MOVEMENT OF EELWORMS

The optimum crumb sizes for movement of potato-root eelworm larvae in a sandy loam, a heavy clay and a peat soil were 150–250 and 250–400 μ. Mobility was very similar in clay and sandy loam, in both of which there was an optimum suction for movement. In the peat, however, mobility increased with suction and no optimum suction was established. Larvae may be able to move in peat at high suctions because friction between the larvae and the peat crumbs is less than between clay or sand crumbs. Larvae moved to the wet end of a moisture gradient in sand, the number increasing with the steepness of the gradient. The rate of spread of larvae in sand 150–250 μ diameter varied between 2 and 3 cm. a day, depending on suction. As pore size increases, any upward movement in a moisture gradient is opposed by falling under gravity. Larvae do not respond to a moisture gradient or fall under gravity in sand where the width of the pore approximates to the diameter of the larva. The presence of host roots also counteracted the response to a moisture gradient; the degree of orientation to the roots increased with the time the roots were in the sand. Direct observation on larvae, newly emerged from cysts, in the presence of host plant roots, suggests that larvae orientate themselves at a distance from the root and do not reach the root by random movement. Many of the movements of eelworms are explicable by considering the relationship between pore size, eelworm diameter and water distribution, and a diagram relates movement and various soil factors.     

MOVEMENT OF EELWORMS

Tracks were plotted of about 300 individual eelworms comprising six species among water droplets on a glass surface. Measurements of the tracks indicated that the product of length and activity of an eelworm divided by its speed was a constant. This supports the hypothesis that the speed of an eelworm among water droplets is a function of its length and activity. This principle can only be applied to movement in soil where the length of the eelworm is less than about three times the particle diameter. Under such conditions the eelworms move in thin films or water droplets over particles. Among smaller-sized particles the speed of the eelworms is influenced by particle size. With increasing eelworm length there is an increase in soil particle size for maximum mobility.     

MOVEMENT OF EELWORMS

Experiments on vertical migration through saturated soil fractions, horizontal migration through soil fractions at different pressure deficiencies and migration in single layers of particles showed that the beet eelworm, Heterodera schachtii Schmidt, attained maximum speed when the pore diameters were between 30–60 μ. Speed of the eelworms increased as lateral displacement of the body was restricted by external resistances acting perpendicularly to the body axis; at the maximum speed there was no lateral movement, each part of the body following the part immediately in front of it. The speed of beet eelworm larvae in water films of various thickness was measured; maximum speed occurred in a film 2–5 μ thick. Four arbitrarily classified types of progression were observed in the pore spaces. It is suggested that the 'moisture characteristic' supplies most of the information required about the physical properties of the soil in relation to eelworm movement. By examining such a curve the pore size distribution can be ascertained and the probable behaviour of beet eelworm larvae in the medium predicted.     

MOVEMENT OF EELWORMS: V. OBSERVATIONS ON APHELENCHOIDES RITZEMA-BOSI (SCHWARTZ, 1912) STEINER, 1932 ON FLORISTS' CHRYSANTHEMUMS

Adults of Aphelenchoides ritzema-bosi tend to migrate up the stems of chrysanthemum plants in stationary water films possibly by a negatively geotropic response. A current of water down the stem opposes such an upward movement. Greatest mobility occurred in thick films of water in places with a high concentration of epidermal hairs as at the top of the stem and on the undersurface of leaves. Ciné films of movement in thick and thin films showed that there were fundamental differences in the type of locomotion in these two environments. Invasion of leaves via stomata was observed and the method of movement is described. The presence of A. ritzema-bosi in leaves appears to render the epidermis permeable to water. During dry weather there is little movement inside the leaf, but after rainfall activity increases as water enters the leaf. Spread of eelworm infestation in the leaf occurs in the mesophyll and across veins although initially these act as barriers. Emergence occurs via the stomata, chiefly on the undersurface. When the leaf is wet, about 50% of the eelworms emerge in the first hour. During wet weather many eelworms were recovered from the surfaces of leaves and it is suggested that eelworms spread mostly under these conditions.     

THE HATCHING RESPONSES OF SOME ROOT EELWORMS OF THE GENUS HETERODERA

An account is given of investigations on the hatching responses, under laboratory conditions, of nine species or varieties of Heterodera , namely, the beet, cabbage, clover, Galeopsis , carrot, hop, potato, oat and pea eelworms. In the first seven of these, marked differences occurred in hatching responses from cysts incubated in root leachings from various plants, these differences being virtually diagnostic for the eelworm species concerned. In general, good responses to leachings from host plants occurred, whereas there was little or no response in non-host leachings; but cases of response to non-hosts and of failure to respond to hosts were encountered. Promising results were obtained in the analysis, by bio-assay, of mixtures of some of these eelworm species. No appreciable response occurred from cysts of oat and pea eelworms incubated in host leachings, but there was some evidence that such a response did occur from pea eelworm cysts under field conditions. Curves for rate of hatching of seven species and for hatching in diluted leachings in five species were found to be of the form described by Fenwick for potato eelworm. Results are given of some experiments on the effect of age of plant on potency of the root leachings, and on the loss of potency during storage.     

Field studies of potato-root eelworm, Heterodera rostochiensis Wollenweber, 1938-40

Field studies of potato crops under different soil and fertility conditions and exposed to attack by diverse concentrations of potato-root eelworm led to the following conclusions:
In the north-western agricultural advisory province potato-root eelworm is not a major cause of failure or partial failure of potato crops on land where the viable cyst concentration is low (under 10/10 c.c. of soil). On such land increased yields can be obtained by improving conditions for plant growth, widening the rotation and increasing the fertility of the soil.
The production of satisfactory crops on land with a low eelworm population is likely to be accompanied by considerable increases in the numbers of root eelworms in the soil.
Crops free from signs of 'potato sickness' or 'potato-root eelworm disease' can be grown on land heavily infested with root eelworms (viable cyst concentration exceeding 25/10 c.c.), if no other adverse condition, such as unsuitable weather, lack of available food or incidence of fungus disease, affects the growth of the crop, but yields from heavily infested land are not likely to be more than moderate.
The treatments used against eelworms in the trials described here did not give results commensurate with their cost.
The practical difficulties of incorporating lethal substances with soil in the field are considerable, and this may have contributed to the disappointing results obtained.
The study of the influence of host variety on cyst production suggests that varietal differences in the host plants may influence the numbers of cysts formed on the crop, but the evidence is not conclusive.     

SOIL AERATION AND THE EMERGENCE OF LARVAE FROM CYSTS OF THE BEET EELWORM, HETERODERA SCHACHTII SCHM

Moisture content, pore size, depth and oxygen consumption in the soil, which influence soil aeration, have been studied in relation to larval emergence from cysts of the beet eelworm. Experiments show that the rate of larval emergence increases with aeration. In studies of larval emergence in the field emphasis should be laid on soil structure rather than on a mechanical analysis. Those factors associated with good soil tilth favour high rates of larval emergence.     

STUDIES IN THE RELATIONSHIP OF EELWORMS AND BACTERIA TO CERTAIN PLANT DISEASES

Two types of organism are known to be present in strawberry plants suffering from cauliflower disease, namely, eelworms, Aphelenchoides ritzema-bosi and/or Aphelenchoides fragariae , and a bacterium, Corynebacterium fascians. Neither type, inoculated separately, has consistently reproduced the disease.
An experimental investigation of a possible obligate etiological relationship between the eelworm and the bacterium is described, in which Aphelenchoides ritzema-bosi and several strains of Corynebacterium fascians were inoculated together and separately.
No abnormalities appeared in the absence of the eelworm. Typical cauliflower symptoms were reproduced only by a combination of the eelworm and strains of C. fascians originally isolated from cauliflower strawberries. Small alaminate leaves and enations appeared in all treatments where the eelworm was introduced, including those where no bacteria were co-inoculated. Subsequent isolations, however, demonstrated the presence of contaminant strains of C. fascians in the latter plants. The severity of the enation symptom was related to the presence of a particular strain of C. fascians , but the alaminate leaves showed no such correlation. Evidence on the source of the contaminant strains found in the experimental plants is discussed.
In parallel field investigations it was established that eelworm-infested strawberries showing no cauliflower symptom frequently bore alaminate leaves, with which C. fascians was consistently associated. It is suggested therefore that cauliflower is simply the least frequent of a range of symptoms of a bacterial disease endemic in strawberry plants infested with Aphelenchoides spp.     

DEPOSITION OF AIR-BORNE LYCOPODIUM SPORES ON CYLINDERS

For the study of deposition of air-borne particles on plant and trap surfaces a small wind tunnel has been constructed giving turbulent or streamline flow up to about 10 m./sec. The efficiency with which cylinders of from 0.018 to 2.0cm. diameter, coated with glycerine jelly, catch Lycopodium spores at wind speeds over the range 1–10 m./sec. has been measured experimentally with a Cascade Impactor, operated iso-kinetically, as standard. Efficiency has been found to increase as wind speed increases, and as cylinder diameter decreases. Similar effects have been observed in the field. Efficiencies observed are lower than predicted by Sell and Glauert, but agree well with those predicted by Langmuir and Blodgett, except with the narrowest cylinders. The standard vertical sticky traps used in routine trappings of fungus spores, pollen, and crop-protectant sprays and dusts have a low trapping efficiency.     

Resuspension studies in cylindrical microcosms: Effects of stirring velocity on the dynamics of redox sensitive elements in a coastal sediment

Effects of resuspension on the release of dissolved, redox sensitive elements (Fe, Mn) was studied in cylindrical microcosms. Effects from changing water stirring velocity in sediment pools were evaluated through measurements of pore water profiles of dissolved Mn, Fe and redox potential. Mn was a good natural marker to follow such effects. At current velocities below the threshold velocity for resuspension (37 cm s-1), Mn release rates to overlying water were 100 times higher compared to steady-state values. Pulse increases in Mn concentration were the result of convective currents inside flow chambers. These results were strongly supported by measurements of Eh profiles in the sediment pore water. Furthermore, impacts from increasing stirring velocity were found down to 1.9 cm depth below the resuspended layer of sediment.     

ON THE STEM AND BULB EELWORM (ANGUILLULINA DIPSACI KUHN) WITH SPECIAL REFERENCE TO ITS OCCURRENCE ON WEEDS OF ARABLE LAND

The importance of wild hosts of Anguillulina dipsaci Kühn as carriers of infection needs no emphasis. The occurrence of the eelworm in the weeds, cleavers ( Galium aparine ) and duckweed ( Stellaria media ) taken in crops of oats and field beans which were themselves suffering from stem eelworm disease, was recorded by Johnson (1936) and Johnson & Thompson (1937); Walton (1937) confirmed the occurrence of infested cleavers plants in eelworm-infested oats. Walton also stated that experiments indicated that seed oats failed to carry eelworm infection, and from circumstantial evidence similar to that recorded by Johnson (1936) stated that "infestations of oat crops which occur, even after the absence of the crop for more than six years, are in all probability attributable to weeds rather than to infested seed". The present writer failed to obtain living eelworms from oat seed from infested crops in Yorkshire. This paper summarizes the results of field observations and experiments accumulated mainly during the past three years.     

A versatile laboratory stream with examples of its use in the investigation of invertebrate behaviour

A versatile and simple laboratory stream was designed and used to investigate the burrowing activity of two insects in response to changes in water velocity and substrate type.Aphelocheirus aestivalis adults were unable to burrow into sand, however, a small proportion of juveniles did burrow in this substrate. The presence of sand in gravel reduced the burrowing success of adults. Steady increases in flow stimulated the burrowing response of both adults and juveniles on gravel and sandy gravel.Ephemera danica was unable to burrow in sand alone at the velocities used in the experiment. The presence of particles greater than 2 mm in diameter in the substrate appeared to be essential for successful burrowing under the test conditions. An increase in flow from 3 to 8 cm s^–1 resulted in an increase in burrowing. The time taken for each specimen to burrow varied widely within replicate tests but most specimens had penetrated the substrate within 150 seconds of introduction. The implications of these observations in influencing the microdistribution of these species are discussed.     

Field trials with D-D mixture against potato-root eelworm

A D-D mixture has been tested against Heterodera rostochiensis at seven 2-acre sites on sands, silts and blackland soils. Apart from a pilot trial where soil was injected in spring, injections were carried out in the autumn, and potatoes were grown the following year. Factors investigated were rate of application (0, 200, 400 and 800 lb. D-D/acre), depth of injection (4 or 8 in.) and the effect of rolling after injection. At the most responsive of the sites (Wainfleet), increases in yield, kill, and the post-crop eelworm population were all roughly proportional to the rate of application. Under favourable circumstances a 50% increase in yield and something like a 50% reduction in eelworm population 4 weeks after injection can be expected from 800 lb./acre, but the reduction is more than made good during the growth of the subsequent crop; accelerated multiplication of eelworm on the treated plots leads to their having a larger population than the untreated controls at lifting time. Of the sites tested, the blackland soils gave a lower eelworm kill and a much lower yield increase from D-D than silts or sands. After autumn injection the nematocidal, and probably the phytocidal, effects of D-D persist in the soil for many weeks. The hypochlorite method of 'hatching' eelworm larvae for counting has proved unreliable.     

Effect of soil permeability on virus removal through soil columns.

Laboratory experiments were performed on four different soils, using 100 cm long columns, to determine the extent of virus movement when wastewater percolated through the soils at various hydraulic flow rates. Unchlorinated secondary sewage effluent seeded with either poliovirus type 1 (strain LSc) or echovirus type 1 (isolate V239) was continuously applied to soil columns for 3 to 4 days at constant flow rates. Water samples were extracted daily from ceramic samplers at various depths of the column for the virus assay. The effectiveness of virus removal from wastewater varied greatly among the different soil types but appeared to be largely related to hydraulic flow rates. At a flow rate of 33 cm/day, Anthony sandy loam removed 99% of seeded poliovirus within the first 7 cm of the column. At flow rates of 300 cm/day and above, Rubicon sand gave the poorest removal of viruses; less than 90% of the seeded viruses were removed by passage of effluent through the entire length of the soil column. By linear regression analyses, the rate of virus removal in soil columns was found to be negatively correlated with the flow of the percolating sewage effluent. There was no significant difference in rate of removal between poliovirus and echovirus in soil columns 87 cm long. The rate of virus removal in the upper 17 cm of the soil column was found to be significantly greater than in the lower depths of the soil column. This study suggests that the flow rate of water through the soil may be the most important factor in predicting the potential of virus movement into the groundwater. Furthermore, the length of the soil column is critical in obtaining useful data to predict virus movement into groundwater.     

THE INFLUENCE OF SOIL MOISTURE ON THE EMERGENCE OF LARVAE FROM CYSTS OF THE BEET EELWORM, HETERODERA SCHACHTII SCHMIDT

The significance of soil moisture in relation to aeration and larval emergence from cysts of Heterodera schachtii Schmidt is discussed. The rate of larval emergence increased as oxygen concentration increased. A comparison of the moisture characteristics of a mass of cysts and of sand of about the same particle size showed that water was removed from between both the cysts and the sand particles at 12–16 cm. of water-pressure deficiency. There was an indication that water was removed from between the eggs within the cysts at 100–135 cm - of water-pressure deficiency. The rate of larval emergence showed a gradual decline is suction was increased beyond 20 cm. of water-pressure deficiency, approaching zero at 175 cm. of water-pressure deficiency. Results suggest that egg hatch is not directly dependent on pressure deficiency and that the presence of free larvae within the cyst inhibits further egg hatch. A technique is described for measuring larval motility in sand. There is a decline in motility when most of the water has been removed from the sand pore-spaces. It is suggested that the relatively low rates of larval emergence at high-pressure deficiencies are due to inhibition of larval migration from the cyst by the surface forces of the water film.     

CALCIUM CHLOROACETATE AS A SOIL DRESSING AGAINST BEET EELWORM: HETERODERA SCHACHTH SCHMIDT, WITH CERTAIN ADDITIONAL OBSERVATIONS

Calcium chloroacetate at 3 and 6 cwt./acre produced significant increases in the yields of sugar beet (both washed beet and total sugar) in a trial on fen soil of the 'skirt' type infested with beet eelworm, Heterodera schachtii Schmidt. However, the material had no effect on the eelworm content of the soil whether measured by cysts, viable cysts or eggs and larvae. An instance is recorded of a significant drop in the level of the beet eelworm infestation in the presence of sugar beet.     

THE STIMULATORY PROPERTIES OF SOME ORGANIC SUBSTANCES ON CYSTS OF THE BEET EELWORM, HETERODERA SCHACHTII SCHMIDT

Techniques for determining the influence of various substances on the rate of larval emergence from cysts of the beet eelworm are described. There is no significant difference between the rates of larval emergence in glutamic acid, galactinol, inositol and water; the rate of larval emergence in beet diffusate, on the other hand, was significantly higher. Studies of larval emergence in carbohydrates suggest that the cyst population used in the experiments was heterogeneous, consisting of two types of cyst which have different reactions to stimulation at different concentrations. The significance of root exudates in relation to beet eelworm is discussed.     

Hydrodynamic analysis of feeding in sand dollars

Summary Subtidal sand dollars, Dendraster excentricus, assume an inclined posture under conditions of moderate water flow (10 cm s^-1 to 2 m s^-1). In this posture, when the test is in the usual position parallel to the water flow, the test acts as a lifting body. Analysis of the hydrodynamic characteristics of the sand dollar test was accomplished by slender body theory. Streamline curvature due to lift moves food particles in the direction of the feeding surface and enhances feeding efficiency. The position parallel to water flow is most advantageous to sand dollars in reversing flows, such as wave surge, whereas sand dollars in unidirectional flows can place the test at a slight angle to the water flow to increase lift. The high camber of sand dollars from a bay habitat characterized by low water velocities was interpreted as an adaptation allowing efficient lift production in low velocity flows; sand dollars from a protected outer coast location characterized by higher water velocities were significantly less cambered.Within sand dollar beds, the animals alter their spacing as a function of the water velocity. This movement is construed as a method for maintaining the optimal gap between sand dollars. Sand dollar tests induce mutual interference effects which vary with the configuration of the animals. In the prevalent configuration, sand dollars are able to take advantage of the streamline curvature induced by neighboring animals. The optimal gap increases as the square of the water velocity. At distances grater than or less than the optimal gap, feeding efficiency was shown to decrease from that at the optimal gap.     

THE EMERGENCE OF LARVAE FROM CYSTS OF THE BEET EELWORM, HETERODERA SCHACHTII SCHMIDT, IN AQUEOUS SOLUTIONS OF ORGANIC AND INORGANIC SUBSTANCES

The rate of larval emergence from cysts of the beet eelworm in a variety of aqueous solutions containing organic and inorganic substances is significantly higher than the emergence rate in water. It is suggested that differences between larval emergence rates in monoamino-monocarboxylic amino-acids may be related to the lipid solubility of these substances and their ability to penetrate the egg membranes. The larval emergence rate in fructose, glucose, sucrose and maltose was significantly higher than that in water, but in raffinose, arabinose and xylose the rate of emergence was no higher than in water. A high rate of larval emergence occurred in sodium chloride, potassium chloride and mercuric chloride, but not in magnesium chloride or calcium chloride. Experiments with several other organic solutions are described. There is an optimum concentration for larval emergence in beet diffusate. The osmotic pressure of the diffusate when maximum emergence occurred was 0·48 atm. Measurements of shrinkage of unhatched larvae in various concentrations of urea, sodium chloride and sucrose showed that decreasing rates of emergence at higher concentrations may be due to changes in the unhatched larvae brought about: by an osmotic effect. High concentrations of beet diffusate may have a similar effect.     

Shear degradation of DNA.

A concentric-cylinder flow-birefringence instrument is used to generate sufficient shear fields to break T2 DNA (M = 1.2 X 10(8)) and E. coli DNA (M = 2.5 X 10(9)) in dilute solution. Breakage is monitored in situ by measuring the change in birefringence relaxation after the flow has been stopped. The breakage of T2 DNA follows first-order kinetics. Rate constants are obtained as functions of shear rate and viscosity (varied by adding glycerol). The data are fitted by a modified Arrhenius equation, assuming that stess increases the rate by lowering the activation energy. The rate increases with temperature, pH, and water concentration, and appears to be a base-catalyzed hydrolysis of the phosphate-ester linkage. La3+ ions catalyze the reaction. E. coli DNA was reduced to half molecules at a shear stress of 0.4 dynes/cm2, which is about 2500 times less than that required for T2. The difference in rates is accounted for in part by the difference in size of the two, but may also reflect the presence of many single-strand nicks in the coli DNA.