Inter-plot competition in yield trials of potatoes (Solanum tuberosum L.) with single-drill plots

An experiment was carried out to assess the significance of inter-plot competition in a yield trial of potato cultivars. Seventeen cultivars were deliberately chosen and assessed for yield in single-drill and four-drill plots. Inter-plot competition for fresh-weight yield was a significant factor in the single-drill plots. It was modelled using a common competition coefficient with a covariate based on neighbour fresh-weight yields. In contrast, there was no statistically significant inter-plot competition for specific gravity. After adjustment for inter-plot competition, varietal ranking in estimated monoculture yield differed little from that based on unadjusted means. However, there was a reduction in the range of yield estimates, and a closer agreement with the observed pure-stand yields from the inner two drills of the four-drill plots. The adjustment for monoculture performance was most pronounced for the higher and lower yielding varieties, as expected from the assumption that the performance of high yielding varieties was enhanced in a competitive environment at the expense of low yielding ones. A general and flexible method of estimating competition coefficients in variety trials, together with a suitable algorithm, was developed and is explained in an appendix. It was used to check for inter-plot competition in a number of potato trials with single-drill plots and a large number of entries. Competition was found in some trials but not in others. Thus, where potato tubers are grown in single-drill plots for assessment of fresh-weight yield, adjustment should be made for inter-plot competition when evidence of inter-drill competition is found.     

Differences in the tolerance of potato cultivars to potato cyst nematodes (Globodera rostochiensis and G. pallida) in field trials with and without nematicides

In field trials Cara, Brio, Maris Piper and Pentland Javelin were consistently more tolerant of damage by Globodera rostochiensis yielding more than Corsair, Pentland Dell, Maris Anchor and Maris Peer, in untreated, heavily infested soil and giving the smallest increases to nematicide treatment. No yield or growth differences were found between plants in untreated and nematicide treated plots at a nematode-free site. The most tolerant cultivars all had a gene (H₁) for resistance to G. rostochiensis derived from Solanum tuberosum ssp. andigena and in soil infested with G. pallida the tolerance of at least one resistant cultivar (Maris Piper) appeared to be lessened. However, some resistant cultivars were comparatively intolerant, even to G. rostochiensis. Early cultivars were generally less tolerant than late maturing cultivars but there were exceptions. Amongst cultivars with resistance derived from Solanum vernei the early cultivar Guardian was more tolerant than the main crop cultivar Corsair. The effect on the yield of several cultivars of a range of densities of G. rostochiensis, produced either by applying different rates of a nematicide or by cropping in the previous year, was examined at two sites. The results indicated that the slope of the regression for yield in relation to nematode density was less for tolerant than intolerant cultivars. At sites infested with G. rostochiensis Maris Piper was found to be consistently more tolerant than Pentland Crown.     

Stem canker (Rhizoctonia solani) on five early and seven maincrop potato cultivars. II Effects on growth and yield

In 1983 and 1984, potato seed tubers of five early and seven maincrop cultivars were inoculated with cultures of Rhizoctonia solani during planting in field experiments to simulate severe seed infection. The size of foliage was assessed during June-August and tuber yields recorded during growth and at harvest in October. Stem canker delayed shoot emergence, decreased the number and length of stems and caused increased variation in stem length; these effects were greatest with Maris Peer and Arran Comet (early cultivars) and King Edward and Pentland Squire (maincrop cultivars). Total weight of foliage was decreased, especially with earlies, dry matter of stems increased and the proportion of foliage on lateral stems increased. With the early cultivars, tuber yield from sprouted Maris Peer seed 11 wk after planting in 1983 was decreased by 24%, and 13 wk after planting in 1984 yields were decreased by 42% (Maris Peer), 40% (Ulster Sceptre), 34% (Estima), 30% (Arran Comet) and 17% (Ulster Prince) with sprouted seed and by 20, 29, 53, 39 and 28% respectively with non-sprouted seed. Decrease in total yield at harvest in October averaged 13% with sprouted seed and 10% with non-sprouted seed. In all cultivars the weight of small tubers was decreased and with Estima the weight of large tubers was increased. Tuber bulking was also delayed with all maincrop cultivars and at harvest yields from sprouted King Edward seed were decreased by 13% in 1983 and by 16% (sprouted seed) and 23% (non-sprouted seed) in 1984; yields of Pentland Squire were decreased by 5, 16 and 21% respectively. Yield losses with other cultivars ranged from 5–13% with sprouted seed and 0–16% with non-sprouted seed. The yields of small tubers were decreased with all cultivars and yields of large tubers were increased with Pentland Squire, Pentland Crown and Cara.     

Effects of potato leafroll virus on the crop processes leading to tuber yield in potato cultivars which differ in tolerance of infection

The effects of infection with potato leafroll virus (PLRV) on the four crop processes leading to tuber fresh weight yield were examined in field plots of four cultivars (Montana, Pentland Crown, Maris Piper and King Edward) differing in tolerance to infection. Averaged across cultivars, infection decreased yield by 50%. This decrease was equally due to less light (total solar radiation) being intercepted, a lower efficiency with which intercepted light was converted into dry weight, and a smaller proportion of dry weight being partitioned to tubers. Dry matter content of the tubers was also diminished but to a lesser extent. The main difference between the cultivars in their response to infection was in the partitioning of dry weight. In Montana and Pentland Crown, harvest index was decreased by 15% in infected plants, whereas in the less tolerant cultivars, Maris Piper and King Edward, it was decreased by 25%. The decline in photosynthetic performance of Montana, a cultivar with slightly earlier maturity than the other three, was delayed in PLRV-infected plants. Effects on number of daughter tubers essentially reflected those on yield to the extent that average tuber weight did not change in Maris Piper, was one third less in King Edward, and the change was intermediate in Montana and Pentland Crown.     

Control of potato cyst nematode (Globodera rostochiensis) by nematicides and a resistant potato cultivar at four sites in Scotland

The control of potato cyst nematode (Globodera rostochiensis) by the oxime-carbamates aldicarb and oxamyl was tested in four fields in Scotland. Dazomet was tested in three of these fields and carbofuran in one. In untreated plots in the three most heavily infested fields Maris Piper (resistant) yielded better than Pentland Crown (non-resistant). All nematicides increased the yields of both potato cultivars but had a greater effect on the yield of Pentland Crown. Dazomet increased yields of tubers most. Heavy nematode infestation reduced yield of tubers more in a sandy soil than in two sandy loams. In a field with few potato cyst nematodes nematicides did not significantly affect tuber yields. Although the nematicides greatly increased yields, they were not completely effective in controlling potato cyst nematodes. In treated plots in the lightly infested field, there were more nematode eggs following a crop of Pentland Crown than before. In contrast, Maris Piper markedly decreased post-cropping populations and except at one site, where dazomet further decreased nematode numbers, combining nematicides with the resistant cultivar failed to decrease nematode numbers further. Nematicides decreased the numbers of larvae invading potato roots by up to 95%, oxamyl at 5–6 kg/ha being consistently the best treatment.     

Effects of potato genotype, oxamyl and the numbers of potato cyst-nematodes, Globodera rostochiensis or G. pallida on tuber yields and nematode increase

A new technique is described for establishing different numbers of the potato cyst-nematode Globodera rostochiensis in field soil, which leaves the soil homogeneous in nutrient status. Field plots established in this way were used to compare yield losses in four potato cultivars (Maris Piper, Pentland Crown, Pentland Dell and Désirée) associated with different numbers of G. rostochiensis. Over the range of 7.4 to 148.4 eggs g^-1 soil at planting, yield losses were 18.7% (Maris Piper), 53.2% (Désirée), 55.7% (Pentland Crown) and 63.5% (Pentland Dell). Similar results were obtained in another experiment on the same field in a different year using only lightly and heavily infested plots. Treating the seedbed soil with oxamyl before planting prevented significant injury to potatoes by G. rostochiensis but increased the yield of Pentland Dell and perhaps Désirée (but not Maris Piper or Pentland Crown) more than expected from nematode control alone. Treating heavily infested soil with such a nematicide cannot therefore be recommended as part of a valid procedure for establishing lightly and heavily infested plots for comparing tolerances of attack by potato cyst-nematodes in a range of potato genotypes. In peaty loam soils moderately or heavily infested with G. pallida, oxamyl at 5.6 kg a.i. ha^-1 incorporated into the seedbed before potatoes were planted generally increased tuber yields, though the effects varied considerably with the cultivar grown. Increase of G. pallida in these soils was controlled better by growing potatoes bred for resistance to it (ZB 35 – 29, Caxton, Santé, Morag, 11233 ab 22, Fingal, A27/23, Cromwell). Increase of G. pallida on susceptible cultivars varied greatly and Romano increased G. pallida no more than the resistant Morag. G. pallida is probably controlled best in peaty loam by growing a resistant cultivar in soil treated with a granular (non-fumigant) nematicide.     

Effects of soil treatments and cultivars on 'early dying' disease of potatoes caused by Globodera rostochiensis and Verticilliurn dahliae

Plots in a field infested with Globodera rostochiensis and Verticillium dahliae were treated in 1971 with methyl bromide, dazomet, aldicarb, benomyl or aldicarb + benomyl and potatoes (cv. Pentland Dell) were grown in four consecutive years. In 1971, all treatments delayed the development of haulm symptoms, decreased soil populations of G. rostochiensis and increased yields. In the second year yields were increased by all treatments except dazomet whereas in the third and fourth years only benomyl or aldicarb + benomyl increased yields. Soil populations of V. dahliae were decreased by methyl bromide in 1971 but not by other treatments or in later years. In 1976, Pentland Dell and Pentland Crown were grown on an infested field in plots fumigated with methyl bromide. Fumigation slightly delayed shoot emergence but increased plant height, ground cover and the size and persistence of the leaves; development of haulm symptoms was delayed and yield increased. Fumigating with methyl bromide at five times the normal rate was more beneficial to Pentland Dell than to Pentland Crown. In a comparison of 10 cultivars in 1975, symptoms developed sooner and yields were smaller on a field infested with G. rostochiensis and V. dahliae than on a non-infested field and on both sites Maris Peer plants died soonest and yielded least. Pentland Crown yielded most on the non-infested field and Maris Piper on the infested field.     

A comparison of inoculation methods for determining potato cultivar reaction to black leg

Two methods of inoculating tubers, one by dipping them in an aqueous suspension of Erwinia carotovora subsp. atroseptica, the other by inserting the end of a wooden toothpick charged with undiluted bacteria into the rose-end near a sprout, were compared over two years for their ability to produce black leg and for their effect on plant growth and yield on a number of cultivars chosen because of expected differences in susceptibility. Although both methods produced a similar rank order for cultivar reaction, the more consistent results were obtained using the toothpick method, clear differences in black leg incidence and yield being evident between the susceptible cultivars Ulster Sceptre and Maris Bard and the more resistant Pentland cultivars. As some cultivars appear more tolerant to black leg than others, yield may be as important a criterion as disease incidence when assessing overall cultivar performance. The inoculation of the base and middle of stems and the infiltration of the bacterium into tubers were also investigated as alternative methods for assessing cultivar susceptibility.     

Healthier seed potatoes. I. Effects of inoculating stem cutting stocks with Polyscytalum pustulans and Rhizoctonia solani on growth, yield and disease

Potato seed tubers of seven cultivars derived from stem cuttings in 1965 (healthier seed) were grown in 1969–72 at two sites, one clay with flints soil (Rothamsted) and the other sandy loam soil (Woburn). Inoculating sprouted tubers at planting with Polyscytalum pustulans did not affect the number of stems/plant or total yield but increased stem base and tuber infection. The yields of large tubers (57–83 cm) were increased and small tubers (>57 cm) decreased, indicating a decrease in tuber numbers. Rhizoctonia solani inoculated at planting decreased numbers of stems/plant and yield by up to 14% at Woburn but not at Rothamsted. At both sites, yields of large tubers were increased with cvs Majestic and Record and decreased with Pentland Crown. Stem canker and tuber infection were increased but infection was also prevalent on tubers from non-inoculated seed at Woburn. When both pathogens were inoculated together yields of large tubers were increased in cvs King Edward, Majestic and Record and decreased in Pentland Crown. Infection of stem bases and tubers was sometimes less than when either pathogen was inoculated singly.     

Development of black dot disease (Colletotrichum coccodes (Wallr.) Hughes) and its effects on the growth and yield of potato plants

Seed tubers of cvs Désirée and Pentland Crown with different severities of black dot were planted in 1988 and 1989 at Rothamsted in fields in 4– or 7-course rotations, respectively. Tubers treated with prochloraz (1988) or imazalil (1989) were planted in some plots, and in others Colletotrichum coccodes inoculum was added to the soil at planting. In further experiments at Mepal, Cambridgeshire in 1989 and 1990 and at Rothamsted in 1990 on sites where potatoes had not been grown for more than 15 years, large amounts of inoculum were added to the soil around disease-free seed tubers of two (1989) or three (1990) cultivars at planting. In all experiments plants were sampled during the season and the effects of treatments on disease development, growth and yield were recorded. Disease on roots, stem bases and tubers was found early in the season and was more severe on Désirée than on Pentland Crown plants from fields in 4– or 7-course rotations. Severity increased throughout the season and with increasing amounts of disease on the seed tubers, especially with Desiree. Disease was also found on plants from disease-free tubers and was more severe in 1988 than 1989. At harvest, black dot on tubers was significantly more severe from severely affected than from disease-free seed, and was most severe where inoculum, especially large amounts, had been added at planting. Fungicide treatment decreased disease early in the season but had no effect on tuber infection at harvest. In 1989 the weight loss of seed tubers during sprouting increased with increasing amounts of black dot, but the disease had little effect on plant size through the season. At harvest the yield of ware tubers (>50 mm) decreased with severe disease but total tuber yields were not significantly affected. However, at harvest in 1988 severely affected seed yielded significantly less than healthy seed. Plants grown from mini-tubers were free from disease on sites where potatoes had not been grown for at least 15 years. Inoculum applied at planting caused severe disease on all cultivars in both years, whereas disease was slight on uninoculated plants. Inoculated plants senesced early at Mepal in 1990, but there were no significant differences in total tuber yield in any experiment. However, yields of ware tubers (>50 mm) were sometimes decreased and the total tuber number per plant increased.     

Effect of the nematicide oxamyl on the multiplication of Globodera rostochiensis and G. pallida and on the haulm growth and yield of six potato cultivars with different levels of resistance and tolerance

The effect of a pre-planting application of oxamyl on the yields of six potato cultivars was studied in co-operative field trials in 1981. Two sites were ‘uninfested’ with potato cyst nematodes (PCN), two were lightly infested (<25 eggs/g soil) and six were moderately to heavily infested (three with Globodera rostochiensis and three with G. pallida). At the uninfested and lightly infested sites oxamyl had little effect on mid-season haulm weights or on final tuber yields. At sites moderately to heavily infested with G. rostochiensis the haulm growth of all cultivars tended to be increased by a similar amount on plots treated with oxamyl, Pentland Dell being least responsive. Yield was increased by different amounts, the increases being least for cvs Cara and Maris Piper and most for Corsair and Pentland Dell. At the sites moderately to heavily infested with G. pallida Cara was again tolerant, its yield being increased little by oxamyl compared with the other cultivars. Maris Piper gave the largest yield increase. Final populations of PCN on non-resistant cultivars were reduced by oxamyl at some sites but not at others. Resistant cultivars also decreased the final numbers of PCN at most sites. Two cultivars derived from Solanum vernei with different degrees of resistance, appeared to be almost equally effective in controlling G. rostochiensis and G. pallida.     

Cultivar Variations of Australian‐Grown Danshen (Salvia miltiorrhiza): Bioactive Markers and Root Yields

Danshen (Salvia miltiorrhiza Bunge, Lamiaceae) is a commonly used and highly valued Chinese medicinal herb grown widely in China. In the present work, we studied cultivar variations of Australian‐grown Danshen in order to select optimal cultivars for local herbal production. Root yields of seven cultivars, V1–V7, were monitored in a one‐year field trial, and bioactive markers, including cryptotanshinone, tanshinone I, tanshinone IIA, and salvianolic acid B, were quantitatively determined using a validated RP‐HPLC method. Significant variations were found in root yields, root production efficiencies, and contents of the bioactive marker compounds. Linear correlations were observed among the contents of three tanshinones but not among those of tanshinones and salvianolic acid B. Among the cultivars, V6 was the best cultivar for production of tanshinones, and V4 and V5 were best for production of salvianolic acid B. The findings indicate that it is possible to achieve optimal root yields, and high contents of tanshinones and salvianolic acid B by selecting specific Danshen cultivars.     

A method for assessing the field susceptibility of potato cultivars to blackleg (Erwinia carotovora subsp. atroseptica)

In experiments to develop a method for assessing the field susceptibility of potato cultivars to blackleg (Erwinia carotovora subsp. atroseptica) seed tubers were stab-inoculated near the stolon (attachment end), with a suspension of the bacterium, or with water, before planting. Disease symptoms were recorded in three years (1980–1982) and plant growth and yield in 1982. Estima and Maris Bard were the most susceptible cultivars with many plants failing to emerge and most of those that did showing disease symptoms. Pentland Crown was the most resistant: few plants failed to emerge and few showed blackleg. Nevertheless compared with water-inoculated plants bacterial inoculation of the seed tubers of this cultivar caused loss of yield and differences in tuber size distribution. Cara, Wilja and King Edward showed intermediate reactions.     

A field study of the relationship between pre-planting density of Globodera pallida and the growth and yield of two potato cultivars of differing tolerance

Non-resistant but tolerant cv. Cara and non-resistant but relatively intolerant cv. Pentland Dell were grown in split plots encompassing a range of population densities of potato cyst nematode, Globodera pallida. Light interception and its efficiency of conversion were estimated by regular ground cover measurements and plant harvests. It was concluded that increasing levels of infestation with G. pallida only slightly decreased the efficiency of utilisation of intercepted radiation. Heavy infestation of G. pallida initially decreased the top growth and light interception of both cultivars by similar proportions, but in later harvests, this adverse effect markedly decreased for Cara whereas it slightly increased for Pentland Dell. This difference was due to the heavily infested Cara eventually achieving and maintaining 100% ground cover whereas the equivalent Pentland Dell never exceeded 75% ground cover. Consequently, final tuber yields were decreased much more for Pentland Dell than for Cara though the decreases in tuber yield were less than those for top growth. The importance of nematode effects on top growth, and hence on light interception, with regard to both yield losses and tolerance differences, were clearly demonstrated. Both linear and logarithmic models were used to describe the relationship between the initial population density of G. pallida and yield, and the implications of differences in tolerance on the parameters in the logarithmic model are discussed.     

Evaluating between-plant interference in field trials for assessing potato genotypes for resistance to late blight

Field trials were done with four cultivars over 3 years to assess the extent to which the amount of late blight on the foliage of a potato plant could be influenced by that on a neighbouring plant of the same or a different cultivar. Drills containing the test plants were interspersed with those of spreader plants (cv. King Edward) which were artificially inoculated with Phytophthora infestans. The intensity of blight on the test plants was recorded on several occasions.
Resistant cultivars tended to be scored as less resistant in mixtures with other cultivars than in pure stands, and susceptible cultivars tended likewise to be scored as more resistant in mixed stands. However, standard analysis of variance indicated no systematic evidence of a significant effect due to neighbour cultivars, nor of interaction between cultivars and neighbour cultivars. In contrast, Kempton's (1982) neighbour model indicated a significant and positive interference coefficient (β) in each trial, which generally decreased over time. Predicted pure stand scores for each cultivar indicated that the adjustment was greatest for the most resistant and most susceptible cultivars. There was no advantage in using two-plant rather than one-plant plots in withstanding neighbour effects.     

Stem canker (Rhizoctoniu solani) on three early and three maincrop potato cultivars: effects of seed tuber size on growth and yield

Large seed tubers (mean 134 g) and small seed tubers (50 g) of three early and three maincrop potato cultivars, spaced respectively 48 and 30 cm apart within rows, were inoculated with Rhizoctonia solani at planting in 1985 and 1986. All seed of early cultivars was sprouted and maincrop seed was either sprouted or not sprouted. In all cultivars, plant emergence was slower from small than large seed and with both was delayed by inoculation. From 11 wk after planting numbers of stems and tubers and weights of foliage and tubers/m² were usually similar from small and large seed when not inoculated, but inoculating delayed plant growth and decreased tuber numbers and yield more from small than large seed. Inoculating decreased mean yields from large and small seed of early cultivars at 11 wk by respectively 24% and 31% (Arran Comet), 12% and 18% (Estima) and by 10% and 28% (Wilja) and losses were greatest with saleable sized tubers (3–4 cm). When grown to maturity inoculating decreased yields by 7% (Arran Comet), 5% (Estima) and 14% (Wilja). With maincrop cultivars, yields in October from large and small seed were decreased by respectively 4% and 10% (DCsirke), 9% and 12% (Maris Piper) and by 14% and 22% (Pentland Squire). In all cultivars yields of tubers < 44 mm and 44–70 mm were decreased and, with Pentland Squire, tubers > 82 mm were increased. The incidence of stem canker and of black scurf on progeny tubers was not affected by seed size but in all cultivars the percentage of greened tubers was slightly increased by inoculation.     

Towards an optimal sampling strategy for assessing genetic variation within and among white clover (Trifolium repens L.) cultivars using AFLP

Cost reduction in plant breeding and conservation programs depends largely on correctly defining the minimal sample size required for the trustworthy assessment of intra- and inter-cultivar genetic variation. White clover, an important pasture legume, was chosen for studying this aspect. In clonal plants, such as the aforementioned, an appropriate sampling scheme eliminates the redundant analysis of identical genotypes. The aim was to define an optimal sampling strategy, i.e., the minimum sample size and appropriate sampling scheme for white clover cultivars, by using AFLP data (283 loci) from three popular types. A grid-based sampling scheme, with an interplant distance of at least 40 cm, was sufficient to avoid any excess in replicates. Simulations revealed that the number of samples substantially influenced genetic diversity parameters. When using less than 15 per cultivar, the expected heterozygosity (He) and Shannon diversity index (I) were greatly underestimated, whereas with 20, more than 95% of total intra-cultivar genetic variation was covered. Based on AMOVA, a 20-cultivar sample was apparently sufficient to accurately quantify individual genetic structuring. The recommended sampling strategy facilitates the efficient characterization of diversity in white clover, for both conservation and exploitation.     

Stability analysis of grain yield in barley (Hordeum vulgare) in the US mid-Atlantic Region

Stability of grain yield performance is an important characteristic in the selection of new crop cultivars. Information from cultivar trials, however, is seldom fully analysed for genotype by environment interactions and, therefore, information on stability of current crop cultivars is lacking. The objectives of this study were to investigate the stability of agronomic traits among genotypes of barley (Hordeum vulgare) across 17 environments (location‐years) in Maryland (USA) from 1994 through 1997 and to examine the effect of locations and years of testing on grain yield performance in this region. Significant differences were observed among barley cultivars and experimental lines for grain yield, plant height, and heading date. Grain yield was positively correlated with plant height and negatively correlated with heading date. Genotype x environment interactions measured through regression analysis were significant for grain yield, heading date and plant height, with the environmental component having the largest effect. Most barley genotypes tested (90%) had regression slopes for grain yield that did not differ from 1.0, indicating good potential for yield response under improving environmental conditions. The most widely grown cultivar in the mid‐Atlantic region, ‘Nomini’, had a regression slope that was higher than 1.0 for grain yield. This indicates that it tends to respond with increasingly higher yields under favorable conditions. In this study, the slope and the standard error of the slope were moderately correlated with grain yield. The genotype's coefficient of variation was not a good indicator of stability for this region. Grain yields of genotype entries common to all years and locations were correlated with corresponding yields at each of the locations and years to assess the relative performance of each location and year. Correlation coefficients across locations were relatively high (r=0.64) within each year of testing. Correlations between years for the same and across locations were generally much lower. The data presented here supports a testing program over more years rather than increased locations to fully characterise the performance of new cultivars.     

Effect of Within-field Variation in Soil Texture on Heterodera glycines and Soybean Yield

The influence of soil texture on Soybean yield in the presence of Heterodera glycines was investigated by comparing yields of susceptible cultivars with a resistant cultivar for 2 years. Soybean yield was negatively correlated with increasing sand content (P = 0.05). Yields of susceptible cultivars were suppressed with increasing sand content. Final nematode population densities were lowest in plots with greatest sand content. Soybean infection by SCN, as determined by the number of cysts 30 days after planting, was not consistently related to soil texture over 2 years. Initial nematode population density was positively related to soybean yield the first year and negatively related to soybean yield the second, probably a result of greater yield suppression by H. glycines in plots with greater sand content.     

Biochemical and bioassay analysis of resistance of potato (Solanum tuberosum L.) cultivars to attack by the slug Deroceras reticulatum (Muller)

A method is described for using young field slugs Deroceras reticulatum (Muller) in a bioassay study of biochemical resistance of potato (Solanum tuberosum L.) cultivars to slugs. Tuber parts or an artificial diet were provided as food sources. Comparisons were made of feeding, survival and weight gain between the susceptible cultivar Maris Piper and the resistant cultivar Pentland Dell. Biochemical analyses were made of these two cultivars and the resistant cultivars Stormont Enterprise and Majestic. Comparisons of tuber sections and peelings as food sources indicated factors affecting growth were located in the surface layers of the tubers. Phenolics and glycoalkaloids were concentrated in the surface layers but the amounts were similar in the susceptible and resistant cultivars and the bioassays indicated that neither acting alone could explain resistance. The amounts and distribution of free amino acids also did not correlate with resistance although when supplied in the artificial diet they partly inhibited feeding. Proteinaceous inhibitors of slug gut proteolytic enzymes were present throughout the tubers but were not concentrated in the surface layers and the amounts were similar in the different cultivars thus they too did not explain the difference in susceptibility between the cultivars. Bioassays using acetone extracts (low molecular weight substances) and acetone powders (high molecular weight substances) either alone or in combination indicated that the resistant cultivar Pentland Dell contained a high molecular weight substance which together with a low molecular weight substance from either the same cultivar or the susceptible Maris Piper could confer resistance. Bioassays using protein extracts supplied in the presence or absence of chlorogenic acid indicated that this mechanism could comprise enzymic oxidation of phenolics. Assays of phenolase confirmed this since activity was highest in the outer layers of the tubers and was highest in the three resistant cultivars. Thus the chief resistance factor identified was high phenolase activity acting rapidly on phenolics when the slug first bites the tuber surface. The quantity of phenolics per se did not control the resistance. Thus while phenolics must be available, resistance is compatible with low blackening on cutting the tuber.