The effect of a few days of rain on the distribution of common scab (Streptomyces scabies) on young potato tubers

At Woburn, Beds, in 1967, potato tubers formed and grew in dry soil except after rain between 23 and 26 June. At lifting, Majestic tubers were severely and uniformly scabbed, except for distinct bands free from lesions. The position of the bands differed depending on the date when tubers started to form; they were further from the stolon attachment when tubers formed early and nearer it when tubers formed late. Samples taken in June and July showed that the scab-free band and the late June rain were related. From 25 June to 1 July the soil was probably wet enough to prevent Streptomyces scabies infection, but only one or two tuber internodes remained free from infection; they were the internodes that began to expand about a week before the rain. Internodes that formed while the soil was wet became scabbed, showing they were still susceptible to S. scabies after the soil had dried again. Recent work on lenticel formation has suggested a hypothesis to explain these results.     

Effects of common scab (Streptomyces scabies) on potatoes

Seed tubers with severe (71% cover) or slight (12% cover) common scab selected from a badly infected crop of cv. Maris Piper were boxed for sprouting in November and February and planted in field experiments at Woburn and Rothamsted in 1980. The severely scabbed seed lost more moisture than the slightly scabbed seed during sprouting and sprouted from a larger number of eyes but total stem numbers were greater on plants from the slightly scabbed seed. Plant growth during the first 6–9 wk after planting was less from the severely scabbed than from the slightly scabbed seed. This was shown by records of ground cover, leaf area index, fresh weights of stems and leaves, total tuber yield (18% lower at 9 wk) and tuber number. Subsequently these differences became non-significant. Seed with slight scab boxed in November produced smaller numbers of progeny tubers than that boxed in February but there were no other consistent effects of boxing date on growth and yield. Scab incidence on progeny tubers was very slight and unrelated to the severity of disease on the seed tubers. It seems unlikely that severe scab infection of seed tubers would significantly decrease tuber yields except perhaps in early potato production.     

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.     

Responses of potato cultivars to the common scab pathogens, Streptomyces scabies and S. turgidiscabies

A glasshouse experiment was conducted to study the responses to Streptomyces scabies and S. turgidiscabies in potato cultivars Bellona, Matilda and Sabina (Solanum tuberosum). Potatoes were grown in a peat‐sand mixture inoculated with one of the two strains of either S. scabies or S. turgidiscabies. Logit models were used to analyse the data on disease incidence and severity, whereas the data on emergence and yield were tested by analysis of variance. S. turgidiscabies, a recently described potato pathogen in Finland, possessed a high ability to cause superficial, raised and pitted lesions on all three cultivars tested. Symptoms induced by S. turgidiscabies were similar to those of S. scabies, regardless of the cultivar, which suggests that the two causal organisms of common scab cannot be distinguished based on symptoms. Infection by S. turgidiscabies and S. scabies delayed emergence, had the tendency to decrease the yield, and increased the proportion of small tubers in the yield, regardless of the potato cultivar. Differences in the levels of resistance to common scab were evident between potato cultivars, since cvs. Matilda and Bellona showed higher disease incidence and more severe scab symptoms than cv. Sabina.     

TxtH is a key component of the thaxtomin biosynthetic machinery in the potato common scab pathogen Streptomyces scabies

Streptomyces scabies causes potato common scab disease, which reduces the quality and market value of affected tubers. The predominant pathogenicity determinant produced by S. scabies is the thaxtomin A phytotoxin, which is essential for common scab disease development. Production of thaxtomin A involves the nonribosomal peptide synthetases (NRPSs) TxtA and TxtB, both of which contain an adenylation (A-) domain for selecting and activating the appropriate amino acid during thaxtomin biosynthesis. The genome of S. scabies 87.22 contains three small MbtH-like protein (MLP)-coding genes, one of which (txtH) is present in the thaxtomin biosynthesis gene cluster. MLP family members are typically required for the proper folding of NRPS A-domains and/or stimulating their activities. This study investigated the importance of TxtH during thaxtomin biosynthesis in S. scabies. Biochemical studies showed that TxtH is required for promoting the soluble expression of both the TxtA and TxtB A-domains in Escherichia coli, and amino acid residues essential for this activity were identified. Deletion of txtH in S. scabies significantly reduced thaxtomin A production, and deletion of one of the two additional MLP homologues in S. scabies completely abolished production. Engineered expression of all three S. scabies MLPs could restore thaxtomin A production in a triple MLP-deficient strain, while engineered expression of MLPs from other Streptomyces spp. could not. Furthermore, the constructed MLP mutants were reduced in virulence compared to wild-type S. scabies. The results of our study confirm that TxtH plays a key role in thaxtomin A biosynthesis and plant pathogenicity in S. scabies.     

Effects of foliar sprays of daminozide on the incidence of potato common scab

Single foliar sprays of the growth retardant daminozide (1.5–12 g/l) approximately halved the incidence of common scab, caused by soil-borne Streptomyces scabies, on potted potato plants in the glasshouse. Two analogues of daminozide (N-dimethylaminomaleamic and N-(dimethylamino)-methylsuccinamic acids) also decreased scab, but others were inactive. Of 22 other unrelated growth regulators and translocated chemicals tested as foliar sprays, only gibberellic acid (0.1 g/l) decreased scab incidence, but many of the tubers were distorted. Chlormequat chloride and chlorphonium chloride, as root treatments, were inactive. In other experiments with daminozide, scab incidence was decreased after application to soil. In tests with two plants per pot, spraying one of each pair decreased its rate of stem extension, but did not affect the other, indicating that little or no daminozide passed into the soil from the roots of the sprayed plant. The decrease in scab brought about by foliar sprays was not altered by varying their timing during the period before symptom development (1 to 5 wk after potting). In agar plate tests, daminozide was only weakly toxic to S. scabies. It is concluded that daminozide probably decreased scab by altering the physiology of the plants, so that scab symptoms did not develop.     

Effect of Streptomyces melanosporofaciens strain EF-76 and of chitosan on common scab of potato

The efficiency of Streptomyces melanosporofaciens strain EF-76, a geldanamycin producer, and of chitosan, a polymer derived from chitin that elicits plant defense mechanisms, to protect potato tubers against common scab was evaluated under both controlled and field conditions (years 2000 and 2001). S. melanosporofaciens EF-76 reduced disease incidence in the greenhouse assay and in the 2001 field assay. EF-76 also reduced symptom severity on potato tubers grown under field conditions. Chitosan provided a protective effect against S. scabies,the causal agent of potato common scab during the 2000 field assay by reducing both disease incidence and symptom severity. Combination of S. melanosporofaciensEF-76 and chitosan ensures a level of protection that was at least equivalent to the protection conferred by one of the two products used alone. In some instances, an additive effect of protection was observed when both products were used in combination. Combination of S. melanosporofaciensEF-76 and of chitosan thus represents a promising method of biocontrol against common scab.     

The effect of applications of manganese sulphate to a neutral soil upon the yield of tubers and the incidence of common scab in potatoes

Summary Potatoes grown on a neutral soil with low water-soluble manganese content gave a normal crop with no signs of manganese deficiency. Treatment with manganese sulphate (0.5 cwt per acre) mixed with a compound fertiliser and applied in the drill at planting time, increased the manganese content of leaves and tubers but had no effect on the total yield of tubers. The manganese treatment, however, increased the average weight of tuber and markedly reduced the incidence of scab. The results suggest a possible method of controlling scab in neutral soils.     

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.     

Management of common scab of potato in the field using biopesticides,fungicides, soil additives,or soil fumigants

Common scab, caused by Streptomyces scabiei is an economically important potato disease worldwide. The potato industry in New Brunswick, Canada experience $1.2 million loss every year due to this disease. Superficial, raised, or deep-pitted brownish lesions on infected tubers reduce the quality and marketability of both fresh-market and processing potatoes, and hence, common scab is considered a priority disease for which adequate control measures are lacking. The objective of this research was to compare various potential treatments in suppressing the disease. Two field experiments were conducted at McCain's Research Farm, Florenceville-Bristol, New Brunswick, Canada, in 2008 and 2009 to assess the efficacy of Bacillus subtilis, Enterobacter cloacae, Chloropicrin, Pic-Plus, manganese sulphate and mustard meal in comparison with the chemical controls fludioxonil and mancozeb against common scab of potato. The disease incidence was significantly reduced by 36% due to the addition of mustard meal to the soil; 35.4% due to fludioxonil seed treatment; 30.0% due to soil fumigation with Pic-Plus; or by 27.2% due to soil fumigation with Chloropicrin. Potato tubers with scab severity ≥5% which are considered unmarketable in Canada were significantly reduced by 56.1% due to seed treatments with B. subtilis; by 57.8% due to fludioxonil; or by 63.1% due to the soil addition of mustard meal. The same treatments significantly increased marketable yield by 32.5%, 24.6%, and 24.6%, respectively. Soil fumigation with Chloropicrin or Pic-Plus increased marketable yield by 9.5% or 7.1%, respectively. These findings indicate that, in addition to the fludioxonil seed treatment, the seed treatment with the biopesticide containing B. subtilis and the soil addition of mustard meal treatments are potential alternatives for managing common scab of potatoes.     

Pectic epitopes are differentially distributed in the cell walls of potato (Solanum tuberosum) tubers

Six monoclonal antibodies (mAbs) were used to map the distribution of pectic epitopes in the cell walls of potato ( Solanum tuberosum L. cvs Kardal and Karnico) tuber tissue in both light and electron microscopes. Unesterified (mAb JIM 5 epitope) and methyl-esterified (mAb JIM 7 epitope) pectins were abundant and equally distributed in all parenchymal and vascular cell walls. Homogalacturonans (HGAs) involved in Ca²⁺-cross-linking (mAb 2F4 epitope) were localised to the middle lamella and abundant at cell corners. The tuber cortex was densely labelled, but parenchymal cell walls in the perimedullary region contained few epitopes of calcium pectate except at corners and pit fields. In contrast, pectic side-chains were not detectable in the middle lamella of all parenchymal cell walls, except in the cortex where mAb LM6 (arabinan epitope) labelled the entire wall. The galactan epitope (mAb LM5) was localised to a zone very close to the plasmalemma in cortical cell walls and was also less abundant at pit fields and in vascular cell walls. MAb CCRC-M2 (rhamnogalacturonan I epitope) did not cross-react. Our results show that the cell walls of potato tubers are not homogeneous structures and that the pectic composition of the walls is spatially regulated.     

The ability of Pseudomonas sp. LBUM 223 to produce phenazine-1-carboxylic acid affects the growth of Streptomyces scabies, the expression of thaxtomin biosynthesis genes and the biological control potential against common scab of potato

Streptomyces scabies causes common scab, an economical disease affecting potato crops world-wide, for which no effective control measure exists. This pathogen produces the plant toxin thaxtomin A, which is involved in symptom development on potato tubers. A biological control approach that can limit S. scabies growth and repress thaxtomin production represents an attractive alternative to classical control strategies. Pseudomonas sp. LBUM 223 produces phenazine-1-carboxylic acid (PCA), an antibiotic that inhibits the growth of plant pathogens and contributes to the biological control of plant diseases. In this study, the involvement of LBUM 223's PCA-producing ability in the growth inhibition of S. scabies, repression of thaxtomin biosynthesis genes (txtA and txtC) and the biological control of common scab of potato was investigated using a mutant defective in PCA production (LBUM 223phzC(-) ). Streptomyces scabies growth was inhibited to a significantly lesser degree by LBUM 223phzC(-) than by the wild type. LBUM 223 also significantly repressed txtA and txtC expression in S. scabies and protected potato against disease, whereas LBUM 223phzC(-) did not. These results suggest that PCA production is central to the ability of LBUM 223 to limit pathogen growth, repress the expression of key pathogenicity genes and control common scab of potato.     

Effects of delaying fungicide treatment on the incidence of gangrene in stored potato tubers

Potato titbers infested with Phoma exigua var. foveata were uniformly wounded and sprayed or dipped in fungicide suspensions either immediately or after periods of up to 21 days' storage at 5, 10, 15 or 20 °C. Tubers were then stored at 5 °C and gangrene assessed after 12 wk. Incidence of gangrene on untreated tubers was progressively decreased by increasing the length of storage at 15 or 20° (curing) but was not affected by 3 days' storage at any temperature. Fungicide treatment immediately after wounding gave best control of the disease; treatment after 3 days' delay was less effective and after 14 or 21 days was usually ineffective. Gangrene was decreased by fungicides more on tubers stored for 3 or 7 days at 5 °C than at higher temperatures. Control of gangrene by curing or fungicides diminished when the amount of inoculum on tubers was increased. Increasing the amount of fungicide applied improved control and fungicides were more effective in decreasing gangrene on cut and crush wounds than on cut wounds. At the arbitrary concentrations used in these experiments imazalil gave better disease control than thiabendazole, prochloraz, carbendazim plus quinolin 8-ol or triadimefon.     

The influence of manganese on the development of potato scab

Summary The development of common scab on potato crops growing in slightly acid to neutral soils has been shown to be greatly reduced by application of manganese sulphate to the soil at planting time. A general improvement in health and appearance of tubers was noted and increased yields of marketable tubers were obtained. The possibility that environmental factors known to influence the development of scab are only important in so far as they affect the solubility of soil manganese is discussed.     

Differentiation of Streptomyces spp. which cause potato scab disease on the basis of partial rpoB gene sequences

In a previous study, we found that the phylogenetic analysis of partial rpoB sequences can be used effectively to phylogenetically differentiate Streptomyces spp. [B.J. Kim, C.J. Kim, J. Chun, Y.H. Koh, S.H. Lee, J.W. Hyun, C.Y. Cha, Y.H. Kook, Phylogenetic analysis of the genera Streptomyces and Kitasatospora based on partial RNA polymerase beta-subunit gene (rpoB) sequences, Int. J. Syst. Evol. Microbiol. 54 (2004) 593-598]. In the present study, we analyzed the partial rpoB gene sequences of 19 reference Streptomyces strains associated with potato scab. Furthermore, to empirically confirm the usefulness of rpoB gene analysis for the phylogenetic differentiation of Streptomyces spp., we applied the proposed system to 27 potato scab isolates obtained from the Korean provinces of Jeju-do and Kangwon-do. Phylogenetic relationships among these isolates using the devised rpoB gene-based methods were generally similar to those reported for 16S rRNA gene-based analysis. Isolates from potato scab lesion in Korea were also clearly differentiated into their phylogenetic groups by this method. In addition, the deduced RpoB amino acid sequences were also found to be useful for differentiating these strains. Our data demonstrate that the rpoB gene-based method can be used as a means of complementing other genetic methods such as 16S rRNA gene analysis or DNA-DNA hybridization to phylogenetically differentiate potato scab related Streptomyces spp.     

Multiplex real-time PCR (TaqMan) assay for the simultaneous detection and discrimination of potato powdery and common scab diseases and pathogens

Aims: To develop a multiplex real‐time PCR assay using TaqMan probes for the simultaneous detection and discrimination of potato powdery scab and common scab, two potato tuber diseases with similar symptoms, and the causal pathogens Spongospora subterranea and plant pathogenic Streptomyces spp. Methods and Results: Real‐time PCR primers and a probe for S. subterranea were designed based on the DNA sequence of the ribosomal RNA ITS2 region. Primers and a probe for pathogenic Streptomyces were designed based on the DNA sequence of the txtAB genes. The two sets of primer pairs and probes were used in a single real‐time PCR assay. The multiplex real‐time PCR assay was confirmed to be specific for S. subterranea and pathogenic Streptomyces. The assay detected DNA quantities of 100 fg for each of the two pathogens and linear responses and high correlation coefficients between the amount of DNA and C_t values for each pathogen were achieved. The presence of two sets of primer pairs and probes and of plant extracts did not alter the sensitivity and efficiency of multiplex PCR amplification. Using the PCR assay, we could discriminate between powdery scab and common scab tubers with similar symptoms. Common scab and powdery scab were detected in some tubers with no visible symptoms. Mixed infections of common scab and powdery scab on single tubers were also revealed. Conclusions: This multiplex real‐time PCR assay is a rapid, cost efficient, specific and sensitive tool for the simultaneous detection and discrimination of the two pathogens on infected potato tubers when visual symptoms are inconclusive or not present. Significance and Impact of the Study: Accurate and quick identification and discrimination of the cause of scab diseases on potatoes will provide critical information to potato growers and researchers for disease management. This is important because management strategies for common and powdery scab diseases are very different.