The production of virus-free clones of some British potato varieties

Virus-free clones of some British potato varieties, of which all commercial stocks are infected, were obtained by culturing apical meristems of sprouts on tubers. The variety Golden Wonder was freed from potato virus A, Arran Comet from potato virus X, and Epicure, Orion and Sharpe's Express from potato virus S. Of the 196 meristems cultured, forty-one (21%) grew and of these twenty developed into plants, of which nineteen were virus-free. More of the excised meristems grew when the first leaf primordium was included than when not, and its inclusion seemed not to increase the chances of the progeny being infected.     

A comparison of Fusarium avenaceum and Fusarium caeruleum as causes of wastage in stored potato tubers

Fusarium avenaceum is reported for the first time as a cause of rotting of potato tubers in Britain. The progress of rotting in tubers infected with F. avenaceum has been compared with dry rot due to F. caeruleum in the laboratory, clamp and potato store. Of the four varieties, Majestic, King Edward, Doon Star and Arran Pilot, tested for susceptibility, King Edward was the most susceptible to F. avenaceum and Doon Star to F. caeruleum.
Optimum temperatures for growth on potato-dextrose agar were 20-25 C. for F. avenaceum and 20 C. for F. caeruleum ; maximum temperatures were > 30 and 30 C. respectively. For infection of wounded potato tubers, cardinal temperatures for F. avenaceum were similar to those on agar, but for F. caeruleum the optimum was 15 C. and the maximum 25 C. The optimum temperature for rotting tended, with both species, to be higher in the more susceptible potato varieties. At low temperatures F. caeruleum caused quicker rotting than did F. avenaceum , even though its rate of growth on agar was scarcely more than half that of the latter.
High humidity favoured rotting especially by F. avenaceum; F. caeruleum was more tolerant of relatively low humidity. Both species caused quicker rotting in the clamp than in store, even though there was no appreciable difference in mean temperature between the clamp and the store. This was attributed to the higher atmospheric humidity in the clamp.     

FIELD EXPERIMENTS ON POTATO SKIN-SPOT DISEASE CAUSED BY OOSPORA PUSTULANS OWEN & WAKEF

In experiments on skin-spot disease of potatoes, caused by Oospora pustulans Owen & Wakef., Kerr's Pink proved the most susceptible of twenty-four commercial varieties; Arran Banner, Majestic and King Edward were highly susceptible; Home Guard and Golden Wonder were highly resistant.
Thymol and tetrachlornitrobenzene, applied as dusts to pits of potatoes at lifting, did not give satisfactory control of the disease. The efficacy of an organo-mercury dip treatment at lifting time was confirmed.
The incidence of the disease was considerably decreased by storing tubers in boxes instead of in pits, by digging the crop about a month before normal harvest, or by cutting the haulms at this time and digging at the normal time.
Ware tubers showed a significantly higher infection than seed tubers of the same crop, indicating that susceptibility of tubers increases towards maturity.
The difference in eye infection between varieties may not be related to that of general superficial infection. It is suggested that, in future, the assessment of varietal susceptibility should be based upon the degree of eye infection, which is in practice the measure of the economic importance of the disease. Tubers having pustules at or near all the eyes gave markedly reduced sprout emergence and plant stand.     

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.     

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.     

Resistance to the potato cyst-nematode, Heterodera rostochiensis, in the plant genus Lycopersicon

Forty-eight lines of Lycopersicon and four lines of Solanum were screened for resistance to twelve Heterodera rostochiensis populations of known patho-type(s). Plant lines were assessed for resistance first by examining the outside of the root ball for cysts and later by washing the root ball to extract all cysts. Possible resistant plant selections were re-tested against three eelworm populations, including the one to which they were first shown resistant. Resistance was discovered in two lines of Lycopersicon pimpinelli-folium, two L. esculentum L. pimpinellifolium crosses, L. esculentum var. cerasiforme, six lines of L. peruvianum, in L. peruvianum var. humifusum, L. hirsutum var. glabratum, and in Solanum indicum. Because resistance was found most commonly in L. peruvianum and because it has already been used as a resistant parent in breeding programmes to incorporate resistance to root-knot nematode (Meloidogyne spp.) in tomato, L. peruvianum seems to be the best source of resistance among plants tested so far. The host-parasite relationships of resistant L. hirsutum var. glabratum (B 6013) were compared with those of a commercial, susceptible tomato, L. esculentum‘Ailsa Craig’. Plants were inoculated with three eelworm isolates; the extent of eelworm invasion, plant reaction and eelworm development were studied. Larvae invaded and penetrated roots of the resistant plant as freely and in as large numbers as they penetrated roots of the susceptible tomato. In the latter, numerous larvae matured while, in contrast, few larvae matured in the roots of L. hirsutum var. glabratum. L. hirsutum var. glabratum was shown to possess a root diffusate as active in hatching larvae of Heterodera rostochiensis as that of L. esculentum‘Ailsa Craig’. The existence of pathotypes of H. rostochiensis, identifiable by their differing abilities to increase on resistant tomato lines, was not clearly revealed in the experiments.     

A test of fruit varieties on entry rate and development by neonate larvae of the codling moth,Cydia pomonella

The rate of entry by neonate larvae of the frugivorous codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), into fruit material was investigated. We used no‐choice bioassays in climate‐controlled rooms to assay larval entry across four host plant species (apple, pear, quince, walnut) and three varieties within a single fruit species (apple). Larvae successfully entering apples were reared to adulthood, and we collected tissue samples from apples which were successfully colonized in order to determine sucrose concentrations. This information was used to evaluate differences in adult moth size, development time, and pulp sucrose concentration due to apple variety. Four important findings emerged: (1) neonate larvae had the highest frequency of entry (86% of larvae) into apple fruits, compared with pear (78%), quince (56%), and walnut (32%); (2) the frequency of larval entry into immature apples differed across apple varieties, and larval entry rate was highest in variety Golden Delicious (72%), compared with Granny Smith (46%) and Red Delicious (64%); (3) on average, adult moths were larger and development times were shorter on the variety with the highest entry frequency (Golden Delicious); and (4) apple pulp sucrose concentrations were higher for Golden Delicious (17.5 μg mg^?1) than for either Granny Smith (15.9 μg mg^?1) or Red Delicious (15.1 μg mg^?1) varieties, which correlates positively with entry and development data. We conclude that host fruit species and varietals within a species affect the entry rate of neonate codling moth larvae in no‐choice assays. We hypothesize that larval development is influenced by mean sucrose concentrations or other phytochemical differences associated with host fruit varieties.     

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.     

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.     

Effects of growth regulators, CCC and B9, on some potato varieties

The effects of CCC and B 9 on the growth habit of potato differed between varieties. CCC diminished stem lengths and dry weight more than Bo because CCC was applied early when shoots emerged from the soil, but B 9 was applied about 3 weeks later when several leaves had formed. In some varieties lateral stem growth was increased by treatment and in others decreased. There was an inverse relation between main stem and lateral stem growth so that varieties with vigorous main stem growth had poor lateral growth and vice-versa. Treatment with the growth-regulators diminished leaf dry weight of main stem leaves less than leaf area, but the degree of magnitudes of the changes depended on the variety. Both regulators lessened net assimilation rate. Net assimilation rate and dry matter per unit area of leaf were inversely related, possibly because accumulation of substances in leaves decreases photosynthesis. Stolon dry weight was positively correlated with main stem leaf area. There was a direct relation between stem length and tuber dry weight, suggesting that tuber initiation occurs at different stem lengths in different varieties. Tubering was earliest in Epicure and latest in King Edward. Epicure had the greatest tuber weight and smallest stem length.     

Isoenzymes of lipolytic acyl hydrolase and esterase in potato tuber

Five varieties of potato (Solanum tuberosum) were shown by gel- and free-flow-electrophoresis to exhibit multiple forms of lipolytic acyl hydrolase (LAH) and esterase enzymes. The electrophoretic patterns of LAH and esterase activities and protein differed with the variety and were characteristic for a given variety. In the variety (Golden Wonder) with the highest LAH activity (p-nitrophenylpalmitate as substrate), this was 200-fold greater than the esterase activity (p-nitrophenylacetate as substrate) and isoenzyme patterns for both enzymes were the most complex. In the variety with a very low LAH activity (Désirée), the LAH and esterase activities were similar and more simple isoenzyme patterns for these enzymes were observed.     

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.     

PÓT TESTS OF NEMATICIDES AGAINST POTATO-ROOT EELWORM

Details are given of nematicidal tests, directed against the potato-root eelworm, in 81. glazed pots of infested soil. The soil was sampled a few weeks after injection to secure data on the kill achieved, by counting the eelworm larvae hatching in root diffusate. A potato tuber was then planted in each pot and yielded information on phytotoxic or soil-amendment effects. A second soil sample at the end of the season showed the degree to which the eelworm population had recovered after treatment. With only duplicate pots, and single batches of 100 cysts from each pot, clear-cut results were achieved with D–D mixture and ethylene dibromide. In the present pilot test, the data were handled by ordinary analysis of variance methods, by which the lowest significant percentage kill was 73%, but the methods could be adapted for purposes of probit analysis.     

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.     

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.     

Dry rot of potato Fusarium caeruleum (Lib.) Sacc.; effect of planting infected and contaminated sets on plant establishment

The effect on plant establishment of planting infected and contaminated sets of susceptible varieties has been studied in field trials from 1942 to 1945.
Healthy whole sets, and whole sets inoculated internally 3 days before planting, yielded good stands, whilst whole sets with obvious dry-rot lesions present at planting time produced gappy crops.
Healthy cut sets gave satisfactory stands except in one test with Doon Star, whilst sets cut with a contaminated knife at planting time yielded only a few misses in 1943 but many in the unfavourable 1944 season and in 1945.     

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.     

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.     

Resistance of kale varieties to attack by Mamestra brassicae

1 The objectives of this work were to study the resistance of six kale ( Brassica oleracea acephala group) varieties to cabbage moth Mamestra brassicae (L.) expressed as antibiosis and to determine the effect of plant age on larval survival and development.
2 The influence of plant age on resistance was determined using leaves from seedlings and from mature plants. Survival and development of M. brassicae larvae and feeding rates were determined in laboratory bioassays.
3 Leaves from seedlings were more suitable than those of mature plants for establishing differences in resistance. There were significant differences between kale varieties in larval survival, growth rate, leaf feeding, and time to pupation but not pupal weight. The varieties MBG-BRS0031, MBG-BRS0351, and MBG-BRS0287 reduced survival of M. brassicae larvae. Larvae that fed on MBG-BRS0060 were the heaviest and took the longest time to pupation. MBG-BRS0031 was consumed significantly less by larvae than were all the other varieties examined. Leaves from mature plants of MBG-BRS0142 and MBG-BRS0170 were defoliated significantly less than those of other varieties.
4 In conclusion, the variety MBG-BRS0031 may be a promising source of resistance to M. brassicae . Leaf antibiotic resistance was shown to play a role in defense against M. brassicae attack but it is not the only possible mechanism of resistance.