The influence of cultivar and isolate on the development of gangrene (Phoma exigua var. foveata) in potato tubers

In studies on the infection of potato cultivars with different field isolates of Phoma exigua var. foveata, cultivar × isolate interactions, although sufficiently large to influence the relative pathogenicities of isolates on different cultivars, were not substantial enough to affect cultivar ranking order. Cultivar rank was markedly influenced by pathogen isolate only when both field and culture collection isolates were compared. This suggests that the complications of cultivar × isolate interactions can be avoided in cultivar screening tests by the use of recent field isolates of high pathogenicity. It was found that cultivar was considerably more important than isolate in determining gangrene lesion size. The need to consider both lesion diameter and depth when estimating rot size following point inoculation of different cultivars was confirmed.     

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.     

Catalase inhibition alters suberization and wound healing in potato (Solanum tuberosum) tubers

In response to wounding, potato ( Solanum tuberosum L.) tubers generate hydrogen peroxide (H₂O₂) in association with suberization, a critical phase of the wound-healing process. In the present study, the effect of aminotriazole (AT), a catalase (CAT, EC 1.11.1.6) inhibitor, on cut tubers was investigated using fresh weight (FW) loss and pathogen attack symptoms as indicators of wound-healing efficiency. Seven days after treatment, AT-treated tuber halves lost more FW and developed infection signs compared with the controls. Thiourea, another CAT inhibitor, as well as exogenous H₂O₂ treatments induced the same effects as AT suggesting that the alteration of the wound healing may be caused by CAT inhibition and the resulting accumulation of H₂O₂. Using transgenic tubers, FW losses 1 week after wounding were either higher (CAT repression) or lower (CAT overexpression) than those of the wild-type. When tuber halves were allowed to wound heal for different periods before treatment, AT had no effect on the progress of their wound healing if wound-healed for at least 3 days. This implies that AT may affect early wound-healing-related events, especially those occurring before or during suberization. A time-course analysis of the effects of AT treatment on wounded tuber tissues revealed that AT prevented the deposition of the polyphenolic domain of suberin in association with CAT inhibition and H₂O₂ accumulation. These data are important in identifying factors that may be required to regulate suberization and contribute to a better understanding of this critical process to hasten its rate and limit wound-related losses in stored potato tubers.     

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 date of haulm destruction and harvest on the development of dry rot caused by Fusarium solani var. coeruleum on potato tubers

Over 3 yr, the development of dry rot, caused by Fusarium solani var. coeruleum, and the efficacy of treating potato tubers with imazalil was examined in relation to the date of haulm destruction and harvest, and the interval between haulm destruction using diquat dibromide and harvest. The effect of these factors on skin set was also assessed. Planting inoculated seed tubers bearing small rots produced more dry rot on the daughter tubers than planting naturally contaminated seed tubers in 1992. The incidence of dry rot was higher on daughter tubers wounded by a standardised method than on those passed over a reciprocating riddle (riddling). The relationship between the two assessment methods was significantly (P<0.05) correlated in 2 out of 3 yr. Overall, the interval between haulm destruction and harvest had less effect on the incidence of dry rot on daughter tubers after riddling than the date of harvest. In 2 out of 3 yr, the incidence of dry rot on riddled tubers was least on those harvested in August and was much greater on September‐harvested tubers. On tubers harvested in October, the pattern was variable, with the incidence declining in 1 yr but increasing in the other. In the third year, the development of dry rot was similar on August‐ and September‐harvested tubers and was least on those harvested in October. Dipping tubers in imazalil gave significant reductions in dry rot although the amount of the reduction was variable and not affected by time of treatment or the amount of fungicide deposited within the range 6.7 to 19.4 mg imazalil kg^?1. Skin set at harvest, as measured by skin strength or the amount of scuff damage, increased the later the tubers were harvested but did not appear to be affected by the interval between haulm destruction and harvest.     

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.     

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.     

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 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).     

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.     

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.     

Control of potato gangrene by thiabendazole in relation to time of harvest after haulm destruction and site of production

The efficiency of thiabendazole in controlling gangrene on potato tubers harvested on different dates was assessed over two years in Scotland (East Craigs) and England (Rothamsted). At East Craigs the incidence of gangrene on wounded and cold-stored tubers was greater than on those tubers kept at ambient store temperatures and graded in January, but the assessments were correlated. The incidence of gangrene increased with delay in harvest after haulm destruction although the pattern differed with year and site. Dipping tubers in thiabendazole or sodium hypochlorite almost eliminated gangrene on all harvest dates at Rothamsted but at East Craigs the incidence of gangrene on dipped tubers was much greater and tended to increase the longer the interval between haulm destruction and harvest. Furthermore, dipping tubers in thiabendazole was much more effective than spraying even though the fungicide residues were twice as large with the latter treatment. All 43 isolates of Phoma foveata collected in Scotland were sensitive to thiabendazole in agar plate tests. The results are discussed in relation to disease development at each of the sites.     

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 harvest date and harvest method on the combustion characteristics of Miscanthus × giganteus

Miscanthus × giganteus is an energy crop with many attributes that make it a potential biofuel feedstock. This study examined the chemical composition of M. × giganteus stems cut at different dates throughout the spring harvest window (January, February and March) and either left in a swath or left flat in a thin layer on the ground and compared the composition to that of the standing crop collected on the same date in April (control). The research then examined the effect of cutting date on the chemical composition of whole plant M. × giganteus biomass (leaf and stem). The parameters examined in both parts of this experiment were lower heating value on a wet basis (LHV_WB), ash, chlorine, potassium, nitrogen, sulphur, carbon and hydrogen content. The range of values recorded for the parameters from both aspects of this trial were LHV_WB 4.84–11.87 MJ kg^?1; ash 1.44–1.97%; Cl 0.07–0.23%; K 0.15–0.32%; N 0.28–0.39%; S 0.13–0.19%; C 46.75–50.00%; H 5.76–6.09%. The length of time that the M. × giganteus remained in the field after cutting affected the LHV_WB (increased with time) of the stem biomass material. Cutting the biomass and leaving it in the field lowered the ash, Cl and C content of the stem material compared to that of the control which was cut and collected on the same date. No differences were observed for the other parameters. Date of harvest affected the LHV_WB, Cl and C content which all improved with later cutting dates. Thus, combustion quality can be improved by delaying the harvest date or by cutting the crop and leaving it in the field for a period prior to collection. Choosing the correct combination of time and harvest method can therefore improve biomass fuel quality.     

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.