In 1989 and 1990, small, medium and large seed tubers, cv. Désirée, were planted 38 cm apart in experiments designed to measure the effects of different combinations of neighbouring plants on the yield and size of tubers from individual plants. Total yield, ware yield (> 150 g), numbers of tubers and numbers of stems increased as seed tuber size increased. Also, decreasing the size of seed tubers decreased the numbers of tubers in all sizes except those > 200 g. Competition from the two neighbouring plants on either side in the same row (first neighbours) increased as the size of seed tubers increased and with all seed sizes yields decreased. For example, plants from large seed benefited from small seed planted as neighbours whereas, with plants from small seed, yield was decreased with neighbouring plants from large seed. Numbers of stems and tubers were not affected by neighbouring plants. Also yields were not affected by the two plants adjacent to the first neighbours (second neighbours) or by the size of seed tubers planted in adjacent rows. Competition from first neighbours also affected tuber size distributions and, with small seed, increasing the competition decreased the number of tubers > 150 g and increased numbers < 50 g. Medium and small seed planted as neighbours of large seed increased numbers of tubers > 150 g. When yields from plants produced by seed of each size with similarly sized seed planted as first neighbours (uniform populations) were compared with mixed populations, yields were always greater from the mixed populations of different seed sizes planted alternately than from the means of uniform populations. Therefore in the mixed populations, there were larger gains from the larger seed than losses from the smaller seed.

SUMMARYPotato seed tubers inoculated with Rhizoctonia solani and Erwinia carotovora subsp. atroseptica, to induce stem canker and blackleg respectively, were planted with uninoculated seed tubers in field experiments designed to measure the effects of the diseases and of neighbouring plants on tuber yield. Gaps were also included. The plant variables total and ware (> 150 g) yields and tuber numbers were affected by disease, and also by competition from the two plants on either side in the same row (first neighbours), and increased as competition from neighbouring plants decreased. Plants adjacent to the first neighbours also influenced yields of plants with stem canker but those in adjacent rows did not have a significant effect with either disease. Both diseases altered the tuber size distributions, which were also modified by neighbouring plants. The data were used to predict total and ware yields for crops containing different proportions of healthy, diseased and missing plants.

Several soil-borne fungal pathogens continue to cause problems in potato production worldwide. The reasons for these problems are illustrated by reference to the pathogens Verticillium dahliae, Rhizoctonia solani, Spongospora subterranea and a group normally considered to be tuber-borne, including Colletotrichum coccodes, Helminthosporium solani and Fusarium species. Generally, the long-term persistence of survival structures, the difficulties in reducing inoculum and lack of good sources of resistance hinder attempts to improve control of soil-borne fungal pathogens. Post-harvest fungicide treatment of seed or ware potatoes does not entirely alleviate storage problems. In the foreseeable future there is no alternative to the use of integrated measures from the time of planting seed to movement from storage. Key research areas that arise as a means of better controlling most of these pathogens are: the role of intraspecific variation in pathogenesis and ecology, problems of detecting and quantifying inoculum, and the need for improved methods for biological control.

Bait plants, comprising micropropagated and commercial seed tubers, were used to estimate the effects of rotation on the density and spatial pattern of inoculum of Rhizoctonia solani in large field plots of potatoes. Soilborne inoculum of R. solani produced economically significant levels of stem canker and its incidence and severity varied with rotation, with most disease in 2‐year and less in 4‐ and 6‐year rotations. The rates of loss of inoculum during intercrop periods differed amongst rotations with a rapid fall to low levels occurring after 1 year in a 6‐year rotation and after 2 years in a 4‐year rotation. Replenishment of inoculum to soil was rapid following the growth of a susceptible crop, with comparatively high levels of infection and disease, even in long rotations. Disease occurred in patches and the size of patches and the density of R. solani within patches differed with cropping frequency. The degree of spatial autocorrelation also differed amongst rotations but there was no evidence for any significant differences in the rate of change of spatial autocorrelation during intercrop periods in the three rotations.

SUMMARYThe effects of 10 fungicides on the growth of Colletotrichum coccodes in agar culture and on the germination of conidia was investigated. In field experiments in 1990 and 1991 the extent to which treating black dot‐affected potato seed tubers with fungicides affected the development of the disease on stem bases, roots and tubers was assessed. Black dot was also assessed on plants from field trials in 1990 which were designed to investigate the efficacy of the soil sterilant 1,3‐dichloropropene (Telone) and two nematicides, aldicarb (Temik) and ethoprophos (Mocap).Prochloraz and fenpiclonil were the most effective fungicides in decreasing the size of C. coccodes colonies on agar. Imazalil, propiconazole and dichlorophen were also effective, but at higher concentrations, whereas tolclofos‐methyl, thiabendazole and benomyl were only moderately effective. Resistant sectors developed from inhibited colonies on agar containing fenpiclonil and tolclofos‐methyl. Conidial germination was prevented at 1 mg/litre fenpiclonil and 5 mg/ litre dichlorophen; imazalil, benomyl and thiabendazole were also moderately effective.Fenpiclonil and propiconazole seed tuber treatments consistently decreased black dot infection on roots, stem bases and daughter tubers early in the season, but only fenpiclonil decreased disease on tubers at harvest in October. Propiconazole also delayed emergence and decreased stem numbers. Soil treatment with 1,3‐dichloropropene, aldicarb or ethoprophos had no effect on black dot but Rhizoctonia solani tuber infection and black scurf were increased.

SUMMARYPotato seed tubers infected or not infected with gangrene (Phoma foveata) were planted at Rothatnsted in 1987 to measure the effect of the disease and of neighbouring plants on yield. The experimental design was constructed so that the effect on growth of six adjacent plants (two nearest neighbours in each direction within rows and one nearest neighbour in each direction across rows) could be estimated for each plant. Total yield, ware (> 150 g) yield and tuber number from individual plants were affected most by the disease but also, in decreasing importance, by the two plants on either side within the same row (first neighbours), the two plants adjacent to the first neighbours (second neighbours) and the two adjacent plants in the rows on either side. Yield and tuber numbers increased as the different combinations of neighbouring plants contained increasing proportions of plants from diseased seed and missing plants; plants compensated for decreasing competition. Tuber size distributions showed that numbers of ware tubers decreased with increasing competition whereas numbers of small tubers were less affected. The fitted model was used to predict yields from crops planted with different proportions of diseased or missing seed tubers.

Summary 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.

In eight experiments in 4 years, inoculating seed tubers withR. solani caused stem canker and in seven experiments decreased the total yield and the yield of tubers 40–70 mm. Yields of larger tubers were usually increased. In most experiments inoculating increased the reducing sugar content of the tubers and darkened the colour of crisps, but did not consistently affect the amounts of sucrose. Treating soil with aldicarb or oxamyl before planting also slightly increased the total reducing sugars.

Plots of potato plants were watered to field capacity one day before harvest on three dates, and tubers from watered and non-watered plots were stored at 7°C and 95% RH immediately or after curing at 15°C either at 80% or 95% RH. Watering increased the severity of silver scurf after storage when tubers were not cured. Curing at 80% RH decreased the disease and amounts were similar from watered and non-watered plants. Black dot was increased by watering whereas black scurf and golding blotch were decreased.

In 3 years, seed tubers were treated or not with imazalil in March or April, sprouted and planted in field experiments. Tubers were harvested on several dates and cured under different conditions for 2 wk. or not cured, before storage for 6 months. In some years effects of irrigation, seed tuber size and seed stock were tested.Fungicide seed treatment decreased silver scurf, and was most effective in 1989 when the seed had little infection and the season was dry, but the treatment increased black dot. Silver scurf and black dot were decreased by harvesting early and also by drying tubers before storage (dry curing), and these practices also sometimes decreased black scurf and skin spot. The effects of fungicide seed treatment and of dry curing were greater the earlier the tubers were harvested. Irrigation decreased silver scurf and in one year decreased black scurf but greatly increased black dot. Silver scurf, black dot and skin spot were more common on tubers from large than small seed whereas black scurf was most common on tubers from small seed.In 2 years tubers were affected with a brown superficial blemish (golding blotch). This developed on the undersides of tubers in soil and was decreased by irrigation. Its cause is not known.

Silver scurf on daughter tubers at harvest was decreased by treating seed tubers with imazalil or thiabendazole in March but significantly increased when the treated tubers were wounded the day before planting in May. The disease was also increased by wounding seed tubers not treated with fungicide. Black dot was decreased by seed treatment with imazalil but was not affected by wounding.

SummaryIn 3 years, seed tubers of cv. King Edward were planted in field experiments and diseases assessed on the daughter tubers after storage for 24–26 wk. In 1991 and 1992, the seed tubers were treated with a mixture of thiabendazole and imazalil at two rates. Tubers were harvested on several dates from late August to early October and dried under different conditions before storage.In 1990, silver scurf after storage was decreased equally by dry curing for 2 wk and windrowing for 2 h whereas skin spot was decreased most by dry curing, and these effects were greater the earlier tubers were harvested. In 1991, dry curing for 2 wk decreased skin spot and silver scurf more than dry curing for 1 wk, and skin spot was slightly decreased by windrowing for 2 h; windrowing improved the control of both diseases when tubers were subsequently dry cured. In 1991 and 1992 fungicide treatment of seed tubers greatly decreased skin spot after storage and was also effective against silver scurf, but increased black dot when tubers were harvested in September or October.Skin spot was controlled equally well by the two rates of fungicide but silver scurf was less well controlled when the dose was decreased by 75%. It is suggested that amounts of fungicide applied to seed tubers could be decreased provided that the efficiency of application is improved and that crops are harvested early and dried before storage.