Infection of potato stems by the gangrene pathogen, Phoma exigua var. foveata

Limited transport of pycnidiospores of Phoma exigua var. foveata within transpiring stems was demonstrated by isolating from cut mature stems placed for 24 h in a spore suspension. Pieces of shoot dipped in spore suspension and subsequently rooted contained the pathogen which was redistributed as the plants grew but without causing symptoms until after desiccation. The fungus was only rarely detected within stems growing from rotted seed tubers suggesting that spore transport in the vascular system was relatively unimportant in these experiments. Stems were readily infected from spore suspensions applied to unwounded epidermis especially when the inoculum was held in cotton wool and secured with wax film. The progressive colonisation of growing stems was demonstrated by isolations made at different positions on the stem and at different times after inoculation. In field experiments in two years stems were inoculated with four spore concentrations on four dates from late June to late August. After desiccation the extent of pycnidial lesions was greatest from the earliest inoculation date and the largest spore concentration and these differences were reflected subsequently in the incidence of gangrene on damaged progeny tubers. Younger stems may be more susceptible and infection in early summer probably results in extensive growth within the stem before desiccation and thus in massive pycnidial production.     

The significance of tuber damage and inoculum concentration of Phoma exigua var. foveata in the development of gangrene in stored potato tubers

The susceptibility to gangrene infection of wounds of various shapes and depths on potato tubers was studied by inflicting wounds using differently-shaped brass teeth and rods of different diameters. Inoculating wounds with spore suspensions or damaging tubers which had been previously contaminated with Phoma exigua var. foveata or which had been recently lifted from plots of field experiments showed that wounds in which tissue was crushed were most susceptible to infection. Over a wide range of inoculum concentrations and in experiments using several different cultivars the incidence of infection of any wound type was compared to that of the standard severe cut and crush wound. Using a probit transformation a linear relationship was established, the slope of the line indicating the relative susceptibility of the wound. In 1977 and 1978, crops of cv. Pentland Dell were surveyed for damage incidence, inoculum and inoculum potential on arrival at a commercial bulk store. Nets of tubers buried among the tuber bulk were recovered after storage and gangrene incidence compared with damage and inoculum assessments. Inoculum potential and incidence of severe damage both influenced disease development but damage incidence was of greater importance, showing that priority should be given to decreasing damage and to curing to promote rapid wound healing in endeavours to control the disease.     

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.     

The influence of potato harvesting and grading machinery on contact spread of Phoma exigua var. foveata on tubers

The incidence of wounds infected by Phoma exigua var. foveata was increased if freshly damaged tubers (recipients) were shaken in a bag with diseased tubers (donors) to simulate the tuber-to-tuber contact that occurs during potato handling. An increase in the number of gangrene rots on damage points also occurred if the recipient tubers were wounded after contact with the diseased tubers, rather than before, and when the donor tubers were heavily infested with P. exigua var. foveata but were free of gangrene lesions. Increasing the proportion of donor to recipient tubers increased the percentage of infected wounds on recipients. Increased incidences of infection in recipient tubers also occurred after they had been passed over an elevator digger when it was lifting stocks of tubers heavily infested with P. exigua var. foveata. When spores of an E +ve isolate of P. exigua var. foveata were sprayed onto the webs of manned potato harvesters, tubers harvested immediately after developed gangrene rots from many of which the E +ve isolate was cultured. An E +ve isolate was also used to demonstrate the transfer of P. exigua var. foveata inoculum from tubers onto soil on riddles of a potato grader and from these soil-coated surfaces onto other tubers during grading.     

Factors affecting the assessment of resistance of potatoes to gangrene (Phoma exigua var. foveata)

Three experiments designed to elucidate the effects and interactions of different inoculation methods, inoculation positions, strains of Phoma exigua var. foveata and clones of potato tubers inoculated were analysed using as data dimensions of the lesions and a more general surface score. The assessment of susceptibility was affected by all these factors. The ranking of varieties for resistance altered considerably depending upon whether the surface area or the depth of lesions was considered. Derivation of more complex scores combining these measurements reduced the relative genetic component and hence would be of no value to potato breeders.     

The effect of time of planting inoculated tubers on the incidence of potato blackleg and gangrene

In a series of trials in the years of 1973-76 inclusive, chitted seed tubers of potato cvs Ulster Sceptre, Majestic and Pentland Crown were immersed immediately before planting, on three dates in each year, in water, suspensions of Erwinia carotovora var. atroseptica, Phoma exigua var. foveata or a mixture of both. The development of blackleg was greatest in cv. Ulster Sceptre and most rapid after late planting. Its final incidence was not closely related to date of planting except in cv. Pentland Crown which was least affected when planted late.
Combined inoculum of P. exigua var. foveata and E. carotovora var. atroseptica increased five-fold the number of plants that failed to emerge and often retarded early growth of the remainder. Failure to emerge was more frequent the earlier seed tubers were planted. Yield was affected most by blanking and blackleg in the Erwinia- inoculated plots and was also reduced by the pre-plant dip in a P. exigua var. foveata suspension, especially after early planting.
Seed dipped in the mixed suspensions yielded progeny that after wounding and cool storage developed a lower incidence of gangrene than progeny from seed dipped in P. exigua var. foveata alone. There was no evidence that planting time influenced the incidence of gangrene during storage.     

Contamination of seed potato tubers grown from stem cutting or microplant-derived tubers by Phoma foveata and Polyscytalum pustulans in Scotland: the influence of site and fungicide treatment at harvest

A survey was made over 5 years of the incidence of contamination by Phoma foveata and Polyscytalum pustulans on 3-year-old virus-tested stem cutting (VTSC) stocks assessed on three occasions during the storage season, and on their daughter tubers at five farms. The contamination occurring on plants grown in small plots from stem-cutting/microplant tubers was also assessed at the farms.
Fungicide treatment soon after harvest reduced the incidence of both pathogens, especially P. pustulans . Tuber contamination was similar for samples collected after fungicide treatment and before planting. The incidence of contamination on daughter tubers, relative to that on the untreated mother tubers, was less with P. foveata but was similar or greater with P. pustulans . Contamination by both pathogens varied between farms and was reflected in the degree of contamination occurring on daughter tubers from stem-cutting/microplant tubers.
The incidence of P. foveata on stems, soil and tubers of seed stocks classified at Foundation Stock grade from various regions of Scotland was assessed over 2 years. The incidence of tuber contamination was correlated ( P < 0.05) with the amount of tuber-borne soil contamination for all comparisons, but was correlated with stem inoculum in only 7 out of 13 comparisons. The largest amounts of stem inoculum of P. foveata were found in stocks in Aberdeenshire.
These results are discussed in relation to the spread of both pathogens to healthy VTSC stocks.     

The susceptibility of tubers of potato cultivars to black dot (Colletotrichum coccodes (Walk.) Hughes)

In experiments with commercial seed of different cultivars at Rothamsted and Woburn, Bedfordshire in 1985 – 88 the severity of black dot on daughter tubers at harvest differed between cultivars. The disease was most severe on Desiree tubers. Amounts of disease were similar at both sites in 1986 – 88 but in 1985 it was more severe at Woburn than at Rothamsted. Disease-free seed of 12 (1987) or 15 (1988) cultivars were planted in experiments at Rothamsted (inoculated with Colletotrichum coccodes or not) and at Mepal, Cambridgeshire (not inoculated) and black dot assessed at harvest in October 1987 and in September and October 1988. There were significant differences in the amount of disease on different cultivars and the order of severity was similar at the two sites, on the two harvest dates in 1988 and in both years. Desiree, Maris Piper, Maris Peer and Record were amongst those cultivars severely affected whereas Cara, Pentland Crown and Romano were least affected. Skin discoloration caused by black dot was more noticeable on white-skinned than red-skinned cultivars and was severe on the Dutch cultivars Estima, Marfona, Santé and Wilja.     

The effect of temperature and duration of exposure of potato tuber moth (Lepidoptera: Gelechiidae) in infested tubers to the biofumigant fungus Muscodor albus

The endophytic fungus, Muscodor albus produces several volatile compounds (alcohols, esters, ketones, acids and lipids) that are biocidal for a range of organisms including plant pathogenic bacteria and fungi, nematodes and insects. We studied the effects of these volatiles on 3-day-old potato tuber moth larvae within infested tubers inside sealed chambers. The length of exposure to M. albus significantly affected mortality of larvae, calculated as percentage of larvae failing to survive to the adult stage. Exposure durations of 3, 7, or 14 days at 24 degrees C followed by incubation in fresh air at 27 degrees C until emergence resulted in mortalities of 84.2, 95.5 and 99.6%, respectively. However, the longer exposures also resulted in increased levels of carbon dioxide (CO(2)) that are unacceptable for tuber storage. Effects of M. albus on larval survival was also monitored at 10, 15 and 24 degrees C, using an exposure duration of 7 days followed by incubation in clean air at 27 degrees C until emergence. Mortality of larvae was sharply reduced at the lower temperatures resulting in 50.8, 76.8, and 95.4% mortality, respectively. Tuber storage conditions, especially cooling rates, are discussed with respect to using M. albus as a fumigant without simultaneously producing unacceptable (for tuber storage) levels of CO(2).     

The effect of soil- and tuber-borne inoculum on the incidence of potato gangrene

Under optimum growing conditions neither tuber- nor soil-borne Phoma exigua var. foveata inoculum appreciably affected stand or yield of the subsequent potato crop. Seed tubers with gangrene rots caused high levels of stem and tuber symptoms when planted in var. foveata contaminated or uncontaminated land; contaminated seed tubers with no rots also produced progeny with a high gangrene potential. Sufficient soil-borne inoculum was carried over in land that produced a gangrene affected crop in the previous year to override the effect of tuber disinfection. Effective gangrene control was achieved by a combination of tuber disinfection shortly after harvest over successive years with a 1 in 5 yr potato crop rotation. Gangrene rots usually developed through injuries to the tuber periderm, rots in other tubers being associated with pustules of powdery scab (Spon-gospora subterranea).     

Effect of neighbouring plants on the yield of potatoes from seed tubers affected with gangrene (Phoma foveata) or from plants affected with stem canker (Rhizoctonia solani)

Healthy seed tubers and seed tubers with gangrene or inoculated with Rhizoctonia solani to induce stem canker were planted 38 or 76 cm apart or alternately at 38 cm spacing in 2-row (1985) or 4-row (1986) plots at Rothamsted.
At 38 cm spacing, stem canker decreased yield by 5% and 8% respectively in 1985 and 1986 and gangrene by 5% and 14%, but usually by twice these amounts when seed was spaced at 76 cm. In all experiments, yields from plots with alternate healthy and inoculated seed (50% disease) were similar to yields from healthy seed, although within these mixed populations individual plant yields from healthy seed were larger and those from diseased seed were smaller than from plots of healthy or inoculated seed respectively. Samples in August showed a similar effect on fresh weight of foliage.
Disease usually decreased the number of stemdplant but in 1985 gangrene increased stem numbers and probably accounted for small amounts of compensatory growth from neighbouring healthy plants.     

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.     

The effect of carnosine on the healing of a lung wound

The influence of carnosine (beta-alanine, alpha-histidine) on the process of lung wound reparation was studied in 90 guinea-pigs. Its efficacy was evaluated microscopically, histologically and by means of electronic microscope. It was established that carnosine, as compared to controls nearly twice accelerates reparative processes in the injured lung by activation of fibroblast proliferation, connective tissue generation and intracellular regeneration. In type II epithelial cells more intensive formation of osmiophilic bodies and lamellar bodies contained in them is observed. The alveolus formation in the wound lips by the 7-8th day after lung injury is going on under the carnosine influence, which is due, probably, to massive excretion of osmiophilic contents from type II epithelial cells and surfactant production. This may be connected with rapid decrease of atelectasis and rapid restoration of lung airness within the wound.     

The tetrachlorodecaoxygen complex reverses the effect of cortisone on wound healing

Immunosuppression induced by the administration of glucocorticoids will prevent normal wound contraction and normal increases in tensile strength. Vitamin A, anabolic steroids, and growth hormone will, in the presence of glucocorticoids, restore mesenchymal cell proliferation, the accumulation of collagen, and the rate of increase of wound tensile strength. They will not, however, antagonize the inhibition of wound contraction. A novel inorganic agent, the tetrachlorodecaoxygen anion complex (TCDO), known to enhance the migration and activation of macrophages, was tested in a rat model of impaired wound healing using high doses of glucocorticoids. Histology, changes in wound contraction, collagen synthesis, and tensile strength were evaluated. Animals receiving cortisone in combination with TCDO displayed markedly enhanced wound healing, including restoration of tensile strength, collagen synthesis, and wound contraction. The results indicate that TCDO could be a potential agent of wound healing in immunosuppressed patients and anergic wounds.     

The incidence of gangrene of potatoes caused by Photna exigua in relation to handling and storage

Damage to potatoes during harvesting and grading increases the incidence of gangrene. Among the factors involved, the condition of the soil and the type of damage caused by the implements may be of importance. Gangrene is most evident in tubers which have been stored cold or moved when cold; warmer storage at some stage usually reduces its incidence and severity. Seasonal incidence is not directly related to rainfall but is determined by factors affecting ripening and destruction of haulm, moisture status of soils at harvest, susceptibility to damage, and exposure during storage and in transit.     

Root-surface mycoflora of cassava (Manihot esculenta) and post harvest rot of the tubers

The root-surface mycoflora of cassava were isolated from roots washed in serial changes of sterile distilled water and plated out on potato-dextrose agar. A small group of fungi which included Aspergillus niger, Botryodiplodia theobromae, Fusarium solani, Penicillium javanicum, Penicillium sp., and Trichoderma sp. were found to be consistently associated with the root surface. While the isolates, B. theobromae and F. solani were found to be aggressively pathogenic on freshly harvested cassava tubers causing extensive rot, A. niger was only mildly so. The root-surface mycoflora, therefore, includes fungi which have been reported as the most important in postharvest deterioration of the tubers. The removal of the rhizoplane microflora by surface-sterilization using calcium hypochlorite or Clorox and subsequent incubation in loosely tied polyethylene bags extended the storage life of the tubers considerably.     

Effects of fungicides applied to seed potato tubers at harvest or after grading on fungal storage diseases and plant development

An assessment was made over 3 years of the efficacy of thiabendazole, 2–aminobutane and a formulation of a mixture of thiabendazole and 2–aminobutane in controlling gangrene, skin spot and dry rot when applied either at harvest or after grading at different dates during the storage season.
After harvest applications, the incidence of gangrene was least on tubers treated with the mixture and generally greatest with thiabendazole. When applied after grading the efficacy of the fungicides was not affected by date of grading. Although 2–aminobutane applications at harvest or after grading were equally effective in controlling gangrene, thiabendazole and the mixture were less effective after grading where gangrene was prevalent in a stock. Thiabendazole and the mixture gave good control of dry rot when applied at harvest but were less effective when applied after grading, particularly late in the storage season. Dry rot was sometimes more prevalent on tubers fumigated with 2–aminobutane, especially those treated after grading. Skin spot was more effectively controlled by the mixture and 2–aminobutane than by thiabendazole but, with applications after grading, the efficacy of all fungicides declined the later the date of grading.
Plants grown from tubers treated with the mixture or its 2–aminobutane glycollate constituent exhibited characteristics associated with genetic variation.     

The effect of plant density, harvest date and method on the yield of seed and components of yield of parsnip (Pastinaca sativa)

Experiments conducted over four years (1980–1983) with parsnip cv. Avonre-sister examined the effects of plant density, harvest date and method on seed yield and the components of yield. In 1980, using the root-to-seed method, the yield of seed increased by 50% for an increase in plant density from 5.6 to 35 plants m^-2; the maximum yield at the optimum harvest date was 3015 kg h^-1. In the other three years, using the seed-to-seed method, seed yield increased to a maximum with an increase in plant density from c. 1 to 10 plants m^-2 but at higher densities yields were lower. At c. 10 plants m^-2, seed yields were 1567, 4029 and 1040 kg ha^-1, in 1981, 1982 and 1983, respectively, when harvested at the optimum time. An increase in plant density increased the number of non-seeding plants in the population, reduced the number of umbels per plant and reduced the number of seeds per umbel. Mean seed weight was, in general, unaffected by plant density and so effects of density on yield reflected changes in seed numbers. Increasing the plant density increased the proportion of primary-umbel seed. Maximum yields of seed were obtained from crops harvested at a seed-moisture content of 50 to 70%, which occurred 46 ± 2·4 days after flowering. Delay in harvesting after this date led to a loss of yield of 33, 35, 139 and 32 kg ha^-1wk^-1 in 1980, 1981, 1982 and 1983, respectively. Plants cut, placed in the windrow to dry and then threshed gave similar yields to those harvested at the same time but dried in bins.