ryegrass. The new DNA test is more reliable, accurate and cost-effective in detecting contamination, with a high sensitivity of 0.04% in a sample size of 5,000 seeds. Use of larger sample sizes (12.5-fold higher compared to SRF test) provided additional accuracy in detecting the level of contamination. The method has produced consistent results in 68 perennial, 26 annual and 14 intermediate ryegrass lines.     

The Effect of Chitosan and Bion on Resistance to Pink Snow Mould in Perennial Ryegrass and Winter Wheat

The effects of chitosan on resistance to pink snow mould (Microdochium nivale) were studied in young winter wheat (Triticum aestivum L.) and perennial ryegrass (Lolium perenne L.) under controlled environmental conditions. In perennial ryegrass, the putative defence activator Bion was also tested. Resistance was measured as regrowth of plants after inoculation with M. nivale and incubation in darkness at 2°C. In winter wheat, pre‐treatment with chitosan at 1000 μg per plant increased resistance to subsequent infection by M. nivale, but this effect was less significant in a replicate experiment. Chitosan‐treated winter wheat plants expressed the gene for the pathogenesis‐related protein chitinase at higher levels than non‐treated plants. Chitinase gene expression was also stimulated by M. nivale infection in winter wheat. Perennial ryegrass pre‐treated with Bion or chitosan and inoculated with M. nivale did not display better regrowth after incubation than non‐treated, inoculated plants. Rather, regrowth was reduced in some of the Bion‐treated plants after incubation. We speculate that the cost or the mechanism of induced resistance makes Bion non‐effective in plants that are not actively growing. Bion at concentrations of 10, 100 and 1000 μg active ingredient per ml, and the highest concentration of chitosan used (2000 μg per ml) reduced in vitro growth of the pathogen, suggesting that both defence activators possess antifungal activity.     

Influences of preceding cover crops on slug damage and biological control using Phasmarhabditis hermaphrodita

In a replicated field experiment, ryegrass, vetch and red clover were grown or the soil was kept bare over a 2–month period in summer to compare the effects of these treatments on slug damage to the following crop (Chinese cabbage) and on the efficacy of nematodes (Phasmarhabditis hermaphrodita) applied as biological control agents to the soil at planting time to protect this crop. Slug damage was significantly (c. two times) greater after red clover or vetch than after ryegrass. Damage on plots without cover crop was intermediate and not significantly different from either extreme. Slug damage was reduced by about one‐third by the nematode treatment. The preceding cover crop did not influence nematode efficacy. Numbers of slugs on harvested plants (mainly Deroceras reticulatum and Deroceras panormitanum) were influenced by an interaction between cover crop and nematode treatment. On subplots without nematodes, more slugs were recorded with than without a preceding cover crop. No such differences were found on nematode‐treated subplots. Soil samples were collected at intervals from 0–99 days after nematode treatment to monitor nematode survival and infectivity in bioassays with D. reticulatum. No significant effects of cover crops were detected in bioassays. Moreover, there were no significant effects of nematodes on slug survival. Their effects on slug food consumption were mostly insignificant and any effects were transient and not consistent. However, significantly more slug cadavers contained nematodes when slugs were exposed to nematode‐treated soil. The implications of these results are discussed.     

Virulence of Meloidogyne spp. and Induced Resistance in Grape Rootstocks

Harmony grape rootstock displays resistance to several Meloidogyne spp. but that resistance is not durable in commercial vineyard settings. A 2-year experiment in a microplot setting revealed host specificities of two virulent populations of Meloidogyne arenaria and an avirulent population of Meloidogyne incognita. In a subsequent split-root experiment, the avirulent nematode population was demonstrated to induce resistance to the virulent nematode population. To quantify the level of resistance, reproduction of the virulent nematode population was determined 63 days after being challenged by an avirulent nematode population using a range of inoculum densities and timeframes. Induction of resistance became apparent when the virulent nematode population was inoculated 7 days after the avirulent nematode population and increased thereafter. The level of induced resistance increased with increased inoculum levels of the avirulent nematode population. Root systems of perennial crops are commonly fed upon simultaneously by multiple nematode species. These two studies indicate that field populations can become preferentially virulent upon one or multiple rootstocks and that co-inhabiting populations may induce existing resistance mechanisms. In perennial crops, it is common for numerous nematode species besides Meloidogyne spp. to be present, including some that feed without causing apparent damage.     

Water budgets and nutrients in a native Banksia woodland and an adjacent Medicago sativa pasture

In southern Australia, the spread of dryland salinity can be traced to increased leakage of water through the root zone to the groundwater, associated with clearing of perennial vegetation and its replacement with annual crops and pastures. Agricultural activity, through fertilizer addition and subsequent nutrient export, has also changed the nutrient status of the soils, often causing increases in soil acidity. In this trial, an area of native vegetation on a deep sandy soil in south-western Australia (dominated by Banksia prionotes Lindley), and an adjacent introduced perennial pasture (Medicago sativa L.), were compared in terms of their water balance and nutrient fluxes for the period between August 1998 and March 2001. Initially, the Banksia woodland vegetation maintained a drier soil profile below 2 m than the establishing lucerne pasture, and leakage beyond 4 m in 1999 was 80 mm under the Banksia woodland and 180 mm under the lucerne. However, in 2000, lucerne's rate of water use during winter was faster than any other vegetation observed on this soil type, possibly due to direct groundwater extraction, and it dried the soil to the same level as the Banksia woodland vegetation. Nutrient (nitrate, phosphate, sulphate, potassium, calcium, magnesium and sodium) fluxes under both systems were generally low, reflecting the inherently low fertility of the soil type. However, sodium and nitrate appeared to accumulate in soil at a depth of 4 m under the Banksia woodland (particularly between the Banksia canopies), but not under the lucerne, possibly due to a history of leaching under the lucerne. Whilst both vegetation types effectively controlled excess water leakage, the differences in nutrient cycling and production levels suggests that some aspects of native perennial vegetation function may not be suitable for incorporation into agricultural systems.