Observations on the biology of the potato-root eelworm, Heterodera schachtii Schmidt

Results are presented of experiments on the effect of low temperature and of soil type upon the potato-root eelworm, Heterodera schachtii.
In some experiments exposure of cysts, to low temperatures had a definite lethal effect, but in others the cysts were unharmed. The reason for this variation was not apparent. Exposure of the larvae of die nematode to even a relatively slight degree of frost resulted in the death of the larvae, provided mat die surrounding medium was completely frozen. In this respect the larvae appear to differ markedly from those of the beet strain of H. schachtii. That the larvae had actually been killed was shown by the fact that they produced no cysts on the roots of potato plants and also by their almost immediate absorption of iodine.
It was confirmed that H. schachtii cysts produced on the roots of potato plants grown in a heavy medium are smaller in size and fewer in number than those formed on the roots of plants grown in a light medium. The heavy medium used was pure clay, the light one pure sand, while equal numbers of eelworm larvae served in each case as inoculum. The difference in cyst size was significant, and fewer larvae emerged from the cysts formed in clay than from those produced in sand.     

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.     

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.     

The influence of crucifers and mustard oil on the emergence of larvae of the potato-root eelworm, Heterodera rostochtensis Wollenweber

Hatching experiments were carried out with cysts of the potato-root eelworm. The effect of solutions of potato-root excretion mixed with root excretions of six types of crucifer seedlings was studied. With cress, white mustard, and black mustard, the emergence of larvae in the mixed excretions was very much less than that from control cysts in potato-root excretion alone. On subsequent return to potato-root excretion alone, larval emergence was found to be unimpaired in the case of white and black mustards; in the case of cress, however, the total emergence differed significantly from control. Similar effects, of both types, were obtained with certain dilutions of ally isothiocyanate, the mustard oil of black mustard seed, in potato-root excretion.     

The effects on a population of potato-root eelworm (Heterodera rostochiensis) of growing potatoes resistant to pathotype B

The effects on a potato-root eelworm (Heterodera rostochiensis Woll.) population of growing potatoes resistant to pathotype B of the eelworm were investigated. The eelworm population, which was originally nearly entirely pathotype A, had been changed into a mixture of pathotypes A, B and C before 1961, when the experiments described in this paper began, by growing potatoes resistant only to pathotype A. The resistant potatoes grown in the years 1961 and 1962 were certain clones of the hybrid triploid species Solarium xjuzepczukii Buk., clones bred from S. multidissectum Hawkes with the gene H₂ and clones bred from S. vernei Bitt. et Wittm. In the years 1963-65 the clone used was D 40 which has both the gene H₂ from Andigena and the gene H₂ from 5. multidissectum and is resistant to both pathotypes A and B of the eelworm. The only potatoes which caused a reduction in the egg population were those bred from S. vernei. The clone D 40, although it had little effect on the egg population, appeared to have more resistance than could be accounted for from possessing genes H₁ and H₂.     

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 cereals root eelworm,Heterodera major (O. Schmidt) Franklin,in North Wales

The cereals root eelworm Heterodera major (O. Schmidt) is shown to be locally distributed in North Wales, the most heavily infested centres being in the arable districts of the eastern counties of the province. The most severe attacks occurred on oats where the crop had been grown too frequently; wheat and barley were apparently unaffected at the cyst populations encountered.     

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.     

Stimulation of larval emergence in Heterodera schachtii Schmidt, by certain concentrations of silver compounds

Certain dilute solutions of organic silver compounds, e.g. 0.001% silver proteinate, acting for a short time on cysts of the potato strain of Heterodera schachtii , cause increased numbers of larvae to emerge from the cysts when the latter are transferred to potato root excretion. More prolonged exposure of cysts to these solutions tends to result in a toxic effect which is also produced by more concentrated solutions. It is concluded that many more eggs in H. schachtii cysts are mature and capable of hatching than can be accounted for by ordinary larval liberation brought about by means of root excretion alone. Increasing periods of pre-treatment of cysts in tap water tend to produce increases in larval emergence, although this stimulation is probably of a different type from that given by silver compounds.