Heterodera Species Research Articles

Serological and morphological studies on the inter- and intraspecific differences of the plant-parasitic nematodes Heterodera and Ditylenchus

Saline extracts of species of Ditylenchus and Heterodera initiated antibody production when inoculated separately into rabbits. On agar gel-diffusion plates these antisera gave a precipitate response against their homologous extracts and against extracts of some of the other nematode species. Up to four precipitation bands occurred, indicating the presence of at least four distinct antigens. Antisera to these extracts did not precipitate against extracts of the host plant or against the extracts of nematodes from other genera. None of the nematode extracts precipitated against normal serum. The precipitation response divided the Heterodera species tested into two groups, one containing H. schachtii, H. trifolii and H. rostochiensis and the other containing H. cruciferae, H. carotae and H. goettingiana with apparently no antigens common to both groups. The three species of Ditylenchus were serologically distinct, and it was not possible to interbreed D. myceliophagus and D. destructor. The extent of the postvulval sac was the most useful morphological character for separating D. myceliophagus from D. destructor. Only slight intraspecific serological differences were detected between five races (narcissus, oat, red clover, tulip and giant) of D. dipsaci and between the two pathotype populations (Colyton and Woburn) of H. rostochiensis. Precipitates occurred at the excretory pore of living larvae and adults of Ditylenchus species and at the vulval aperture of white females of Heterodera species placed in their homologous antisera.We thank Dr A. J. Gibbs for much advice.

Cytological Investigations in the Genus Heterodera

Meiosis and the early cleavage divisions of the fertilized egg were studied in several species of Heterodera. In H. scheichtii, H. avenae, H. cruciferae and H. carotae, two divisions were seen in meiosis and reproduction in these species is probably amphimictic. The diploid chromosome number of H. schachtii, H. avenae and H. cruciferae was found to be 18 but there may be some variation in the chromosome number of this last species. Some observations were made on H. goettingiana.

Methods for the Extraction of Heterodera Cysts From Not Previously Dried Soil Samples

An elutriator for the separation of cysts from not previously dried soil samples to determine the population density of Heterodera species is described. With this apparatus a mixture of cysts and organic debris is obtained. For further separation this mixture is dried and the cysts are then separated from the debris by flotation with a modification of Buhr's method using water or a mixture of three parts of acetone and one part of carbon tetrachloride.

The hatch of beet eelworm (Heterodera schachtii Schm.) larvae in rivanol, and its use for the estimation of field populations

SUMMARYThe hatch of beet eelworm (Heterodera schachtii Schm.) larvae from cysts in 0–1% aqueous solutions of rivanol (3,9‐diamino‐7‐ethoxyacridine lactate) was greatest and fastest in 1 % rivanol, which also inhibited the growth of micro‐organisms. The hatching effect of rivanol was specific to beet eelworm out of five Heterodera species tested. Larval hatch from cysts on nylon sieves at the surface of 1% rivanol was not lessened by a thin layer of debris, nor was hatch affected by soil storage and drying times except with new cysts.Measuring the density of beet eelworm infestations in fields by larval hatch in rivanol would avoid errors from inclusion of related species, observer bias, and difficulties of determining viability. The number of cysts used could easily be increased to help reduce sampling errors.

The decline of a pea root eelworm (Heterodera göttingiana Liebscher) population in the absence of host plants

1. The decline of a pea root eelworm (Heterodera göttingiana Liebscher) population, of which most eggs were formed in 1955, was measured annually from 1955 to 1958 in twenty-two microplots containing loamy sand, and measurements were continued in one of these microplots from 1960 to 1962.2. Sampling errors increased when egg populations were more than 4 years old.3. Two crop rotations (fallow, and maize, fallow, carrots) had no effect on decline rate from 1955 to 1958, nor did initial eelworm density.4. After 3 years the egg population was still 48% of that in 1955. Subsequently the population declined on average by more than 50% per annum. It is suggested that host crops should not be grown more than once every five years.5. Cysts declined more slowly, with a mean annual rate of 26%.6. A sharp decline in eggs during the first year followed by at least one year's relative quiescence may be general in Heterodera species; with H. göttingiana unexpected crop failures may be associated in part with the prolongation of this relative quiescence for more than 1 year.

Effect of Temperature on Hatching in the Cereal Root Eelworm

UNLIKE most other species of Heterodera, the larvae of the cereal root eelworm (Heterodera avenae Wollenweber, 1924) do not hatch in response to diffusates of the host plant root1. The mechanism controlling hatching in this species has never been demonstrated, although it has been known for many years that in the field larvae invade the roots of the host plant in the greatest numbers in spring and early summer2. Experiments carried out at the Welsh Plant Breeding Station during the summer and winter of 1961/62 indicate that, in the laboratory, hatching of the cereal root eelworm occurs in response to a rise in temperature after a period of low temperature.

Assessing the Damage caused by Heterodera rostochiensis Woll.

PREVIOUSLY there have been no published accounts of the effect of any species of Heterodera on the yield of the host crop under field conditions. Jones1 attempted to obtain conditions approaching those found in the field by the use of small, artificially constructed plots or microplots. However, these were too small to give a realistic estimate of yield per acre. For insect pests differential population density-levels have been established by the use of insecticides. This is impractical, for eelworms as nematicides, in addition to affecting the eelworm population, also often exert an effect on plant growth. I have therefore investigated the effect on the yield of potatoes of differences in initial density of eelworm population. This method, possibly with some modification, should prove suitable for other Heterodera species or for any species of nematode for which reliable population estimates can be made.

The Invasion and Development of Some Species of Heterodera in Plants of Different Host Status

Das Wirts-Parasit-Verhaltnis zwischen Heterodera schachtii und 13 Pflanzenarten aus den Familien der Chenopodiaceen und Cruciferen wurde untersucht. Die Pflanzen wurden mit sterilisiertem Kompost in 6 cm Topfen angezogen und jede mit 2000 Rubennematodenlarven beimpft. Eine Woche spater wurden die Wurzeln vorsichtig von Boden gesaubert und die Pflanzen mit unverseuchtem Kompost in 10 cm Topfe uberfuhrt. In wochentlichen Abstanden wurden 4 Pflanzen jeder Art 5 Wochen lang vom Tage der Beimpfung an gerechnet aufgenommen, die Wurzeln fixiert und in Formol-Eisessig-Alkohol aufbewahrt. Nach Farben mit Saurefuchsin und nach Mazerierung in einem Mixer erfolgte ihre Untersuchung auf Nematoden. Larven III wurden in allen Pflanzenarten gefunden, wenn auch ofters nur in kleiner Zahl; Larven IV traten in Beta webbiana, B. patellaris, B. procumbens, Hesperis und Matthiola auf. Mannchen fanden sich nicht in B. procumbens, Hesperis oder Matthiola und nur einmal in B. webbiana. Weibchen wurden in Wurzeln von B. procumbens, B. patellaris, Hespiris und Matthiola nicht gefunden; sehr wenige Weibchen traten in Radies, Chenopodium album und Brassica sinapis und nur einmal in Beta webbiana auf. B. trigyna ist eine bessere Wirtspflanze als Zuckerrube; anfallige Chenopodiaceen-Arten sind besser als anfallige Cruciferenarten. Die Zahl der Larven, die anscheinend schon tot waren, bevor die Wurzeln fixiert wurden, war hoch, selbst in anfalligen Arten; die meisten Toten scheinen im zweiten und dritten Larvenstadium aufzutreten. Wenn geschlupfte Larven von H. schachtii, H. rostochiensis und H. gottingiana Topfen zugefugt wurden, in denen Pflanzen stehen, die keinen Schlupfreiz produzieren, werden alle Arten befallen, wenn auch zuweilen nur von sehr wenigen Larven. Die Anwesenheit von Wirtspflanzenwurzeln wirkt nicht auf die Zahl der Larven von H. schachtii, die Erbsenwurzeln befallen, welche selbst keinen Schlupfstoff fur diesen Nematoden bilden. Es werden die Faktoren diskutiert, die auf das Verhaltnis Wirt-Parasit einwirken. Eine Veranderung der Jones'schen Klassifizierung von Pflanzen in Kategorien ihres Wirtspflanzengrades wird angegeben. Die Beziehung wird am besten als Frequenz ausgedruckt, z.B., wenn eine Impfung mit Q Larven erfolgt und P Larven die Reife erzielen und sich vermehren an der Sorte X der Pflanze Y, dann ist P/Q die Frequenz. Sie kann o sein, wenn Y eine Randwirtspflanze und Q klein ist; sie kann aber einen bestimmten Wert haben, wenn Q gross ist.

Chromsoome number in the sugar-beet nematode Heterodera schachtil Schmidt.

MULVEY1 made a preliminary study of oogenesis in a species of Heterodera (probably H. schachtii var. trifolii Goffart, 1932) from the roots of hairy vetch (Vicia villosa Roth), this being the first record of oogenesis in a cyst-forming nematode. This is a report on the chromosome number and arrangement during meiosis of the sugar-beet nematode, H. schachtii Schmidt, 1871.

Chromosomes of the Potato Root Eelworm

THE systematics of the cyst-forming eelworms of the genus Heterodera are difficult, and in several cases distinctions between species depend largely on the host-range of the eelworms. In the taxonomy of the higher plants, chromosome numbers have often been useful in determining the limits of species and the relations between species in difficult groups. It was therefore thought that an investigation of the chromosomes of Heterodera species might be valuable. Details are given below of a technique that can be used for determining the chromosome number of the potato root eelworm, Heterodera rostochiensis Woll.

Seasonal Variations in the Hatching Responses of the Potato-root Eelworm, Heterodera rostochiensis Wollenweber, and Related Species

There is a winter dormancy in the hatchability of some species of Heterodera. The dormancy varies in degrees with eelworm species, being slight in the case of beet eelworm and more pronounced in the case of potato-root eelworm and some other species. It is not wholly dependent on immediate environmental conditions but possibly is induced by previous conditions. The dormancy can be shortened by storing infested soil at certain temperatures, but the cessation of dormancy in spring is not necessarily associated with rise in soil temperature.

Further Studies on the Identification of Heterodera Species by Larval Length. Estimation of the Length Parameters for Eight Species and Varieties.

In their preliminary paper (Fenwick and Franklin, 1942) on the identification of the cyst-forming species of the nematode Heterodera by means of the length of the larvae, the present authors studied methods of preparing the larvae for measurement and of making the measurements, and considered the form of sample to be taken from a population of Heterodera larvae which would give the most reliable estimate of the mean length of the whole population. Those studies were made on the larvae of H. rostochiensis only, but the ultimate purpose was to obtain strictly comparable estimates of the mean lengths of the larvae of all the known species of Heterodera in order to find out whether larvae of the different species could be identified by their length.

OBSERVATIONS ON THE BEET EELWORM AND OTHER CYST‐FORMING SPECIES OF HETERODERA

An account is given of observations made between 1944 and 1948 on beet eelworm and related species of Heterodera. The host ranges of beet‐, potato‐, pea‐, carrot‐ and Galeopsis‐root eelworms were studied in the field, on sites where previous information had been obtained of the species of Heterodera present. The beet eelworm attacked plants in the Polygonaceae, Chenopodiaceae, Amarantaceae, Aizoaceae, Caryo‐phyllaceae, Cruciferae, Onagraceae and Labiatae. Considerable differences were observed in the intensity of attack upon different hosts. The Cruciferae contains many efficient hosts and is the most important host family in the British flora. The Galeopsis eelworm, like the beet eelworm, attacked plants in several families, including the Chenopodiaceae, Caryophyllaceae and Labiatae, which also contain hosts of the beet eelworm. The other root eelworms were more specific, the carrot eelworm attacked only cultivated and wild carrots (Daucus carota L.), the potato eelworm attacked only plants in the genera Solanum and Ly copersicum (potato, Solatium dulcamara L. and tomato), while the pea eelworm attacked only peas (Pisutn), beans and tares (Vicia). The potato eelworm did not attack Solanum nigrum L. or S. sarrachoides Sendt. and there was no evidence that potato eelworm is capable of attacking weeds in natural orders other than the Solanaceae. In pot‐tests, the hop eelworm attacked only hops and nettles and the cabbage eelworm only plants in the family Cruciferae. The rates of development of cysts of the beet, potato, pea and carrot eelworms were studied in the field. In all four species, the rate of development was similar and varied with the soil temperature, being slow in spring, more rapid in summer, falling off during the autumn and becoming negligible during the winter. In the pea eelworm, drought caused delay in the invasion of rootlets by eelworm larvae and reduced the intensity of attack. Assuming adequate moisture, the maximum number of generations for all four species is between two and three per year. In practice, the number of generations is also governed by the vegetative period of the host crop. During the host‐range trials and observations on rates of development, fresh cysts were examined in large numbers. Some differences in shape were observed between the larger cysts of the various species. There were also differences in colour and in the number of eggs extruded into the gelatinous egg sac. Analysis of length and breadth measurements suggested that size ranges and regression coefficients are somewhat more valuable than average lengths and breadths, and length‐breadth ratios, which are usually given. The carrot‐root eelworm (Heterodera carotae Jones) is described, and the status of the Galeopsis‐root eelworm is discussed. The latter is distinct from beet eelworm and appears to be identical with a form found by Goffart in 1936, for which the name Heterodera galeopsidis is proposed. The observations here recorded do not support the theory of adaptability of root eelworms to new hosts.

Soil populations of beet eelworm (Heterodera schachtii Schm.) in relation to cropping

Between 1934 and 1936 beet eelworm was found to be prevalent on a farm in the Isle of Ely. The virtual absence of other species of Heterodera and the workable nature of the soil made the farm ideal for the study of soil populations. In 1937 a sampling technique for estimating the cyst, viable cyst and egg content of soil was developed and standardized. The first part of the paper deals with the sampling technique and the investigation of the various steps of which it is composed. When put into operation, the technique gave consistent results, but difficulty was experienced with counts of viable cysts and eggs owing to bias between different observers.The second part of the paper gives the results obtained by the application of the technique to twenty‐four fields during the period 1937‐42. There appears to be a correlation between the general level of the eelworm population and the frequency with which susceptible crops are grown, and also between the general level of the population and the amount of damage to the crop, ‘beet sickness’ being most extensive and most severe where populations are greatest. Marked increases in the eelworm population follow the planting of susceptible crops upon heavily infested land, while smaller increases occur in fields with low eelworm populations. Fields which were completely ‘beet sick’ required very long periods of rest from susceptible crops (more than 10 years) before the eelworm populations in them fell to a low level and before they could be regarded as safe to grow sugar beet once more. The general effect of non‐susceptible root crops is to bring about a lowering of the eelworm population. Cereals have the same general effect, but to a lesser extent. In some instances, rises were recorded after the growing of cereals. These appeared to be due to weeds, or to susceptible crops growing as weeds amongst the cereal crops. Claying had a beneficial effect.The results of the investigation indicate the difficulties which may arise where close cropping is practised with crops susceptible to beet eelworm, and the desirability of avoiding widespread ‘beet sickness’ in important beet‐growing areas. Brassicae are capable of increasing beet‐eehvorm populations, although they do not as a rule show signs of distress when growing on land where sugar beet develops the symptoms of ‘sickness’.A strong correlation was observed between the viable cyst and egg content of bulk samples. This suggests that viable cysts are a better measure of the general level of the eelworm population than might have been supposed. Cysts decay slowly and remain in the soil long after they are devoid of living contents. For this reason cysts alone are better as a guide to the past history of an infestation than as a measure of its present intensity. The eehvorm population of any field is complex and has highly individual characteristics. The best measure of this population so far devised is an estimate of the encysted egg population. While this may be adequate where it is desired to note changes in the level of the population following a particular crop or treatment, it is not nearly so satisfactory where a forecast of damage to an ensuing crop is required.In examinining cysts for viability, fungal hyphae and the fruiting bodies of other organisms were found inside. One of these, a fungus of the Order Chytridiales, had fruiting bodies similar to microcysts (Triffitt, 1935), and it is concluded that some part at least of the microcyst population of soils is composed of the fruiting bodies of fungi of this Order. Another organism, probably a fungus, caused blackening of the eggs. This was almost certainly a parasite, and its rather high incidence suggested that it might be of importance in reducing egg populations.In conclusion, a number of suggestions are made for the adaptation of the technique for advisory purposes. Experience suggests that the general level of the egg population above which it is unsafe to grow sugar beet or mangolds on Fen soils is about 10 eggs per g. of air‐dried soil or approximately 0.1 viable cysts per g.

Identification of Heterodera Species by Larval Length. Technique for Estimating the Constants determining the Length Variations Within a given Species

A complete investigation of the variations likely to occur in the lengths of larvae of Heterodera rostochiensis (for example) under all conditions would involve measuring larvae from a great number of varieties of potato grown in a great number of different localities, and also of larvae from tomatoes of different varieties grown under various conditions. The results obtained so far for this species of Heterodera make but a small contribution towards the mass of information which might be accumulated. Until such further data become available, certain conclusions have been drawn as to the best method of taking samples of larvae for measurement, but it must be remembered that these conclusions may need revision at a later date in the light of new information. According to our present knowledge it appears that the variety of potato on which a cyst has developed does not significantly affect the mean length of the larvae within, but that the mean length of the larvae from a single plant, irrespective of the variety, may differ significantly from that of larvae from another plant. It is also deduced that the variations in the mean lengths of the larvae from different cysts are greater than the variations within the individual cysts. From these facts it follows that if one wishes to estimate parameters for larval lengths of a population of H. rostochiensis larvae, a better estimate will be obtained if one takes a small number of larvae from each of a large number of cysts rather than the same total number of larvae from a small number of cysts. Similarly the cysts should have come from a large rather than a small number of plants, but apparently it is not necessary that the plants should be of as many varieties as possible, though this last assumption is based on information derived from only seven varieties of potato, all of them maincrop varieties, and it may need modification when larvae from other varieties have been examined.

THE ESTIMATED NUMBER OF NEMAS IN THE SOILS OF MANITOBA

Samples of soil, taken from 29 different sections of the arable portions of Manitoba, have been analyzed. Twelve species of eight genera of nematodes have been identified. No species of the genus Heterodera were found.An endeavor has been made, first, to determine the approximate degree of infestation, and second, to correlate such factors as hydrogen ion concentration, moisture equivalent, and organic matter content with the nema counts.The nematode population is somewhat higher in the soils of Manitoba than in those of many parts of the United States, but considerably lower than in those of North China.The number of species seems to be limited.The degree of infestation appears to be negatively affected by either hydrogen ion concentration or moisture equivalent, but is decidedly influenced by organic matter content.