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.     

Hatching agents for the potato cyst-nematode, Heterodera rostochiensis Woll

Of 444 compounds tested for their ability to stimulate eggs of Heterodera rostochiensis to hatch, forty-five did so to varying extents. Many compounds that hatched H. schachtii Schm. eggs did not hatch H. rostochiensis eggs. The most effective compounds with H. rostochiensis were picrolonic acid, anhydro-tetronic acid and vanadates. Tests with analogues of picrolonic and anhydro-tetronic acids suggested some structures that may be concerned in hatching H. rostochiensis and H. schachtii. The simple analogues hatched the eggs of H. schachtii but not those of H. rostochiensis. Suitably placed polarisable atoms seemed to be an essential feature. Picrolonic acid contains pairs of polarizable atoms, about 4 and 6·7 Å apart, that may respectively be associated with the hatching of H. schachtii and H. rostochiensis. Other features of the more active hatching agents were a cis and co-planar arrangement of the two polarizable atoms with respect to the rest of the molecule. The function of hatching agents is discussed.     

Soil teachings and the hatching of larvae of the potato-root eelworm, Heterodera rostochiensis Wollenweber

Leachings were taken from fallow soil brought in from an outside plot at monthly intervals throughout a year, and tested for their ability to stimulate hatching of the potato-root eelworm. Some of the leachings showed stimulatory action, the results indicating a seasonal pattern of activity with a peak in August. As noted previously, this suggests that soil organisms produce substances able to stimulate hatching and that these are probably responsible for the so-called ‘spontaneous’ hatching.     

Influence of earthworms on larval emergence in the potato-root eelworm, Heterodera rostochiensis Wollenweber

Hatching experiments were carried out with potato-root eelworm cysts ( a ) from plant pots containing different numbers of earthworms, and ( b ) from worm casts. Cysts from these sources differed from controls in the following ways: (1) more larvae emerged, per cyst; (2) the larvae emerged sooner; (3) more cysts produced larvae; (4) the number of emerging larvae, per cyst, increased with the number of earthworms.
Examination of the contents of cysts isolated from worm casts showed that, compared with controls, 145% more eggs hatched; it also indicated some of the limitations of the 'per-cyst' basis of expressing results.     

The chemical composition of the egg shells of the potato cyst-nematode, Heterodera rostochiensis Woll

1. Eggs of the potato cyst-nematode (Heterodera rostochiensis Woll.) were isolated by sieving a suspension of crushed cysts. Eggs were broken open by ultrasonic vibration and the egg shells separated from the released larvae by centrifuging in a potassium tartrate density gradient. About 1 mg. of dried egg shells was obtained from 1000 cysts. 2. The major constituent of the egg shells was protein (59%, calculated from nitrogen content). About 80% of the egg shells went into solution on acid hydrolysis. Of the 18 amino acids determined with the Technicon Auto-Analyser, proline was most abundant and, with aspartic acid, glycine and serine, made up about 64% by weight of the total amino acids. The small amounts of aromatic and sulphur-containing amino acids, and the presence of hydroxy-proline, indicate a collagen-like protein. 3. The egg shells gave a positive van Wisselingh colour test for chitin, and glucosamine was detected in their acid hydrolysate by chromatography. The glucosamine content of the egg shells, determined by the Elson-Morgan colorimetric method, was 7%, corresponding to about 9% chitin. 4. Dried egg shells contained about 7% of lipid, 6% of carbohydrate and 3% of ash. Polyphenols (3% by weight of the egg shells) were detected in the acid hydrolysates. 5. Neither the collagen nor the chitin showed evidence of crystallinity when examined by X-ray diffraction.     

Assessment of the damage caused to potatoes by potato cyst eelworm, Heterodera rostochiensis Woll

Thirty field trials were carried out from 1960 to 1967, mainly on peat and silt soils in the eastern counties of England, to study the relationship between potato yield and numbers of H. rostochiensis. The population range of most interest to advisers (0–60 full cysts/100 g (0–150 eggs/g)) was encountered by chance fairly frequently. The relationship between yield and initial eelworm densities over this range could be expressed equally well untransformed or transformed (using square root transformation for cyst counts and log. transformation for egg counts). Sigmoid curves could not often be fitted to the graphs of yield against log. egg count because densities large or small enough were not often encountered and small numbers cannot be estimated with sufficient accuracy. Assuming straight-line regressions of yield on untransformed eelworm densities certain generalizations can be made. The slopes vary somewhat, but on average losses are 1·2 tons/acre per ten full cysts/100 g and 0·85 tons/acre per twenty eggs/g. Linear regressions tend to be parallel over a large range of potential yields which means that yield loss is independent of potential yield. In five trials there was some evidence that the yield had reached a minimum varying between 5 and 10 tons/acre when eelworm populations were very dense. In two trials, estimates of maximum and minimum yield were obtained. Both gave the difference (maximum loss) as 7 tons/acre and the ratio of minimum to maximum as 0·50 and 0·44 respectively. Yield losses were often great without obvious symptoms appearing in the haulm.     

The inhibition of hatching of potato root eelworm (Heterodera rostochiensis Woll.) in partially sterilized soil

Experiments on the hatching of Heterodera rostochiensis have shown that the addition of ammonium carbonate to potato root water markedly inhibits hatching when the concentration of ammonia introduced is approximately 100 p.p.m. The strong acid salts of ammonia in equivalent amounts have no such inhibitory effect.
These observations are linked with experiments on the effect of partial sterilization of soil on the hatching of H. rostochiensis , and it is demonstrated that delay of hatching in such soils is only effective so long as the ammonia concentration within the soil is maintained at a sufficiently high level.     

Observations on periodicity of hatching of eggs of the potato cyst nematode, Heterodera rostochiensis Woll.

Soil containing new-generation cysts of Heterodera rostochiensis was taken from the field at monthly intervals during late summer and autumn and kept in various conditions for up to a year. The number of eggs that hatched in the stored cysts was compared each month with the number that hatched in cysts taken directly from the field. Eggs did not hatch readily when stimulated during the late autumn and early winter, although more did so in cysts taken from the field before August than after. A few more eggs hatched in cysts stored in air-dried soil than in cysts stored in moist soil. Some cysts were kept at 15 or 20 °C continuously and others at 5, 15 or 30 °C for 6 weeks followed by 20 °C continuously. Storage at 30 °C caused eggs to hatch sooner, but otherwise the temperature of storage had little effect on hatch at any time of the year. Warmth also increased the hatch of H. cruciferae sooner, and some synthetic hatching agents did so with both of these species. When freed from new cysts, more eggs of H. rostochiensis hatched than in intact cysts and hatch was further increased when the fragments of tanned cyst-wall were left with the freed eggs. Puncturing the cyst-wall of new brown cysts had little effect on the hatch in potato root diffusate. Like eggs in new cysts, those in 1-year-old cysts stored out of doors ceased to hatch during the autumn and winter. The term ‘dormancy’ is inadequate to describe the inability of eggs of H. rostochiensis and other Heterodera spp. to hatch in the appropriate stimulant and the term ‘facultative diapause’, as applied to insects, better fits the phenomenon.     

Control of the potato-root eelworm, Heterodera rostochiensis Wollenweber, by allyl isothiocyanate, the mustard oil of Brassica nigra L

Previous work showed that, in the presence of root excretioci of certain crucifen, larvae of the potato-root eelworm do not emerge from the cysts when stimulated by potato-root excretions; a similar lack of response is also shown in the presence of dilute solutions of allyl isothiocyanate, the mustard oil of black mustard. The present paper describes a small-scale field trial in which this mustard oil is applied with peat to the drills at the time of planting. An increase in yield of about 100% resulted from a dressing of the oil equivalent to o.1 cwt/acre.     

Orientation of male Heterodera rostochiensis Woll. and H. schachtii Schm. to their females

A chemical attractive to their males is emitted by females of Heterodera rostochiensis and H. schachtii. The attraction is evident in agar plates when the females have been killed or removed from the agar, and on 3 mm. thick agar it takes more than 15 min. at 20° C. to diffuse sufficiently to be perceptible to males 5 mm. from the females. The perception is gustatory and seems to elicit klinokinetic aggregation behaviour and klinotaxic orientation. The males respond to a decrease in stimulation by stopping, followed by either a reversal and turn, or a swing and turn. Fatigue of the receptors, leading to loss of acuity of perception, occurs at greater concentrations of the attractant and decreases the frequency of turning. Then the intensity of turning increases and the approach to the female is less well orientated. Females of H. schachtii secreted a less labile or more concentrated attractant; thus their males were more frequently less well orientated and removal of the females caused more direct attraction. Male behaviour was efficient enough to ensure mating even of isolated females.     

The interrelationships of the causal fungus of brown root rot of tomatoes and potato root eelworm, Heterodera rostochiensis Woll.

In a study of the association between the causal fungus of brown root rot of tomatoes and Heterodera rostochiensis Woll., it was demonstrated that the nematode did not increase the susceptibility of the roots to invasion by the fungus; however, the fungus decreased the hatch of the potato root eleworm, the invasion of the host plant by the nematode, and number of new cysts subsequently produced.     

Association between the potato cyst-nematode, Heterodera rostochiensis Woll., and Verticillium dahliae Kleb. in the early-dying disease of potatoes

Investigation of a possible association between Verticillium dahliae and H. rostochiensis (pathotype E, British notation) was based on field observations and an examination of disease development in single-stemmed potato plants grown in pots. An association was found in the distribution of the nematode and the fungus in the field, and the disease was far more severe with a combined infection than with either pathogen alone. Studies on leaf-area development and yield reduction suggest there is synergism between fungus and nematode, the reductions produced by the combined infection exceeding the sum of those produced by either pathogen alone. Fungal mycelium and the extent of host colonization by V. dahliae were greatly increased by the presence of the nematode. The possible benefits to V. dahliae in the fungus-nematode complex are discussed.     

On cyst colour changes, bionomics and distribution of potato cyst-eelworm (Heterodera rostochiensis Woll.) pathotypes in the East Midlands

Pot tests and field trials in 1966 confirmed earlier observations that pathotype A potato cyst-eelworm (Heterodera rostochiensis Woll.) showed marked colour differences from pathotypes B and C during development on the roots of susceptible potatoes. In hatching tests started in July, newly formed brown cysts of pathotype A released four times as many larvae as those of pathotypes B and C, without entering an immediate diapause; the latter produced larvae after a rest period of 18–21 days. Subsequent second-generation cyst production in glass-tube culture was greater from pathotype A. Field observations suggested that all pathotypes mature at about the same time and give rise to only one flush of cysts on both early and maincrop potatoes in the growing season. Cyst colour surveys in commercial crops, and pot and field trials using resistant potato selections, confirmed that pathotypes B and C are both dominant and widespread in the East Midlands.     

The effects of soil sterilants on the cereal cyst-nematode (Heterodera avenae Woll.), take-all (Ophiobolus graminis Sacc.) and yields of spring wheat and barley

In a 3-year field experiment on sandy loam at Woburn, methyl bromide, chloropicrin, D-D mixture, dazomet, formalin and mercury salts were applied in the first year only, or in the first and second years, before drilling spring wheat. In the third year, their residual effects on spring barley were measured. All sterilants except mercury decreased cereal cyst-nematode ( Heterodera avenae ) numbers and take-all ( Ophiobolus graminis ) incidence, and increased crop yields in the year they were applied. Dazomet gave the best control of H. avenae in the first year and controlled O. graminis best in both years of application. In the third year, O. graminis increased in all plots previously treated (except after two successive D-D treatments), but H. avenae increased only after formalin. Two formalin applications more than trebled the H. avenae egg count by the end of the third year and depressed the yield of barley. Two successive applications of chloropicrin gave the best nematode control.
Other fungus diseases, Fusarium foot rot, Pythium root rot, eye-spot ( Cercosporella herpotrichoides Fron.) and sharp eye spot ( Rhizoctonia solani Kühn), were uncommon and sterilants had no detectable effects on them. The largest aggregate straw yield (3-year total) was obtained from two applications of dazomet, but the best yields after treatment in the first year only were given by D-D and methyl bromide.