Polyscytalum Pustulans Research Articles

Occurrence of Skin Blemish Diseases (Scab and Scurf) in Norwegian Potato Production

A survey of the prevalence of skin blemish diseases in potatoes after the growing seasons of 2008 and 2009 was carried out on 247 potato lots representing different cultivars and production regions in Norway. The results showed the presence of silver scurf (Helminthosporium solani) in all lots. Skin spot (Polyscytalum pustulans) and black scurf (Rhizoctonia solani) were found in 80% of the lots, and black dot (Colletotrichum coccodes) and common scab caused by Streptomyces spp. were present in 50–70%. Also, powdery scab (Spongospora subterranea) occurred in 65–80% of the lots, and root-lesion nematodes (Pratylenchus spp.) were detected in 60% of the sub-samples that exhibited symptoms of common scab.

A Quantitative Polymerase Chain Reaction Assay for the Detection of Polyscytalum pustulans, the Cause of Skin Spot Disease of Potato

AbstractSkin spot disease of potato caused by the pathogen Polyscytalum pustulans is likely to become more important with the withdrawal of 2‐aminobutane as a fungicide, and new methods of control will need to be found. As part of a disease control strategy, it will be necessary to study the disease in more detail, to utilize host resistance and to identify stocks where problems are likely to arise. Existing methods for the detection and quantification of P. pustulans are time‐consuming and require specific expertise. Real‐time PCR assays have been developed for many pathogens of potato and have subsequently been used as tools for the study of the epidemiology and control of disease. The development of a real‐time PCR assay for the detection and quantification of P. pustulans is described. The specificity of the assay was demonstrated and detection was shown to be reliable at levels as low as 20–250 fg/μl DNA, (equivalent to 60–680 pg DNA/g) in soil and on symptomless tubers at attogram (ag) levels. These values are in line with previously developed tests.

Effects of Fungicide and Rate of Application on the Development of Isolates of Polyscytalum pustulans Resistant to Thiabendazole and on the Control of Skin Spot

Resistance to thiabendazole, sometimes of varying degrees, has occurred in a number of fungal species causing diseases of potato tubers, including skin spot caused by Polyscytalum pustulans. The effect of applying a range of concentrations of thiabendazole, imazalil or a formulated mixture of thiabendazole and imazalil on the development of isolates of P. pustulans partially or fully resistant to thiabendazole and on the control of skin spot over a number of successive generations of seed potato multiplication was examined in a series of experiments over 3 years. Seed tubers were inoculated with a sensitive, partially resistant or fully resistant isolate in the first year of an experiment and combinations of fungicide and a range of concentrations were applied to the seed tubers annually before planting over two or three successive generations. Immersing seed tubers inoculated with the partially resistant isolate in a relatively high concentration of thiabendazole, 0.7 g a.i. l-1, did not result in the development of fully resistant isolates. Moreover, the effect of all fungicides in controlling eye contamination and skin spot did not differ between partially and fully resistant isolates. For seed tubers inoculated with the sensitive isolate, applying thiabendazole resulted in the occurrence of resistant isolates on first-generation daughter tubers, but the effect of fungicide concentration on the frequency of resistant isolates, partially or fully resistant, was inconsistent. In one experiment, the highest frequency of resistance was found at the greatest concentration, 0.4 g a.i. l-1, but, in the second experiment, the frequency was unaffected by the concentration of thiabendazole. On second-generation daughter tubers derived from thiabendazole-treated seed tubers, virtually all isolates were resistant and the frequency of partially and fully resistant isolates was similar to that in the first generation. Applying a mixture of thiabendazole and imazalil to seed tubers inoculated with a sensitive isolate resulted in the development of some resistant isolates in the first year but the frequency did not generally increase with successive applications of the mixture. A mixture of thiabendazole and imazalil, and imazalil alone gave similar control of eye contamination when applied at equivalent concentrations. However, increasing the concentration of imazalil from 0.1 to 0.7 g imazalil l-1 did result in an increasing impairment of plant growth, but the deposits achieved within that range of concentrations were greater than that recommended by the manufacturer. These results demonstrate that control of P. pustulans by thiabendazole, imazalil and a mixture of thiabendazole and imazalil applied at a range of concentrations was similar for isolates partially or fully resistant to thiabendazole.

Pathogen resistance in aerial tubers of potato cultivars

Increases in resistance of aerial tubers (cultivars Désirée, Epicure and King Edward) against nine pathogenic potato fungi were determined in vitro using freeze‐dried powder derived from subterranean and aerial tubers as a culture medium, and in vivo by inoculating both tuber types with the storage pathogens Fusarium sulphureum,F. solani var. coeruleum,Phoma exigua var. foveata and Polyscytalum pustulans. Significantly reduced growth rates and spore and conidiophore numbers were recorded when pathogens were cultured on freeze‐dried powder derived from aerial tubers. No effects on spore size of F. solani var. coeruleum were recorded but spore and conidiophore lengths of remaining pathogens were reduced and width increased, except for Colletotrichum coccodes, where the diameter of acervuli increased, and P. exigua var. foveata, where spore length and width were reduced. Reduced disease incidence, severity and spore numbers were recorded on aerial compared with subterranean tubers inoculated in vivo with the four storage pathogens. Spore length of P. pustulans was reduced and spore width increased, while spore length and width of P. exigua var. foveata were reduced on aerial compared with subterranean tubers inoculated with these pathogens. No effects on spore size of Fusarium spp. were recorded. Concentrations of glycoalkaloids, chlorogenic acid, calcium and magnesium were higher in aerial than in subterranean tuber tissue, which may have contributed to increased resistance and effects on size and morphology of fungal spores. The implications of using aerial tubers as an alternative technique for the production of seed potatoes and their importance to the potato industry are discussed.

Assessing progenies of potato for resistance to skin spot (Polyscytalum pustulans) in the glasshouse

Crosses were made between six parents differing in susceptibility to skin spot and glasshouse-grown tubers from true seedlings of the nine resulting progenies assessed for resistance to the disease. The assessment was evaluated by a two-year comparison with the behaviour of field-grown tubers of the progenies and their parents. The correlations between progenies in the glasshouse and field were r=0.911 and 0.753 and those between the mid-parental values and the progeny means were high (r>0.864), indicating that parental differences were heritable. It is concluded that correlations were sufficient for glasshouse-grown tubers to be used for assessing large numbers of progenies in genetical studies and for eliminating highly susceptible progenies at an early stage in a breeding programme.

The occurrence of thiabendazole‐resistant isolates of Polyscytalum pustulans and Helminthosporium solani on seed potato tubers in relation to fungicide treatment and storage

Tubers infected with thiabendazole‐sensitive isolates of Polyscytalum pustulans and Helminthosporium solani were treated annually for 4 years with benomyl, thiabendazole or a formulated mixture of thiabendazole and imazalil, and grown at three farms in Scotland. The proportion of thiabendazole‐resistant isolates of H. solani increased, and isolates producing black colonies became more common with successive annual applications of thiabendazole or benomyl. Silver scurf was not reduced after three annual applications. When these fungicides were applied once to untreated seed the incidence of resistant isolates of H. solani was much less in 1988 than in 1991 when tubers had been grown on farms for 3 years from untreated seed.The proportion of resistant isolates of P. pustulans increased with the number of successive applications of thiabendazole or benomyl but at differing rates on each farm. At one farm, skin spot was not reduced by three annual applications of these fungicides whereas at the other farms it was reduced by 90–100% by four annual applications.A smaller proportion of resistant isolates of P. pustulans and H. solani was obtained after applying the mixture of thiabendazole and imazalil than after benomyl or thiabendazole alone. Their occurrence was not related to the number of fungicide applications. The mixture also reduced both diseases by more than 75% over the 4 years.At one farm where resistant isolates of H. solani were present, tubers were infected when stored on trays but not when stored in bags.

Infection of potato stem bases, stolons and tubers byPolyscytalum pustulans (Owen & Wakef.) Ellis and development of sclerotia

Polyscytalum pustulans spread from infected buds of seed tubers on the nodal scale leaves of elongating stems and stolons. More eyes were infected at the apical end of developing tubers than at the stolon end.

Effect of substrate and plant maturity on the incidence of infection of potato roots by pathogenic and non-pathogenic fungi

Colonization of roots of field-grown potato plants by pathogenic and non-pathogenic fungi was examined by plating surface-sterilized roots onto low nutrient medium. Plants which originated from micropropagated nodal cuttings (MP plants) or from seed-grade potato tubers (ST plants) were subjected to nine pre-plant treatments designed to test the effect of different potting substrates and different levels of natural tuber inoculum on the root mycoflora. The experiment was repeated in a second year but at a different site; in each year roots were collected once in July, August and September. Cylindrocarpon destructans, Colletotrichum coccodes, Trichoderma viride, Microdochium bolleyi and Fusarium tabacinum showed the highest mean frequencies of colonization; Verticillium tricorpus, Rhizoctonia solani, Polyscytalum pustulans and Gliocladium roseum were mainly confined to roots of ST plants or MP plants with added tuber inoculum, and Trichoderma viride, Chrysosporium pannorum, Penicillium spp., Aureobasidium pullulans and Phoma leveillei were dominant in roots of MP plants raised in peat-based potting substrates. There was evidence of suppression of the potato pathogens R. solani, P. pustulans and C. coccodes, and of several non-pathogens in roots of the same plants. Most of the fungi recovered from potato roots were also typical components of the soil microflora and occurred in both field sites. Exceptions were Fusarium oxysporum and V. dahliae, which were found in one year only. Possible associations and dissociations between different combinations of fungi occurring together on the same root system were evaluated using a correlation matrix, and spatial relationships of different groups demonstrated by principal coordinate analysis. The influence of the indigenous mycoflora on root colonization by particular potato tuber pathogens is discussed.

Resistance to thiabendazole in isolates of Polyscytalum pustulans (skin spot) and Fusarium solani var. coeruleum (dry rot) in Scotland

Between 1988 and 1991, isolates of Polyscytalum pustulans were obtained from potato tubers grown in Scotland and tested for sensitivity to thiabendazole (TBZ) on agar. Sensitive isolates did not grow at 1 mg TBZ/1, whereas growth of resistant isolates was unaffected at 1 mg and reduced by 10–30% at 10 mg/1. Resistant isolates were present on all classes of seed potatoes, and were also recovered from the air of stores during grading. Isolates from a stock of seed tubers from Northern Ireland were resistant. All 44 isolates of Fusarium solani var. coeruleum collected in 1991 were sensitive to TBZ.

Effect of inoculum source on root and tuber infection by potato blemish disease fungi

The incidence of potato pathogens on healthy roots of micropropagated (MP) and seed tuber (ST) plants was examined on successive dates during the growing season in two field experiments. Microplants were grown in a glasshouse for 4–5 weeks in perlite or peal‐based substrates, and exposed or not to natural inoculum before planting in the field. The seed tubers originated from stocks of visually clean or moderately blemished tubers and were surface‐sterilized or not before planting. Polyscytalum pustulans and Helminthosporium solani only infected roots of ST plants and inoculated MP plants. The incidence of P. pustulans was affected by seed tuber‐borne inoculum and, in I year, by the substrate. H. solani was detected infrequently on roots. Rhizoctonia solani was present at low frequencies in most root samples, and more ST than MP plant roots were colonized; there were no substrate effects. In 1 year, increased inoculum levels increased root infection, but only in MP roots. Colletotrichum coccodes occurred at high frequencies and was most common in roots of ST plants. Progeny tubers showed some treatment effects when tested in September and after storage for 6 months, but there were no consistent relationships between root and progeny tuber infection.

Effects of rotation length, fungicide treatment of seed tubers and nematicide on diseases and the quality of potato tubers

SummaryIn 1982 – 88, potatoes were grown in 2‐, 4‐ and 6‐course rotations with spring barley on a field infested with Globodera rostochiensis. Severity of stem canker and black scurf increased with increasing frequency of previous potato crops, and seed tuber treatment with tolclofos‐methyl became less effective in controlling diseases. This suggested that previous crops had increased the amounts of soil‐borne inoculum of Rhizoctonia solani. Oxamyl soil treatment increased stem canker in one year and decreased black scurf in four years.Seed tuber treatment with imazalil or prochloraz decreased stem base infection by Polyscytalum pustulans and skin spot and silver scurf on tubers. Black dot was prevalent on tubers in all years and was not affected by seed tuber treatment or previous cropping. Oxamyl increased black dot and common scab in five years and decreased % tuber dry matter in six years.Cysts of G. rostochiensis were found attached to Désirée but not to Maris Piper tubers in August. At harvest tubers of both cultivars were affected by superficial pitting and its severity was related to soil populations of G. rostochiensis at planting. This damage was controlled by oxamyl. It is suggested that the pitting developed from holes made in the tuber skin at larval invasion.In 1989, Désirée seed tubers and healthy mini tubers were planted in all plots and severity of stem canker and black scurf increased with increasing proximity of previous potato crops and with the number of previous crops. Black dot on stems and tubers was not affected by previous cropping but was much less severe in a plot that had not grown potatoes during the seven years of the experiment. The severity of common scab generally decreased as the number of preceding potato crops increased.

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

SummaryA 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.

Occurrence of Polyscytalum pustulans, Phoma foveata and Fusarium solani var. coemleum in field soils in Scotland

The effectiveness of various methods for detecting three fungal potato pathogens was compared with artificially infested soil, naturally infested tuber‐borne soil and field soil. In the spring of 1985 and 1986 field soils from 30 farms in north‐east Scotland were sampled just before planting a seed potato crop and 6 months after harvesting such a crop. The minimum statutory gap between crops is 5 years. Polyscytalum pustulans was recovered from 32 out of 60 field soil samples taken 6 months after harvest while from fields sampled in the spring before a potato crop was planted the fungus was isolated from 10 out of 30 soils in 1985 and five out of 30 in 1986. Phoma foveata was isolated from only one out of 60 pre‐planting soil samples but Fusarium solani var. coeruleum was recovered from eight of these soils.Microplant bait plants were grown over 3 years at an experimental farm near Edinburgh in various fields at different intervals after a previous potato crop. Contamination by P. pustulans was not related to interval after potatoes between 1 ‐ 7 years. No contamination was recorded in fields where potatoes had not been grown for more than 30 years.

Effects of delaying fungicide treatment and of curing and chlorpropham on the incidence of skin spot on stored potato tubers

SUMMARYPotato tubers wounded with pin pricks and inoculated with Polyscytalum pustulans were not treated or treated with fungicide on the following day or after holding them in dry or damp conditions at 5, 10 or 15°C for periods up to 21 days. Tubers were then stored in damp conditions at 5°C and skin spot assessed after 24 wk. Disease was progressively decreased on untreated tubers by increasing the holding time and temperature in dry, but not damp conditions. Curing at 15°C for 14 days in dry conditions decreased the incidence of skin spot from 70% prick wounds infected to 4% but in damp conditions to only 53%. Fungicides controlled disease best when applied the day after inoculation, and more disease developed on tubers treated after holding in damp rather than dry conditions.Incidence of skin spot in store on naturally infected tubers increased the later they were harvested. Carbendazim controlled the disease when applied immediately after harvest or up to 2 wk later and efficacy was greater after early than late harvest.More skin spots developed on inoculated tubers stored at 5 than at 10°C but applying the sprout suppressant chlorpropham (CIPC) increased the number and size of spots, especially on tubers stored at 10°C. The disease was increased most by CIPC applied 2 days after inoculation; treatment after 5 but not after 10 wk also increased it. CIPC also slightly increased skin spot on tubers previously treated with carbendazim.These experiments with naturally infected or inoculated tubers showed that skin spot in store can be decreased by early harvesting, by curing in dry conditions and by applying fungicide to tubers soon after harvest. They also indicated that when tubers are harvested damp, the disease can be controlled by drying or, if fungicides cannot be applied immediately, the tubers should be kept cool before treatment.

Sampling potatoes for the incidence of tuber diseases and levels of inoculum

SUMMARYIn three years, between three and six seed stocks of cv. Pentland Crown were planted in field experiments. Seed tubers, stem bases and progeny tubers from the growing crop and tubers at harvest and after storage were sampled intensively and estimates of inoculum and disease made for five principal tuber diseases: the blemishes skin spot (Polyscytalum pustulans), silver scurf (Helminthosporium solani) and black scurf (Rhizoctonia solani) and bacterial soft rot (Erwinia carotovora).The data were principally used to calculate sample variances after transformation of % values to logits. A variance of 0.010 was selected as giving acceptable accuracy for most purposes and to achieve such values substantial numbers of samples would have been needed. Five to ten 50‐tuber samples of seed tubers or 15–20 samples of a harvested or stored crop would have been required for assessing disease symptoms and similar numbers of samples for assessing gangrene potential by a standard damage test. The eye plug test for assessing inoculum of the three blemishing diseases used 15 or 20‐tuber samples and five such samples of seed or about ten from the growing crop would usually have been adequate. Variances were nearly always greater for black scurf assessments than for other blemishing diseases.The data also illustrated changes in inoculum and disease levels during the season and in store. Their significance for understanding the epidemiology of the diseases and for storage disease forecasting is discussed.

Fumigation of potatoes with 2-aminobutane for control of gangrene and skin spot

In Scotland 2-aminobutane (2-AB) has been used successfully for the control of gangrene (Phoma exigua var. foveata) and skin spot (Polyscytalum pustulans) in potatoes stored both in boxes and bulk. Gangrene and skin spot can be particularly troublesome in some areas of Scotland and under the prevalent harvesting conditions 2-AB provides a high level of control while other materials have been less effective.

Incidence of tuber diseases in classified seed potatoes harvested In England and Wales, 1974–77

Seed potato tuber inspections were inaugurated in England and Wales as a training exercise daring 1974–77, preparatory to the operation of seed potato classification under statutory authority as from 1978. The number of stocks inspected varied from 359 for the 1976 harvest (38% of those classified) to 536 for the 1975 harvest (52%). The proportion of stocks meeting the required standards progressively increased from 82% for 1974 crops to 91% for 1977 crops. The diseases most commonly causing the failure of stocks to meet the required standards were gangrene (Phoma exigua var. foveata) for crops harvested in 1974, 1975 and 1977, and non‐bacterial soft rots for 1976 crops. Over the 4 years the proportion of stocks failing to meet the required standards for diseases were respectively: 1.2% for tuber blight (Phytophthora infestans); 0.6% for bacterial soft rots (Erwinia carotovora subspp.); 2.0% for non‐bacterial soft rots (including Phytophthora erythroseptica and Pythium ultimum); 0.8% for dry rots (Fusarium spp.); 4.0% for gangrene; 0.1% for skinspot (Polyscytalum pustulans); 0.1% for black scurf (Rhizoctonia solani); 0.1% for powdery scab (Spongospora subterranea) and 1.6% for common scab (Streptomyces spp.).

Testing potato cultivars for susceptibility to skin spot (Polyscytalum pustulans)

Three potato cultivars established in commerce and twenty-two cultivars submitted for National List Trials were tested from 1978 to 1980 for their susceptibility to skin spot (Polyscytalum pustulans) by a laboratory tuber dip test and by a field test measuring the transmission ofP. pustulans from uniformly inoculated seed tubers to stems and progeny tubers. In the field test the susceptibility of a cultivar to stem base infection was related to the percentage eyes infected on progeny tubers but cultivar susceptibility to stem infection and to eye infection was not significantly correlated with the percentage surface area affected by skin spot in the laboratory test.

Methods of testing fungicides for controlling skin spot (Polyscytalum pustulans) on potatoes during storage

Suspensions of 12 fungicides were evaluated for controlling skin spot on stored potatoes in two experiments. Tubers produced from seed tubers inoculated withPolyscytalum pustulans (Experiment 1) or tubers wounded with pin pricks before dipping in mycelial suspensions ofP. pustulans (Experiment 2) were immersed for 5 minutes in suspensions of different fungicides at 10 mg l. 100 mg 1 or 1 g l a.i. After incubation in sealed plastic bags at 5°C the numbers of skin spots developed at random over the tuber surface (Experiment 1) or at pin pricks (Experiment 2) were recorded after 20 or 24 weeks.

Summary of results of experiments on the control of the potato tuber fungal diseases gangrene, dry rot and skin spot with various chemical substances, 1966–80

Over the years 1966–80, experiments were done to compare the effectiveness of a number of fungicides and other chemical substances for the control of the post-harvest potato tuber diseases gangrene (Phoma exigua var.foveata), dry rot (Fusarium solani var.caeruleum) and skin spot (Polyscytalum (Oospora) pustulans). Generally good control of gangrene and skin spot was achieved by fumigation with 2-aminobutane (sec-butylamine). Infection levels with dry rot were too low for a definite conclusion to be reached. Thiabendazole mists and fog, and thiophanate-methyl and imazalil mists did not give as good a control of gangrene as gaseous 2-aminobutane. However a mist formulation of a mixture of thiabendazole and a salt of 2-aminobutane was as effective as gaseous 2-aminobutane against gangrene, but the formulation proved unstable and unsuitable for general use. Although none of the materials tested controlled all diseases equally well, there are several active fungicides whose use can be adapted to the particular needs and storage facilities of farmers and merchants. In some cases where fungicides did not control pathogens, failure was traced to too low a residue or poor distribution of the chemical over the tuber surface, suggesting faulty treatment or unsatisfactory methods of application.