The Potential for Mapping Nematode Distributions for Site-specific Management

The success of site-specific nematode management depends on a grower or advisor being able to afford to make a map of an infestation that is accurate enough for management decisions. The spatial dependence of nematode infestations and correlation of soil attributes with nematode density were assessed to investigate the scale of sampling required to obtain correlated observations of density and the use of soils data to reduce the cost of sampling. Nematodes and soil were sampled on a 76.2 × 76.2-m grid in two irrigated corn (Zea mays) fields for 2 years. Nematodes of each of three species were found in 36% to 77% of the cores from a field. Spatial dependence was detected for 10 of 16 distributions, and 22% to 67% of the variation in density within a field could be attributed to spatial correlation. Density was correlated to distances of 115 to 649 m in the directions of 0, 45, 90, and 135° from the crop row, and distances varied with direction. Correlations between nematode density and soil attributes were inconsistent between species and fields. These results indicate a potential for mapping nematode infestations for site-specific management, but provide no evidence for reducing the cost of sampling by substituting soils data for nematode counts when making a map.     

Interrelationship of Heterodera schachtii and Meloidogyne hapla on Tomato

Invasion of tomato (Lycopersicon esculentum L.) roots by combined and sequential inoculations of Meloidogyne hapla and a tomato population of Heterodera schachtii was affected more by soil temperature than by nematode competition. Maximum invasion of tomato roots, by M. hapla and H. schachtii occurred at 30 and 26 C, respectively. Female development and nematode reproduction (eggs per plant) of M. hapla was adversely affected by H. schachtii in combined inoculations of the two nematode species. Inhibition of M. hapla development and reproduction on tomato roots from combined nematode inoculations was more pronounced as soil temperature was increased over a range of 18-30 C and with prior inoculation of tomato with H. schachtii. M. hapla minimally affected H. schachtii female development, but there was significant reduction in the buildup of H. schachtii when M. hapla inoculation preceded that of H. schachtii by 20 days.     

外来入侵甜菜孢囊线虫在新疆的适生性分布及风险评估

[目的] 甜菜孢囊线虫是世界性的重要进境检疫性有害生物,每年造成严重的经济损失。甜菜是新疆重要的优势经济作物,总产量占全国总产量一半以上。对新疆开展甜菜孢囊线虫适生性分析及风险评估,旨在为我区重大外来有害生物的科学预警及防控提供指导。[方法] 采用BIOCLIM模型对甜菜孢囊线虫在新疆的具体潜在分布区进行预测,并采用多指标赋值运算,对其入侵的风险进行定量分析。[结果] 甜菜孢囊线虫在新疆荒漠绿洲区的适生区域主要集中在阿勒泰市、塔城、博乐市、伊犁州、乌鲁木齐市、昌吉州、库尔勒市、阿克苏乌什县和拜城县和哈密市的巴里坤哈萨克族自治县。甜菜孢囊线虫的危险性综合评估风险值R为2.397,属于高度危险等级。[结论] 新疆主要甜菜种植区为甜菜孢囊线虫的高度适生区,需要高度重视,其预测结果为建立甜菜孢囊线虫长效预警与防控机制提供必要的理论基础。     

Effect of Plant Age and Transplanting Damage on Sugar Beets infected by Heterodera schachtii

Sugar beet (Beta vulgaris L. cv. Monogerm C.S.F. 1971) seeds sown into Vineland fine sandy loam, infested with 15,500 H. schachtii juveniles/pot, showed little growth during an 11-week test in the greenhouse. Seedlings transplanted at 2, 4, and 6 weeks of age had 32, 30, and 31% less top weight and 71, 68, and 59% less root weight, respectively, compared to controls grown in nematode-free soil. Nematode reproduction in both direct-seeded and transplanted sugar beets was limited and related to root weight. Shoot/root ratios were increased by the nematodes in all nematode-infected beets compared to those grown in soil without nematodes. In contrast to seeding or transplanting sugar beets into nematode-infested Vineland fine sandy loam, an inoculation of Beverly fine sandy loam supporting 0 (seeds), 2-, 4-, and 6-week-old sugar beet seedlings with 7,400 juveniles/pot, followed by 11 weeks of growth in the growth-room, resulted in top weight losses of only 13, 3, 18, and 15% and losses in root weight of 44, 38, 36, and 38%, respectively. Nematode reproduction was high and all shoot/root ratios were increased by the nematode compared to the noninoculated controls. These experiments have shown that sugar beets sown into nematode-infested soil are damaged much more heavily by H. schachtii juveniles than seeds inoculated with the nematode immediately following sowing. Results indicate that an increase in tolerance of sugar beets to attack by H. schachtii does not occur beyond the first 2 weeks of growth and that transplanting damage lowers the tolerance of seedlings to nematode attack.     

The relationship between numbers of Heterodera schachtii and sugar beet yields on a mineral soil, 1978-81

In a 4-yr field experiment on a mineral soil infested with beet cyst nematode (Heterodera schachtii) the relationship between root yield of sugar beet (y) and initial population of H. schachtii (P_i) fitted the equation: y=y min +(y max - y min) Z^Pi-T y min = 7·7 t/ha, y max = 39·4 t/ha, 2 = 0·99938 and T= 0 eggs/100 g soil. From this information and that obtained during a recent survey of 2766 beet fields, the total national root yield loss caused by H. schachtii was estimated as approximately 10 000 t/annum on mineral soils and (assuming a similar yield loss relationship on all soil types) 30 000 t/annum on organic soils. No consistent differences in efficiency of extracting cysts were found between the Fenwick Can and the flotation column. A bioassay technique was as effective as cyst extraction techniques in identifying infested soils.     

Manipulation of two α‐endo‐β‐1,4‐glucanase genes,AtCel6 and GmCel7, reduces susceptibility to Heterodera glycines in soybean roots

Plant endo‐β‐1,4‐glucanases (EGases) include cell wall‐modifying enzymes that are involved in nematode‐induced growth of syncytia (feeding structures) in nematode‐infected roots. EGases in the α‐ and β‐subfamilies contain signal peptides and are secreted, whereas those in the γ‐subfamily have a membrane‐anchoring domain and are not secreted. The Arabidopsis α‐EGase At1g48930, designated as AtCel6, is known to be down‐regulated by beet cyst nematode (Heterodera schachtii) in Arabidopsis roots, whereas another α‐EGase, AtCel2, is up‐regulated. Here, we report that the ectopic expression of AtCel6 in soybean roots reduces susceptibility to both soybean cyst nematode (SCN; Heterodera glycines) and root knot nematode (Meloidogyne incognita). Suppression of GmCel7, the soybean homologue of AtCel2, in soybean roots also reduces the susceptibility to SCN. In contrast, in studies on two γ‐EGases, both ectopic expression of AtKOR2 in soybean roots and suppression of the soybean homologue of AtKOR3 had no significant effect on SCN parasitism. Our results suggest that secreted α‐EGases are likely to be more useful than membrane‐bound γ‐EGases in the development of an SCN‐resistant soybean through gene manipulation. Furthermore, this study provides evidence that Arabidopsis shares molecular events of cyst nematode parasitism with soybean, and confirms the suitability of the Arabidopsis–H. schachtii interaction as a model for the soybean–H. glycines pathosystem.     

Arabidopsis HIPP27 is a host susceptibility gene for the beet cyst nematode Heterodera schachtii

Sedentary plant‐parasitic cyst nematodes are obligate biotrophs that infect the roots of their host plant. Their parasitism is based on the modification of root cells to form a hypermetabolic syncytium from which the nematodes draw their nutrients. The aim of this study was to identify nematode susceptibility genes in Arabidopsis thaliana and to characterize their roles in supporting the parasitism of Heterodera schachtii. By selecting genes that were most strongly upregulated in response to cyst nematode infection, we identified HIPP27 (HEAVY METAL‐ASSOCIATED ISOPRENYLATED PLANT PROTEIN 27) as a host susceptibility factor required for beet cyst nematode infection and development. Detailed expression analysis revealed that HIPP27 is a cytoplasmic protein and that HIPP27 is strongly expressed in leaves, young roots and nematode‐induced syncytia. Loss‐of‐function Arabidopsis hipp27 mutants exhibited severely reduced susceptibility to H. schachtii and abnormal starch accumulation in syncytial and peridermal plastids. Our results suggest that HIPP27 is a susceptibility gene in Arabidopsis whose loss of function reduces plant susceptibility to cyst nematode infection without increasing the susceptibility to other pathogens or negatively affecting the plant phenotype.     

Heterozygote deficits in cyst plant‐parasitic nematodes: possible causes and consequences

Deviations of genotypic frequencies from Hardy–Weinberg equilibrium (HWE) expectations could reveal important aspects of the biology of populations. Deviations from HWE due to heterozygote deficits have been recorded for three plant‐parasitic nematode species. However, it has never been determined whether the observed deficits were due (i) to the presence of null alleles, (ii) to a high level of consanguinity and/or (iii) to a Wahlund effect. The aim of the present work was, while taking into the possible confounding effect of null alleles, to disentangle consanguinity and Wahlund effect in natural populations of those three economically important cyst nematodes using microsatellite markers: Globodera pallida, G. tabacum and Heterodera schachtii, pests of potato, tobacco and sugar beet, respectively. The results show a consistent pattern of heterozygote deficiency in the three nematode species sampled at the spatial scale of the host plant. We demonstrate that the prevalence of null alleles is weak and that heterozygote deficits do not have a single origin. Our results suggested that it is restricted dispersal that leads to heterozygote deficits through both consanguinity and substructure, which effects can be linked to soil movement, cyst density, and the number of generations per year. We discuss potential implications for the durability of plant resistances that are used to protect crops against parasites in which mating between relatives occur. While consanguineous mating leads to homozygosity at all loci, including loci governing avirulence/virulence, which favours the expression of virulence when recessive, the Wahlund effect is expected to have no particular effect on the adaptation of nematodes to resistances.     

Impact of soil suppressiveness on various population densities of Heterodera schachtii

Infectivity of second‐stage juvenile (J2) populations of Heterodera schachtii was assayed with radish.The numbers of J2 in three‐day‐old seedlings were proportional to the numbers of J2 in two differently textured soils.In a microplot trial with a known H.schachtii‐supprcssivc soil, half of the plots contained untreated suppressive soil, the other half contained the same soil, but methyl iodide‐fumigated and therefore conducive.Both soils were infested with cysts introducing the equivalents of 0, 30, 60 or 120 H.schachtii eggs g‐¹ soil, kept moist for 2 months, and then planted to Swiss chard.The numbers of J2 in radish roots were proportional to the numbers of H.schachtii eggs introduced into the microplots, at a low level of detection in suppressive soil and at a high level in conducive soil.Growth of Swiss chard was not different at increasing infestation levels in suppressive soil, but growth was reduced in conducive soil proportionally to increasing nematode infestation level.     

Nematophagous Fungi Associated with Root Galls of Rice Caused by Meloidogyne graminicola and its Control by Arthrobotrys dactyloides and Dactylaria brochopaga

Root galls of rice caused by Meloidogyne graminicola were examined for natural colonization by nematophagous fungi from four fields with different nematode infestations. Old galls from severely infested fields had a higher frequency of Monacrosporium eudermatum and Stylopaga hadra than young galls. The frequency of Arthrobotrys oligospora, Arthrobotrys dactyloides, Dactylaria brochopaga and Monacrosporium gephyropagum was lower. A greater proportion (%) of root galls were colonized by nematophagous fungi in those fields in which rice roots had a greater root gall index. This indicated that disease severity supported the colonization of galls by nematophagous fungi. In vitro predacity tests of four fungi showed that A. dactyloides was most effective in capturing and killing J₂ of Mel. graminicola followed by D. brochopaga and Mon. eudermatum. Application of inocula of A. dactyloides and D. brochopaga in soil infested with Mel. graminicola, respectively, reduced the number of root galls by 86% and of females by 94%, and eggs and juveniles by 94%. The application of these fungi to soil increased plant growth: shoot length by 42.7% and 39.8%, root length by 45.5% and 48.9%, fresh weight of shoot by 59.9% and 56.7% and fresh weight of root by 20.3% and 25.1%, respectively, compared to these parameters for plants grown in nematode‐infested soil.     

Surveys of pest incidence on oil-seed rape in south central England

Surveys of pest incidence on oil-seed rape in the south of England in 6 yr have shown that Meligethes aeneus can cause a considerable loss of pods on some fields. There is some evidence that control measures are often applied too late to obtain the best results; preliminary observations on the probable effect on yield are also presented. Ceuthorhynchus assimilis was recorded on the majority of fields with an average of 19% pods infested on spring-sown rape. Very heavy pod infestations were found on farms where rape had been grown regularly for some years. Dasyneura brassicae is at present not an important pest with an average of less than 1% pods infested. However, in each year infestations of up to 11% pod attack were recorded on a few fields.     

Dactylella oviparasitica parasitism of the sugar beet cyst nematode observed in trixenic culture plates

The nematophagous fungus Dactylella oviparasitica is considered the primary cause of a sugar beet cyst nematode (Heterodera schachtii) population suppression in a field at the Agricultural Operations, University of California, Riverside. Parasitism of H. schachtii by the ascomycete D. oviparasitica was studied using both Arabidopsis thaliana (type Landsberg erecta) and cabbage as host plants in gnotobiotic agar culture. Suitability of Arabidopsis as a host for H. schachtii was confirmed using seedlings grown with the nematode in axenic sand culture. Both developing males and females of H. schachtii broke through the Arabidopsis root surface during late juvenile stages and both were susceptible to D. oviparasitica parasitism. In contrast to Arabidopsis, developing juvenile males remained in nearly all observed cases enclosed within the cabbage root tissues while the larger body expansion of the female juveniles caused the root cortex to split; consequently only the latter ones were accessible to the fungus. In the presence of D. oviparasitica, the number of females with eggs was reduced by more than 95% and the number of eggs per female by almost 60% as compared to females developing on plates without the fungus. Viable eggs were not susceptible to parasitism while more than 90% of heat- or cold-killed eggs were rendered susceptible. These observations suggest that parasitism of developing juveniles may be the essential mode of action in the population suppression of H. schachtii.     

Effects of Soil Moisture on Control of Heterodera schachtii with Aldicarb

Soil moisture and the nematode population density in aldicarb-treated soil influenced control of the sugarbeet nematode, Heterodera schachtii. Greater numbers of nematode larvae infected 14-day-old sugarbeet seedlings growing in aldicarb-treated soil at 20-30% than at 80-100% field capacity (F. C.), and plant growth was inversely related to nematode infection and the nematode population density. Compared with that of control plants, plant growth increase also was greater at 80-100% F. C. when the nematode population was above 1.8 larvae/gm soil. A nematode population of 1.8 larvae/gm soil did not significantly affect sugarbeet yields. Aldicarb gave less control when soil moisture levels dropped to 20 and 50% F. C. at nematode populations of 3.5 and 6.2 larvae/gm soil. More effective control was obtained wth soil moisture levels at or above 80% F. C. This difference was attributed to continued activity of the toxicant in the rhizosphere at the high moisture level.     

Pathological Differences in Heterodera schachtii Populations

Five populations of Heterodera schachtii Schm. from Oregon, Idaho, and Utah did not differ significantly in seedling penetration and rate of emergence and virulence. Another Utah H. schachtii population (Utah 2), however, differed from these five populations in all of the above-mentioned characteristics. More H. schachtii larvae of the Utah 2 population than the other populations penetrated sugarbeet seedlings at 10, 15, 20, and 25 C. Root and top weights of sugarbeet plants were signiticantly less when roots were parasitized by the Utah 2 population than when they were parasitized by larvae of the other nematode populations under similar experimental conditions. Also, the period of larval emergence was shorter in the Utah 2 population than in any of the other H. schachtii populations.     

Antagonistic Interaction of Heterodera schachtii Schmidt and Fusarium oxysporum (Woll.) on Sugarbeets

In a field experiment, nematicides controlled the disease of sugarbeets caused by Heterodera schachtii and Fusarium oxysporum. Biocides that were both fungicidal and nematicidal also controlled the disease, but sugar yields were no higher than those obtained with the plain nematicides. In greenhouse experiments, the interaction between H. schachtii and F. oxysporam was disadvantageous to the nematode. Damage to sugarbeets was less when the fungus and the nematode were present than when only the nematode was present. The fungus inhibited nematode invasion and development in sugarbeet seedlings, thereby decreasing the number of nematodes that matured about 3-fold.     

Differences in the Response of Certain Weed Host Populations to Heterodera schachtii

Significant differences (P = 0.05) in nematode reproduction were observed among populations of Heterodera schachtii and weed collections of black nightshade, common lambsquarters, common purslane, redroot-pigweed, shepherdspurse, and wild mustard from Colorado, Idaho, Oregon, and Utah. Colorado weeds supported the greatest nematode development (P = 0.05). Weeds collected from Idaho and Utah were similar with respect to their response to H. schachtii with the exception of shepherdspurse. At increasing soil temperatures, a Utah redroot-pigweed collection showed a higher percent susceptibility to a Utah nematode population than to nematode populations from the other states (P = 0.05). There was a higher percentage of susceptible plants when the weed host population was collected from the same geographical area as the nematode inoculun.