Relationship Between Heterodera schachtii,Meloidogyne hapla,and Nacobbus aberrans on Sugarbeet

Heterodera schachtii, Meloidogyne hapla, and Nacobbus aberrans either alone, or in various combinations with each other, can, when inoculated at a concentration of 12 second-stage juveniles/ cm³ of soil, cause a significant (P = 0.01) suppression of growth of sugarbeet (cv. Tasco AH14) seedlings. M. hapla and H. schachtii decreased growth of sugarbeet more than N. aberrans over a 60-day period. The adverse effect of N. aberrans on the final population/initial population (Pf/Pi) ratio for either M. hapla or H. schachtii was dependent on time, and was more accentuated on that of M. hapla than on that of H. schachtii. Neither M. hapla nor H. schachtii had an adverse effect on the Pf/ Pi ratio of N. aberrans. N. aberrans is considered to be less aggressive on sugarbeet than either H. schachtii or M. hapla. Sections of sugarbeet roots infected simultaneously with H. schachtii and N. aberrans showed scattered vascular elements between the N. aberrans syncytium located in the central part of the root and that of H. schachtii in the peripheral position.     

The Relationship of Heterodera schachtii Population Densities to Sugarbeet Yields

Sugarbeet yields were contpared with field populations of Heterodera schachtii Schmidt. The correlation between sugarbeet yields and viable larvae/g of soil was negative and high, but that between sugarbeet yields and viable cysts/g of soil was lower. Sugarbeet yields were also compared with H. schachtii populations by years of rotation with a nonhost crop. The coefficients of correlation (r) between yield and viable larvae/g of soil were negative and high: 0 yr of rotation, -0.935; 1 yr, -0.922; 2 yr, -0.954; 3 yr, -0.935; and combined years, -0.965, with 95% confidence limits of -0.91 to -0.98 for combined years. The comparable correlation coefficients between yield and "viable cysts"/g of soil were negative and lower: 0 yr of rotation, -0.151; 1 yr, -0.022; 2 yr, -0.490; 3 yr, -0.456; and combined years, -0.586, with 95% confidence limits of -0.22 to -0.80 for the combined years.     

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.     

Assessment of Parasitic Activity of Fusarium Strains Obtained from a Heterodera schachtii-Suppressive Soil

This study assessed the potential impact of various Fusarium strains on the population development of sugarbeet cyst nematodes. Fungi were isolated from cysts or eggs of Heterodera schachtii Schmidt that were obtained from a field suppressive to that nematode. Twenty-six strains of Fusarium spp. were subjected to a phylogenic analysis of their rRNA-ITS nucleotide sequences. Seven genetically distinct Fusarium strains were evaluated for their ability to influence population development of H. schachtii and crop performance in greenhouse trials. Swiss chard (Beta vulgaris) seedlings were transplanted into fumigated field soil amended with a single fungal strain at 1,000 propagules/g soil. One week later, the soil was infested with 250 H. schachtii J2/100 cm³ soil. Parasitized eggs were present in all seven Fusarium treatments at 1,180 degree-days after fungal infestation. The percentage of parasitism ranged from 17 to 34%. Although the most efficacious F. oxysporum strain 471 produced as many parasitized eggs as occurred in the original suppressive soil, none of the Fusarium strains reduced the population density of H. schachtii compared to the conducive check. This supports prior results that Fusarium spp. were not the primary cause of the population suppression of sugarbeet cyst nematodes at this location.     

Pathological Interaction of a Combination of Heterodera schachtii and Meloidogyne hapla on Tomato

Increased culturing of a tomato population of Heterodera schachtii (UT1C) on tomato for 480 days (eight inoculation periods of 60 days each) significantly increased virulence to ''Stone Improved'' tomato. A synergistic relationship existed between Meloidogyne hapla and H. schaehtii on tomato. A combination of H. schachtii (UTIC) and M. hapla significantly reduced tomato root weights by 65, 64, and 61% below root weights of untreated controls, and single inoculations of M. hapla and H. schachtii, respectively. This corresponded to root reductions of 42, 44, and 46% from a combination of H. schachtii (UT1B) and M. hapla. Antagonism existed between H. schachtii and M. hapla with regard to infection courts and feeding sites. The root-knot galling index dropped from 6.0 with a single inoculation of M. hapla to 4.3 and 3.3 with combined inoculations of M. hapla plus UT1B and M. hapla plus UTIC cyst nematode populations. The pathological virulence of H. schachtii to sugarbeet was not lost by extended culturing on tomato; there were no differences in penetration, maturation, and reproduction between sugarbeet populations continually cultured on sugarbeet and the population continually cultured on tomato.     

Resistance of Trisomic and Diploid Hybrids of Beta vulgaris and B. procumbens to the Sugarbeet Nematode,Heterodera schachtiis Arnold

Trisomic and diploid hybrids of sugarbeet (Beta vulgaris L.) X wild beet (B. procumbens Chr. Sin.) inherited the gene for resistance to Heterodera schachtii Schm. from B. procumbens. The hybrids showed partial resistance to H. schachtii, manifested in failure of larvae to reach maturity. Although significantly greater numbers of female nematodes developed on plants inoculated with populations from the Netherlands or Italy than on plants inoculated with a population from the Salinas Valley, California. the totals for all populations on resistant plants were small. Greater numbers of males than females developed on root-slice cultures of resistant hyhrids when compared to a susceptible cultivar.     

Suppression of Heterodera schachtii Populations by Dactylella oviparasitica in Four Soils

The effects of Dactylella oviparasitica strain 50 applications on sugarbeet cyst nematode (Heterodera schachtii) population densities and plant weights were assessed in four agricultural soils. The fungus was added to methyl iodide-fumigated and nonfumigated portions of each soil. The soils were seeded with Swiss chard. Four weeks later, soils were infested with H. schachtii second-stage juveniles (J2). Approximately 1,487 degree-days after infestation, H. schachtii cyst, egg and J2 numbers and plant weights were assessed. In all four fumigated soils, D. oviparasitica reduced all H. schachtii population densities and increased most of the plant weights compared to the nonamended control soils. In two of the nonfumigated soils (10 and SC), D. oviparasitica reduced H. schachtii population densities and increased most plant weight values compared to the nonamended control soils. For the other two nonfumigated soils (44 and 48), which exhibited pre-existing levels of H. schachtii suppressiveness, fungal applications had relatively little impact on H. schachtii population densities and plant weights. The results from this study combined with those from previous investigations suggest that D. oviparasitica strain 50 could be an effective biological control agent.     

Population Dynamics of Dactylella oviparasitica and Heterodera schachtii: Toward a Decision Model for Sugar Beet Planting

A series of experiments were performed to examine the population dynamics of the sugarbeet cyst nematode, Heterodera schachtii, and the nematophagus fungus Dactylella oviparasitica. After two nematode generations, the population densities of H. schachtii were measured in relation to various initial infestation densities of both D. oviparasitica and H. schachtii. In general, higher initial population densities of D. oviparasitica were associated with lower final population densities of H. schachtii. Regression models showed that the initial densities of D. oviparasitica were only significant when predicting the final densities of H. schachtii J2 and eggs as well as fungal egg parasitism, while the initial densities of J2 were significant for all final H. schachtii population density measurements. We also showed that the densities of H. schachtii-associated D. oviparasitica fluctuate greatly, with rRNA gene numbers going from zero in most field-soil-collected cysts to an average of 4.24 x 10⁸ in mature females isolated directly from root surfaces. Finally, phylogenetic analysis of rRNA genes suggested that D. oviparasitica belongs to a clade of nematophagous fungi that includes Arkansas Fungus strain L (ARF-L) and that these fungi are widely distributed. We anticipate that these findings will provide foundational data facilitating the development of more effective decision models for sugar beet planting.     

The Relationship of Plant Age,Soil Temperature,and Population Density of Heterodera schachtii on the Growth of Sugarbeet

There were direct relationships between inoculum density of Heterodera schachtii Schm. (nematode population density), initial soil temperature, the growth of sugarbeets in the greenhouse under controlled temperatures, and nematode populations. Heterodera schachtii was least pathogenic on plants inoculated at 6 wk of age and most pathogenic on plants grown from inoculated germinated seed (0 wk of age). In the field, H. schachtii was least pathogenic on sugarbeets grown at an initial soil temperature of 6 C and most pathogenic on those grown at an initial soil temperature of 24 C. The growth period for sugarbeets at the different soil temperatures was determined by heat units; since penetration of sugarbeet roots by H. schachtii larvae is accelerated at soil temperatures above 10 C, each hour-degree ahove 10 C was counted as one effective heat unit (HU). Using this guideline it was determined that root weight depressions in the greenhouse, for each degree-unit population (HU-UP) where unit population = one larvae/g soil, were 0.052, 0.09, 0.12, and 0.17 mg at initial soil temperatures of 6, 12, 18, and 24 C, respectively. Root weight depressions were 0.28, 0.23, 0.15, and 0.086 mg when plants were inoculated at 0, 2, 4, and 6 wk of age.     

In-Vitro and In-Vivo Effects of Aldicarb on Survival and Development of Heterodera schachtii

Aqueous solutions of 5-500 μg/ml aldicarb inhibited hatching of Heterodera schachtii. Addition of hatching agents, zinc chloride, or sugarbeet root diffusate, to the aldicarb solutions did not decrease the inhibition of hatching. When cysts were removed from the aldicarb solufions and then treated for 4 wk in sugarbeet root diffusate, larvae hatched and emerged. Treatments of newly hatched larvae of H. schachtii with 5-100 μg/ml aldicarb depressed later development of larvae on sugarbeet (Beta vulgaris). Similar treatments with aldicarb sulfoxide had less effect on larval development, and aldicarb sulfone had no effect. Numbers of treated larvae that survived and developed were inversely proportional to concentration (0.1-5.0 μg/ml) and duration (0-14 days) of aldicarb treatments. Development of H. schachtii on sugarbeet grown in aldicarb-treated soil was inversely proportional to the concentration of aldicarb in the tested range of 0.75 - 3.0 μg aldicarb/g of soil. Transfer of nematode-infected plants to soil with aldicarb retarded nematode development, whereas transfer of plants first grownin treated soil to nematode-infested soil only slightly suppressed nematode development. Development of H. schachtii was inhibited in slices of storage roots of table beet (B. vulgaris), sugarbeet and turnip, (Brassica rapa), that had grown in soil treated with aldicarb.     

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.     

Host Suitability of Rapeseed for Heterodera schachtii

Because rapeseed, especially canola, has the potential to be grown in rotation with sugarbeet in the north-central region of the United States, this study was initiated to assess its susceptibility to infection by Heterodera schachtii and to develop a screening method for Brassica germplasm. Existing methodology was adapted for growing Brassica juncea, B. napus, B. rapa, Brassica hybrids, and sugarbeet, Beta vulgaris, in H. schachtii-infested soil to count the females that developed on the roots. Cysts on sugarbeet contained a mean of 130 eggs compared with 240 for B. napus, lowest for the Brassica. Viability of eggs produced was assessed in soil planted with Brassica and sugarbeet and infested with with 0, 100, 1,000, 3,000, and 5,000 eggs to count resulting females and cysts. Number of females (y) was related linearly to infestation rate (x) by the regression equations y = 2.82 + 0.07(x) for the Brassica lines (R² = 0.79; P < 0.001) and y = 0.43 + 0.04(x) for sugarbeet (R² = 0.69; P < 0.007). These data indicated the potential for H. schachtii population increase if the two crops are used in rotation. All of the 111 germplasm lines tested were susceptible. The methodology developed during this research would benefit attempts to develop rapeseed cultivars resistant to H. schachtii.     

Comparative Morphometrics of Eggs and Second-Stage Juveniles of Heterodera schachtii and a Race of H. trifolii Parasitic on Sugarbeet in The Netherlands

Measurements of second-stage juveniles of Heterodera schachtii from California and The Netherlands and a race of H. trifolii from The Netherlands were obtained and compared to determine if these populations can be differentiated by morphometrics. Juvenile lengths of 10 specimens from each of 10 cysts of each population were measured. Dimensions of tail regions of 20 juveniles from individual cysts of H. schachtii (California) and a like number of juveniles of H. trifolii (The Netherlands) were also obtained. The mean lengths of juveniles of H. schachtii from California and The Netherlands were not significantly different, but similar measurements of H. schachtii and H. trifolii were different (P = 0.05). Mean dimensions of tail lengths, tail widths, tail hyaline lengths, and tail length/tail width were significantly greater for H. trifolii than for H. schachtii. Also, dimensions of eggs of H. trifolii were significantly greater than dimensions of H. schachtii eggs. The investigations established that H. schachtii can be readily differentiated from H. trifolii by morphometrics of eggs and juveniles, Minimum sample sizes required for specified confidence intervals for each criterion measured are provided.     

Factors Affecting the Biology and Pathogenicity of Heterodera schachtii on Sugarbeet

A direct relationship exists between soil temperature and Heterodera schachtii development. The average developmental period of two nematode populations from Lewiston, Utah, and Rupert, Idaho, from J2 to J3, J4, adult, and the next generation J2 at soil temperatures of 18-28 C were 100, 140,225, and 399 degree-days (base 8 C), respectively. There was a positive relationship (P < 0.05) between nematode Pi, nematode generations, and sugarbeet yields. The greatest sugarbeet growth inhibition (87%) occurred when sugarbeets were exposed to a Pi of 12 eggs/cm³ soil for five generations (1,995 degree-days), compared with a 47% inhibition when plants were exposed to the same Pi for two generations. There was a negative correlation (P < 0.05) between the Pi, Pf, and sugarbeet yield for each population threshold. The smaller the Pi, the greater the sugarbeet yields and the greater the Pf. Root yields were 80 and 29 t /ha and Pf were 8.4 and 3.6 eggs/cm³ soil when sugarbeet seeds were planted at Pi of 0.4 and 7.9 eggs/cm³. respectively, at a soil temperature of 8 C. The number of years rotation with a nonhost crop required to reduce the nematode population density below a damage threshold level of 2 eggs/cm³ depends on the Pi. A Pi of 33.8 eggs/cm³ soil required a 5-year crop rotation, whereas a Pi of 8.4 eggs/cm³ soil required a 2-year crop rotation.     

A Simulation Model of Heterodera schachtii Infecting Beta vulgaris

A simulation model of a single sugarbeet, Beta vulgaris L., plant infected by the sugarbeet cyst nematode, Heterodera schachtii Schmidt, was developed using published information. The model is an interactive computer simulation programmed in FORTRAN. Given initial population densities of the nematode at planting, the model simulates nematode population dynamics and the growth of plant tap and fibrous roots. The driving variable for nematode development and plant growth is temperature.     

Effects of Oxime Carbamate Nematicides on Development of Heterodera schachtii on Sugarbeet

Treatment of sugarbeet, Beta vulgaris L., with aldicarb, aldicarb sulfoxide, or aldicarb sulfone 10 days after plants were inoculated with Heterodera schachtii prevented development of the nematode, but second-stage larvae penetrated the roots. These chemicals had no measurable effects on nematodes in plants treated 15 days after inoculation. The tests established that soil treatments of aldicarb are directly or indirectly lethal to larvae developing within roots of sugarbeet. Heterodera schachtii failed to develop on root slices of red table beet grown in soil treated with aldicarb or aldicarb sulfoxide. Similar treatment of plants with aldicarb sulfone or oxamyl did not affect subsequent development of H. schachtii on root slices of treated plants.     

Sequential Decision Rules for Managing Nematodes with Crop Rotations

A dynamic model of nematode populations under a crop rotation that includes both host and nonhost crops is developed and used to conceptualize the problem of economic control. The steady state of the dynamic system is used to devise an approximately optimal decision policy, which is then applied to cyst nematode (Heterodera schachtii) control in a rotation of sugarbeet with nonhost crops. Long-run economic returns from this approximately optimal decision rule are compared with results from solution of the exact dynamic optimization model. The simple decision rule based on the steady state provides long-run average returns that are similar to the fully optimal solution. For sugarbeet and H. schachtii, the simplified rule can be calculated by maximizing a relatively simple algebraic expression with respect to the number of years in the sequence of nonhost crops. Maximization is easy because only integers are of interest and the number of years in nonhost crops is typically small. Solution of this problem indirectly yields an approximation to the optimal dynamic economic threshold density of nematodes in the soil. The decision rule requires knowledge of annual nematode population change under host and nonhost crops, and the relationship between crop yield and nematode population density.     

A Comparison of the Hatching of Juveniles from Cysts of Heterodera schachtii and H. trifolii

The effects of root diffusates of selected plants within the families Chenopodiaceae and Cruciferae and the hatching agent zinc chloride were tested for their effects on hatching and emergence of juveniles from cysts of Heterodera schachtii and a race of H. trifolii parasitic on Chenopodaceae and Cruciferae in The Netherlands. Although all diffusates strongly stimulated hatching of juveniles of H. schachtii, their effects on H. trifolii were less evident.     

The Effects of Hot Water Treatments on Survival of Heterodera schachtii

Cysts of Heterodera schachtii were treated in a water bath at constant temperatures ranging from 45 - 62.5 C for 1 sec to 28 hr. Treated and untreated cysts were incubated 8 weeks in sugarbeet root diffusate at 24 C to measure emergence of surviving larvae. Within the temperature range of 49 - 54 C, the minimum lethal temperature was proportional to the log time of treatment. No larvae emerged from cysts exposed 10 sec at 60 C. Although treatment of cysts for 8 hr at 45 C significantly reduced emergence, increasing the treatment period to 28 hr did not completely suppress emergence.     

Population Dynamics of Heterodera schachtii on Tomato and Sugar Beet

Experiments showed that development of male and female Heterodera schachtii on tomato and sugarbeet are disproportionately influenced by the nematode inoculum level and root size, which together determine the density of invading larvae. Slight overcrowding favored development of males over females, whereas severe overcrowding equally affected development of males and females. Differential population changes of host-selected races on tested cultivars was attributable to selective development of male and female nematodes.