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 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.     

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.     

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.     

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.     

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.     

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.     

Extraction, standardization and assessment of the volatility of the sex attractants of Heterodera rostochiensis Woll. and H. schachtii Schtn.

A bioassay using five males of Heterodera spp. on an agar plate was sensitive enough to estimate the relative concentrations in washings of females of substances attractive to males. It showed that these substances include volatile components able to diffuse through air and accumulate on agar blocks.     

Interactions between Heterodera rostochiensis Woll. and Verticillium dahliae Kelb. on potatoes and the effect of CCC on both

H. rostochiensis (British pathotype A) increased the severity of the disease caused by Verticillium dahliae in potato plants grown in pots when there were more than ten eggs/g of soil and both pathogens parasitized the root system together. The growth-suppressing chemical CCC, which severely stunted plants, prevented the disease and decreased H. rostochiensis reproduction. An estimate of foliage symptoms measured the effects of the disease more precisely than did tuber yields.     

Field experiments in the control of Verticillium dahliae and Heterodera rostochiensis on potatoes

Potatoes were grown in soil infested with Verticillium dahliae and Heterodera rostochiensis and treated with methyl bromide, aldicarb or benomyl. Successive crops were grown in subsequent years on the same plots but without further treatment. Largest yields in the year of treatment were from plots fumigated with methyl bromide but in the second crop benomyl-treated plots yielded most. Neither methyl bromide nor aldicarb affected yields from trie third crop. After lifting the first crop, soil from plots treated with methyl bromide or benomyl contained less V. dahliae than that from plots receiving aldicarb or nothing. Soil was also less infective after the second crop following methyl bromide fumigation, but not the third. Plots treated with methyl bromide or aldicarb contained many fewer H. rostochiensis than untreated plots after the first crop, slightly less after the second and equal numbers after the third crop. H. rostochiensis were also fewer in benomyl-treated plots than in untreated after the first crop, but, surprisingly, were fewest after the second crop. The increased yield after applying benomyl to soil seems to depend more on its effect on H. rostochiensis than on V. dahliae. Fumigating with methyl bromide decreased common scab and black scurf on progeny tubers but increased infections by Oospora pustulans.     

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.