Susceptibility of potato tubers to infection by Phytophthora infestans

Phytophthora infestans infects King Edward potato tubers more readily through inoculated eyes than through lenticels, but more lenticels than eyes became infected when whole tubers were sprayed with inoculum. The resistance of lenticels but not of eyes increased as tubers aged. The spores did not infect through intact periderm. The likelihood of tubers on plants grown in pots becoming infected by sporangial suspension poured on to the soil increased the nearer the tubers were to the soil surface, the stem, or the side of the pot. Naturally infected tubers, and those sprayed with sporangial suspension, had most eyes infected at the rose end, and most lenticels infected on the middle region of the tuber. Of naturally infected tubers, on which the site of infection could be identified, most were infected through eyes at the rose end.     

Potato tuber lenticels: susceptibility to infection by Erwinia carotovora var. atroseptica and Phytophthora infestans

In laboratory tests, proliferation increased the susceptibility of lenticels on mature tubers to infection by Erwinia carotovora var. atroseptica. No differences in susceptibility to E. c. atroseptica and Phytophthora infestans were detected on tubers lifted at different stages of growth from soil kept wet or dry for 3 wk prior to each sampling. As tubers aged, fewer lenticels became infected by either pathogen or were penetrated by fluorescein dye, presumably because cells had been suberized and cork barriers formed.     

Studies on the lenticel development, surface microflora and infection by common scab (Streptomyces scabies) of potato tubers growing in wet and dry soils

The mechanisms of control of potato common scab (Streptomyces scabies) by irrigation were investigated in 5 years by studying the growth and surface microflora of young potato tubers developing in wet or dry soils. Direct examination of the tuber surface by light or scanning electron microscopy showed that the newly formed internodes near the apex, which carry stomata, had a very sparse microflora. In dry soil the older internodes, which carry lenticels at a susceptible stage, were increasingly colonised by actinomycete hyphae and discrete, although sometimes large, bacterial colonies. In wet soil, actinomycetes were rarely seen on tuber surfaces but bacteria were generally scattered over them, differences not always shown by isolation from periderm pieces onto water agar. When dry scab-infested soil was wetted, scabs did not subsequently develop on the two youngest tuber internodes (A-1, A-2), which may be an example of disease escape rather than inherent resistance of the stomata. The scab control achieved in wet soil was probably caused by some form of microbial antagonism, but whether through competition or antibiosis was not established.     

Effects of water potential on the infection of potato tubers by Streptomyces scabies in soil

When soil was maintained at a mean water potential (Slatyer & Taylor, 1960) of the order of -80 J kg^-1 at 25 cm depth throughout the growing period, much infection of potato tubers by Streptomyces scabies occurred, but when soil was irrigated to maintain it at potentials greater than - 13 J kg^-1 at 10 cm depth infection was negligible. Until about 5 weeks after initiation, tubers were very susceptible to infection; irrigation during this period reduced scab considerably, but subsequent irrigation reduced it only slightly. At the low water potential, actinomycete populations on lenticels were high and bacterial populations low; these effects were reversed at the high water potential. Since also actinomycetes were more frequently isolated in the absence than in the presence of bacteria it was deduced that there was an interaction between these two groups. It is suggested that irrigation may decrease the population of S. scabies in tuber lenticels by increasing populations of bacteria antagonistic to it.     

Persistence of Phoma exigua xar. foveata and Polyscytalum pustulans in dry soils from potato stores in relation to reinfection of stocks derived from stem cuttings

The incidence of Phoma exigua var. foveata and Polyscytalum pustulans in dry soil and dust from potato stores was assessed at 10 farms in Scotland producing virus-tested stocks of seed potatoes derived from stem cuttings (VTSC). Samples were collected on three occasions during 1976: during the storage period (February-March), soon after the potatoes were planted (May) and just before the new crop was lifted and stored (August-October). Both pathogens were detected at all three sampling times but P. exigua var. foveata was more frequently detected at the last one than P. pustulans. Soil and dust from various sites within the stores, such as floors, ledges, graders and boxes, were contaminated by these pathogens. Propagules of both pathogens remained airborne for at least 12–17 min after the floors were swept. P. exigua var. foveata remained viable for at least 16 months in dry soil from tuber surfaces stored at 4–6°C. When VTSC tubers were dusted before planting with a dry store soil contaminated by P. exigua var. foveata the gangrene potential of the progeny tubers was much greater than that from untreated tubers or from tubers treated with an uncontaminated soil. The role of inoculum surviving in dry soil is discussed in relation to reinfection of VTSC stocks.     

Factors affecting the field infection of potato tubers of different cultivars by blight (Phytophthora infestans)

The relationships between rain and blight (Phytophthora infestans) were studied in unsprayed crops of cultivars differing widely in foliage and tuber susceptibility. The occasions when tubers were infected depended on rain and not cultivar, but numbers of tubers infected after rain was affected by the blight susceptibility of the cultivar. Infected tubers were first found when less than 5 % (BMS key) of the potato foliage was infected but few fresh infections occurred when 50–75% of the foliage had been destroyed. Some tubers were infected after 8 mm rain (tubers near the surface with even less) but large increases in numbers of tubers infected usually occurred only after 25 mm or more had increased soil moisture to above ‘field capacity’ around the tuber for at least 24 h. The most susceptible cultivars Ulster Ensign and Arran Banner had all plants with some tuber blight, and some plants with all tubers affected and often many lesions per tuber. Cultivars of intermediate susceptibility, King Edward and Up-to-Date, had some plants without blighted tubers, many with a few and very few with all. The more resistant cultivars Majestic and Arran Viking had many plants without infected tubers and many lesions that aborted while still necrotic threads, so that the fungus did not spread. Most infections occurred through tuber eyes, lenticels or sometimes growth cracks. The distribution of blight lesions on tubers differed in the different seasons, for example, lenticels were most commonly infected on Arran Banner and Ulster Ensign and eyes on King Edward, Majestic and Arran Viking. In late or slowly developing attacks, lesions on stems became more numerous and larger than in fast, early attacks and were prolific sources of spores on King Edward and Up-to-Date but not on Majestic and Arran Viking. Because much rain water runs down the stems of Up-to-Date and King Edward, stem lesions can provide an important source of inoculum for tubers.     

Sites of contamination and numbers of Erwinia carotovora present in stored seed potato stocks in Scotland

Forty-eight bulk and crate stored commercial seed potato stocks from the east of Scotland were examined for contamination by Erwinia carotovora in 1967 and 1968. Both E. carotovora var. carotovora and E. carotovora var. atroseptica were present, the latter, the blackleg pathogen, being detected four times more frequently. Most contamination was located on the tuber surface and in the lenticels; it was rare in the vascular ring and the organism was never detected in the cortex. The level of contamination of lenticels was only slightly affected after winter storage, which, however, substantially reduced surface contamination. More than 10³ cells were detected on the surface of tubers in over half the stocks and in half the stocks each lenticel sampled contained more than 10² bacteria.     

Cloned allospecific human helper T cell lines induce an MHC-restricted proliferative response by resting B cells

To analyze helper T (Th) cell-induced B cell proliferation in man, we have cloned allospecific Th cells, grown them as long-term IL 2-dependent T cell lines (TCL), and analyzed their phenotypic and functional properties. The two TCL described in this report, A-7 and A-57, are both composed exclusively of T3+, T4+, T8- T cells blasts. In proliferative assays, with a panel of x-irradiated allogeneic stimulator cells, A-7 was found to proliferate in response to DR3-bearing cells, whereas A-57 responds to DR2-positive stimulators. Both TCL are capable of providing MHC-restricted polyclonal help for allogeneic B cells, as measured in the reverse hemolytic plaque assay. Of greater interest, x-irradiated A-7 and A-57 cells are capable of inducing a proliferative response by allogeneic B cells that is absolutely MHC restricted at the inductive (Th-APC) level. Thus, x-irradiated A-7 cells only trigger proliferation by DR3+ B cells, whereas A-57 cells selectively activate DR2+ B cells. In contrast, after antigen-specific activation, x-irradiated A-7 and A-57 cells can recruit a significant proliferative response by allogeneic B cells bearing "irrelevant" DR antigens. The possibility that Th-induced B cell proliferation may be restricted at the effector (Th-B cell) level was addressed by fractionating B cell populations into "activated" and "resting" subsets by discontinuous Percoll density gradient centrifugation and further purification by employing a monoclonal antibody directed against an antigen expressed on activated B cells (4F2). These studies demonstrate that activated B cells are readily and nonspecifically recruited to proliferate by activated Th cells, whereas optimal proliferative responses by resting B cells require MHC restricted Th-B cell interaction.     

Factors affecting the development of spraing in potato tubers infected with potato mop-top virus

A larger proportion of tubers of Arran Pilot potato growing at the surface of soil infested with potato mop-top virus (PMTV) showed spraing symptoms (brown rings) at harvest than of tubers from below the surface. Infected tubers with or without spraing developed a spraing ring when stored in darkness, first for 1–2 wk at 18 d?C and then for 1–2 wk at any constant temperature between 5 and 13 d?C. Only a faint surface ring developed when either of these periods was decreased to 1 day; 4-day periods were needed to induce distinct symptoms. Internal tuber symptoms developed more slowly than surface symptoms, and their formation was favoured by cutting the tubers in half. Additional pigmented surface rings were produced outside the first ring by successive cycles of treatment at 18 and 9d?. Spraing did not develop when the first stage of treatment was at 22–25d?, when the tubers were kept first at 10d? and then at 5d?, when the treatment at 5–13d? preceded that at 18d?, or when the tubers were kept at constant temperatures ranging from 5 to 25d?. When tubers of six potato varieties were grown in PMTV-infested soil and then stored at temperatures designed to induce symptoms, the varieties known to be the most susceptible in the field were those which had the greatest tendency to develop spraing during storage. When infected tubers were exposed to light, typical spraing symptoms were not induced, but greening of the tuber surface was much delayed in localized ring-shaped areas, so that pale weals appeared. Spraing symptoms were produced, in favourable conditions, by the reaction of cells at the periphery of the PMTV-invaded zone. Internal spraing did not prevent PMTV invading tissue outside the brown arcs; its rate of spread was about 10 μm/h at 14–18d?.     

Salt hardiness and dye reduction by potato tissue and mitochondrial fractions as influenced by previous storage of the tubers

Oxygen uptake and tetrazolium reduction occurred at higher rates in discs from potato tubers (Solanum tuberosum L.) stored at 0° than in discs from tubers stored at 12.8°. Tetrazolium reduction was at a higher rate in mitochondrial fractions from tubers stored at 0° than in mitochondrial fractions from tubers stored at 12.8°. These physiological activities were more resistant to hypertonic KCl treatments in tissue and mitochondrial fractions from tubers stored at 0° than in tissue and mitochondrial fractions from tubers stored at 12.8°. Inhibition of O₂ uptake and tetrazolium reduction progressively increased with increasing concentrations of KCl for tissue and mitochondrial fractions from tubers stored at 0 and 12.8°, but inhibition was more severe and occurred at lower concentrations of KCl for the material from tubers stored at 12.8°. Tissue from tubers stored at 0° was at the same time more sensitive to hypotonic solutions and more resistant to hypertonic solutions than corresponding tissue from tubers stored at 12.8°. Adaptive changes brought on in the tubers by the stress of cold storage were demonstrated in the discs and mitochondrial fractions prepared from cold-stored tubers.     

Factors affecting the occurrence of gangrene (Phoma exigua) in potatoes

The presence in soil from Scotland and England of Phoma exigua f. sp. exigua and P. exigua f.sp. foveata, which cause gangrene, is confirmed by isolation, and it is established that infection of tubers occurs before lifting, and after lifting from soil adhering to tubers. The distribution of the disease is related to soil moisture, gangrene being most prevalent in tubers from the north-eastern counties of Scotland where the moisture content of arable soils remains high throughout the growing season. The incidence of the disease may be affected by the haulm, either as a source of infection—though not an important source—or by its effect on the maturity of the tubers. The incidence of gangrene is less where haulm destruction is rapid. The incidence of gangrene in a crop is not related to its incidence in the seed tubers planted and, unless infection is severe, the effect on yield is not serious. Symptoms of skin necrosis were associated with tubers from acid soils, infected with P. exigua f.sp. foveata and stored at low temperature.     

How past and present influence the foraging of clonal plants?

Clonal plants spreading horizontally and forming a network structure of ramets exhibit complex growth patterns to maximize resource uptake from the environment. They respond to spatial heterogeneity by changing their internode length or branching frequency. Ramets definitively root in the soil but stay interconnected for a varying period of time thus allowing an exchange of spatial and temporal information. We quantified the foraging response of clonal plants depending on the local soil quality sampled by the rooting ramet (i.e. the present information) and the resource variability sampled by the older ramets (i.e. the past information). We demonstrated that two related species, Potentilla reptans and P. anserina, responded similarly to the local quality of their environment by decreasing their internode length in response to nutrient-rich soil. Only P. reptans responded to resource variability by decreasing its internode length. In both species, the experience acquired by older ramets influenced the plastic response of new rooted ramets: the internode length between ramets depended not only on the soil quality locally sampled but also on the soil quality previously sampled by older ramets. We quantified the effect of the information perceived at different time and space on the foraging behavior of clonal plants by showing a non-linear response of the ramet rooting in the soil of a given quality. These data suggest that the decision to grow a stolon or to root a ramet at a given distance from the older ramet results from the integration of the past and present information about the richness and the variability of the environment.     

SOME FACTORS AFFECTING THE OCCURRENCE AND DEVELOPMENT IN POTATOES OF GANGRENE CAUSED BY PHOMA SOLANICOLA PRILL. & DELACR.

It is shown that gangrene is a soil- and haulm-borne disease and that infection of the tubers can take place through cuts, bruises, 'eyes' and lenticels. Low temperature and dry, dark storage were found to be most favourable to development of the disease, but rotting was more severe if the tubers had been kept at a higher temperature before infection took place. Storage at 21° C. for 10 days was sufficient to arrest rotting. Gangrene developed on all the potato varieties tested, but different varieties showed differences in susceptibility to the disease. No difference was observed in the susceptibility of early and late varieties but susceptibility increased with advancing maturity of the tubers.     

Effects of treating seed potatoes from commercial and stem cutting stocks with benomyl, thiabendazole and 2-aminobutane on yield and disease

Potato seed tubers of six cultivars from commercial stocks and from stocks derived from stem cuttings (healthier seed) were fumigated with 2-aminobutane 2 wk after lifting or treated with benomyl or thiabendazole in January. 2-aminobutane prevented skin spot and gangrene developing on treated tubers. Experiments were planted at Rothamsted (clay with flints soil) and at Woburn (sandy loam soil) in 1973–75. Healthier seed produced more stems/plant than commercial stocks and yielded on average 8% more at Rothamsted in 1973 and 1974 and respectively 5 and 10% more at Woburn in 1973 and 1975. Seed treatments did not consistently affect stem numbers or increase yield although all treatments tended to decrease tuber size. Infection of stem bases and tubers by Polyscytalum pustulans and Rhizoctonia solani was usually less from healthier than from commercial seed and was decreased by benomyl and thiabendazole in 1973 and 1974. Infection by Helminthosporium solani of the skin around tuber eyes was greater from healthier than from commercial seed but was decreased by benomyl and thiabendazole. 2-aminobutane sometimes decreased infection of tubers by P. pustulans and R. solani but neither of stem bases nor of tubers by H. solani. Gangrene on tubers uniformly wounded at lifting was not consistently affected by seed source or seed treatment. Treating seed with benomyl or thiabendazole in 1975 decreased skin spot and black scurf in tubers stored until March 1976. These treatments also decreased silver scurf on the produce of commercial seed at Rothamsted but gangrene was not consistently affected by seed treatments.     

Laboratory studies of potato tuber resistance to infection by Phytophthora infestans

Eyes and lenticels on tubers of cvs King Edward, Record, Maris Peer and Majestic gradually became more resistant to infection by Phytophthora infestans as the growing season progressed. Lenticels of Record and Majestic, and eyes of Maris Peer and Majestic were more resistant than those of King Edward. Differences in resistance between cultivars were apparent at all concentrations of inoculum.     

Modification of growth habit of Majestic potato by growth regulators applied at different times

The effect of gibberellic acid, CCC (2-chloroethyltrimethylammonium chloride) and B 9 (N-dimethylaminosuccinamic acid) was studied on growth of potato plants in pots. Growth was analysed on four occasions and changes in habit defined in terms of internode lengths, leaf areas and growth of lateral branches. Soaking seed pieces for 1 hr. in GA solution caused six internodes to elongate greatly, an effect not prevented by CCC applied when the shoot emerged from the soil. The effects on internode extension were determined by the length of the interval between GA treatment and CCC treatment. Treatment at emergence with CCC shortened all internodes and more CCC applied 4 weeks later had no effect. Late application of CCC or B9 shortened the top two lateral branches, usually very long in untreated plants. The regulators affected leaf growth differently from internode growth: usually growth regulators had less effect on leaf growth. Effects on growth depended on when the regulators were applied. Treatment with GA alone inhibited bud development at higher nodes than in untreated plants; when followed by late treatment with CCC, lateral growth at higher nodes was also less. CCC retarded development of lateral branches especially when applied early. B9 had a similar effect to CCC applied late. When regulators retarded growth of lower laterals, upper laterals often grew more than in untreated plants. Treatments did not affect the number of leaves on the main stem at first but later GA hastened senescence. GA increased the number of leaves on lateral stems, and the effect was enhanced by CCC. CCC alone increased the number of first- and second-order lateral leaves. GA lengthened and CCC shortened stolons. The effect of CCC persisted throughout the life of the plant. CCC or B 9 shortened stolons whenever applied. CCC hastened tuber initiation but slowed tuber growth. CCC at first retarded formation of lateral tubers but had no effect on the ultimate numbers of lateral and terminal tubers. The value of E (net assimilation rate) did not alter with time. CCC applied at emergence increased E, probably because it hastened tuber initiation and temporarily increased sink capacity. Although tubers formed earlier with CCC, their growth was slower and their demand for carbohydrate was also less. The increase in second-order laterals in CCC-treated plants indicates that they utilize carbohydrate that would normally go to tubers. This experiment also demonstrates that crowding leaves by shortening stems did not diminish E, possibly because another over-riding process (bigger sinks) offsets the effect of shading.     

Soaking of seed potatoes in phosphate solution to economise on the fertilizer input

Summary The effect of soaking seed tubers in P solution on tuber yield, P uptake and fertilizer economy was investigated on acid brown hill soil and alluvial soil from 1972 through 1975. Nondormant whole seed tubers soaked in 0.4 percent P solution of mono- and diammonium phosphate and of filtered superphosphate increased tuber yield and P uptake. The effect of the soaking on increasing the P composition of seed tubers was most pronounced in the case of monoammonium phosphates solution. Cut seed tuber pieces absorbed more P than the whole seed tubers. On highly P-deficient acid brown hill soil, soaking affored to economise on P application by about 25 kg P₂O₅/ha, applied to soil whereas on marginally deficient alluvial soil, a soaking treatment was adequate and obviated the need of soil application of P.     

The effect of disinfecting and bruising seed potatoes on the incidence of dry rot (Fusarium caeruleum (Lib.) Sacc.)

The effect of bruising and of disinfecting ('dipping') seed potatoes with a proprietary organo-mercury preparation on the incidence of dry rot in them was tested in field trials during three seasons. The tubers used were of the susceptible variety Ninetyfold, taken from crops grown in contaminated soil, harvested immature in July to early August each year under farm conditions, and stored in boxes.
Seed tubers not deliberately bruised, whether dipped or not at lifting time, remained practically sound until planting time in the fallowing season, if left undisturbed in their boxes.
Tubers deliberately bruised, either at digging time or 1-2 weeks later, but not dipped, developed severe dry rot with few exceptions. The disease had run its course by mid-October. When undipped, sound tubers were bruised in October, they contracted severe dry rot, but dipping such tubers immediately before bruising reduced the loss satisfactorily in five out of six trials.
Tubers bruised at digging time and immediately dipped suffered little from dry rot in almost all cases. Delayed dipping of bruised tubers checked the disease in some trials but not in others. Seed tubers severely bruised 1-2 weeks after being dipped remained practically sound except in one instance, whereas tubers severely bruised approximately 3 months after being dipped, subsequently developed severe dry rot in four out of six tests, unless they had been redipped immediately before they were bruised.
Inoculation of healthy tubers with soil samples showed that the fungus is widely distributed in potato fields in Cheshire. Dipping killed all, or almost all, of the fungus in the soil adhering to the seed tubers.
The results are discussed and suggestions are made for further investigations and for practical control measures.     

Dry rot of potato [Fusarium caeruleum (Lib.) Sacc.]. Investigation on the sources and time of infection

Inoculation with soil samples proved that the fungus causing dry rot is frequently present in field soils in Cheshire and in soil adhering to imported seed tubers. The fungus was viable in soils having a wide range of p H values and in fields which had not grown potatoes for 5-6 years. Other sources of infection include lofts, used sacks, seed boxes, diseased tubers, and knives used for cutting seed potatoes.
Bruised tubers stored in heavily contaminated boxes developed much dry rot; far less disease occurred in unbruised tubers. In boxes containing own-saved seed, healthy tubers in contact with diseased ones remained sound. Bruised tubers in contact with, or contaminated by, diseased tubers contracted dry rot. Cutting seed with a contaminated knife increased the disease seven-fold.
Inoculation of tubers attached to the parent plant showed that little or no infection occurred before lifting. In field trials severe dry rot developed in several varieties 6-8 weeks after lifting.
The results are discussed in relation to seed treatment.     

Treatment of seed and ware potato tubers with thiabendazole for control of storage diseases

In March 1977 and 1978 King Edward seed potatoes from three commercial stocks and one stock derived from stem cuttings (healthier seed) were treated with thiabendazole at 320 g a.i./t and 46% of the material applied was deposited on tubers. Seed was stored on trays to sprout and in April samples were planted in replicated plots on two farms in Lincolnshire. Treatment did not consistently affect numbers of stemslplant or total yields but decreased infection of underground stems by Polyscytalum pustulans and Rhizoctonia solani and of tubers at harvest by P. pustulans, R. solani and Helminthosporium solani. In samples stored for 20 wk at Sutton Bridge Experimental Station seed treatment decreased the incidence of skin spot from 25 to 4%, of black scurf from 36 to 20% but had less effect on silver scurf (untreated 52% tubers, treated 47%) and did not affect the incidence of black dot (Colletotrichum coccodes) on stored tubers. In another series of experiments 1 t samples of seed from six King Edward stocks (1977) and four King Edward and one Maris Piper stocks (1978), collected from different farms in Lincolnshire in March, were treated with thiabendazole at 40 g a.i./t. Tubers retained 48% of the material applied. The treated seed was planted on the respective farms in a single large plot adjacent to untreated seed similarly stored and sprouted. Seed treatment significantly decreased the incidence of skin spot, black scurf, silver scurf and gangrene in tubers stored at Sutton Bridge in 1977–78. After lifting the experiment in 1978 samples from treated and untreated seed were treated with thiabendazole (40 g a.i./t) before storage (ware treatment). Skin spot was decreased more by seed treatment than ware treatment whereas silver scurf was controlled best by ware treatment. Incidence of black dot was not consistently affected by seed or ware treatment.