Substituted benzoic and picolinic acids as foliar sprays against potato common scab

Glasshouse tests on potato cv. Arran Banner measured the effects of single early foliar sprays of 18 substituted benzoic and two dichloropicolinic acids on the severity of common scab, caused by soil-borne Streptomyces scabies. The monosubstituted benzoic acids tested (25 mM) did not affect scab. However, anti-scab action was shown by some 2,5-disubstituted acids. Decreases in scab severity were about 35% from 1·6 mM 5-bromo-2-chloro- and 2,5-dimethylbenzoic acids, and over 50% from 1·6 mM 2,5-dichloro-, 2,5-dibromo- and 5-chloro-2-nitrobenzoic acids and from 0·05 mM 3,6-dichloropicolinic acid (clopyralid), which is structurally similar to 2,5-dichlorobenzoic acid. None of the spray treatments affected yield or shape of tubers. No visible effects on foliage were caused by two of the benzoic acids active against scab (5-chloro-2-nitro- and 2,5-dimethyl-), but the other active acids caused some distortion. In tests of the six dichlorobenzoic acids against S. scabies in culture, the 2,5-isomer (which was the most effective against the disease in plants) was one of the least toxic. With other 2,5-disubstituted benzoic acids, it probably decreased scab indirectly by altering the response of the host to infection.     

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 Translocation of 3,5-Dichlorophenoxyacetic Acid in Relation to its Effect on Potato Common Scab

The translocation of 3,5-dichlorophenoxyacetic acid (3,5-D)in the potato plant was studied to clarify the mode of actionagainst potato common scab. 3,5-D was translocated to the tuberfaster and metabolised less than 2,4-D. Translocation to tuberswas proportional to growth and a concentration of 10–25nmol 3,5-D g^–1 fr. wt was attained 8 d after a foliarapplication. Previous evidence indicates this concentrationwas insufficient to be directly toxic to the pathogen Streptomycesscabies and the results suggest that 3,5-D acts in the tuberby altering the host's response to infection.     

Effect of chitosan and a biocontrol streptomycete on field and potato tuber bacterial communities

Four treatments applied on potato seed tubers i.e. talc only, chitosan only, Streptomyces melanosporofaciens strain EF-76 in talc and S. melanosporofaciens strain EF-76 in chitosan were compared for their efficiency to reduce common scab incidence. As previously reported the joint application of the geldanamycin-producing S. melanosporofaciens strain EF-76 and chitosan was effective to control common scab of potato. Nevertheless, field application of S. melanosporofaciens EF-76 did not allow the selection of geldanamycin-resistant actinomycetes (GRA) in the bulk soil during the potato growing season. The number of GRA on harvested potato tubers was, however, significantly higher in treatments that contained chitosan than in other treatments suggesting that chitosan might promote the establishment of the antagonistic actinomycete on progeny tubers. Biolog EcoPlates were used to determine the metabolic profiles of the bacterial soil communities. A permutation MANOVA analysis detected significant differences within the metabolic profiles of the bacterial communities at the potato flowering period but not at the beginning of the season or a week before harvest. A combination of S. melanosporofaciens EF-76 and chitosan thus represents a promising tool against common scab, with low short-term impact on soil bacterial communities.     

Optimal rates of 2,4‐dichlophenoxyacetic acid foliar application for control of common scab in potato

Applications of the synthetic auxin 2,4‐dichlorophenoxyacetic acid (2,4‐D) to the foliage of potato plants can reduce common scab, a tuber disease. However, in prior research effective applications at 200 mg L^?1 2,4‐D resulted in phytotoxic side effects with reduced tuber yield and quality. This study showed that minimal significant threshold rates from 8.3 to 23.6 mg L^?1 2,4‐D reduced disease incidence in pot trials, and from 10.8 to 41.0 mg L^?1 minimised disease severity in both pot and field trials. In only one pot trial, significant phytotoxicity was found with rates of 100 mg L^?1 or greater, reducing mean total tuber mass per plot and 38 mg L^?1 or greater, reducing mean mass per tuber. Notably, within the field trial, a more reliable plant growth system for estimation of yield, no significant impacts were observed. Disease control was associated with decreased sensitivity of tubers to thaxtomin A, the phytotoxin produced by the common scab pathogen essential for disease induction. The amount of residual 2,4‐D in tubers at harvest varied with cultivar, Russet Burbank accumulating more 2,4‐D than Desiree. Application rates less than 100 mg L^?1 resulted in levels of 2,4‐D below the Australian standard maximum residue limit. These data suggest that applications of 2,4‐D at low rates could provide a commercially suitable control strategy for common scab.     

Growth Promotion of Fluorescent Pseudomonads and Control of Potato Common Scab in Field Soil with Non-antibiotic Actinomycete-biofertilizer

Field studies on the control of potato common scab were done with non-antibiotic Actinomycete biofertilizer, which was manufactured from swine feces with coprophilous actinomycetes and acidified to pH 5.0 with sulfuric acid. The field soil at pH 5.0 was mixed with 0.56 kg of the acidified biofertilizer per subplot (1.0 x 1.2 m) to a depth of the 15 cm. The scab severity, in terms of the percent coverage of the total surface area of potato tubers, was 10.7%, in the control, while the lesions were slight in the biofertilizer subplot, scab severity being only 2.30/0, and the potato production was increased 3 times with the biofertilizer. The viable counts of Streptomyces scabies in the biofertilizer subplot decreased to 1 × 10³ per gram soil from 5 × 10⁵ by 12 weeks, while the inhabitant fluorescent pseudomonads increased to 2.4 × 10⁷ per gram soil from 3 × 10⁴, and the increased fluorescent pseudomonads were antagonistic to the pathogenic strains.     

Glasshouse tests of chemicals for control of potato common scab

Quintozene is the only chemical used successfully in practice to control soil-borne Streptomyces scabies, the cause of potato common scab. However it may be carcinogenic, and the aim of this work was to find a substitute for it. About 100 chemicals, most of which had known fungitoxic action or were related to such chemicals, were tested in the glasshouse by growing potato plants in soil with which the chemicals had been mixed, usually at 50 ppm. Many of the chemicals failed to decrease the incidence of scab, or decreased the yield of tubers, or both. The most effective chemical was captafol, which was as effective as quintozene and, unlike quintozene, did not decrease yield. Captan and folpet, which are very closely related to captafol, were ineffective.     

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.     

Inoculum Density and Population Dynamics of Suppressive and PathogenicStreptomycesStrains and Their Relationship to Biological Control of Potato Scab

SeveralStreptomycesstrains are capable of suppressing potato scab caused byStreptomyces scabies.Although these strains have been successful in the biocontrol of potato scab in the field, little is known about how populations of pathogenicStreptomycesin the potato rhizosphere are influenced by inoculation of the suppressive strains. The effects of inoculum densities of pathogenic and suppressiveStreptomycesstrains on their respective populations on roots and in rhizosphere soil were examined during the growing season. The relationships between inoculum density or rhizosphere population densities and disease severity were also investigated. Populations of suppressiveStreptomycesstrain 93 increased significantly on roots with increasing inoculum dose. At its highest inoculum dose, the suppressive strain reached a population density greater than 10⁶CFU/g root 14 weeks after planting. The ability of the suppressive strain to increase its populations with increasing inoculum density was hindered at high inoculum doses of the pathogen, suggesting that density-dependent competitive interactions may be occurring between the two antagonists. Strain 93 was most effective at preventing scab early in the growing season (8 weeks after planting), when tubers were most susceptible to the scab disease. Population densities of the suppressive strain in soil were more highly negatively correlated with scab severity than were populations on roots, suggesting that rhizosphere soil rather than potato roots may be the primary source of inoculum of the suppressive strain for tubers.     

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.     

The effect of harvest date and duration of wound healing conditions on the susceptibility of damaged potato tubers to infection by Phoma exigua (gangrene)

In experiments extending over 3 yr, King Edward tubers harvested on various dates from early September until early November were uniformly wounded and kept at 15 ^oC and c. 95% r.h. for periods of up to 21 days for wounds to cure. When tubers were subsequently inoculated with Phoma exigua var.foveata and incubated at 5 ^oC for 12 wk, curing for 3 and 7 days decreased gangrene incidence to respectively c. 13 and 4% of the non-cured controls. When tubers were inoculated before curing and immediately after wounding, curing for 7 days decreased gangrene incidence to only c. 68% of the non-cured controls. Curing was also progressively less effective the later the date of harvesting, suggesting that there would be advantages in harvesting potato crops early in the autumn.     

Chemical control of potato common scab disease under field conditions

The aim of this study was to evaluate certain fungicides against Streptomyces scabies (Thaxter), the main causal agent of common scab disease in potato and other crops, in vitro and in vivo. Fourteen isolates of S. scabies were isolated from naturally infected potato tubers showing common scab symptoms. All isolates were pathogenic to potato tubers and produced typical symptoms of common scab. Isolate (No. 11) caused highest disease index (DSI) followed by isolates 10, 8 and 5 (61.13%). Seven fungicides (Rizolex, Capitan, Moncut, Ridomil, Maxim, Topsin, and Oxyplus) were screened in vitro for their toxicity against the pathogen isolate (No. 11). Results showed that four of them (Rizolex, Capitan, Moncut, and Ridomil) exhibited inhibition zone ranging from 5.33 to 26.33?mm. Capitan, Ridomil, and Rizolex were able to reduce DSI under field condition but they varied in their effects. Capitan was the best fungicide which aids in the reduction of disease (33.8%) followed by Ridomil (31.5%) while Rizolex (21.2%) was the lowest one.     

Glasshouse tests of quinones, polyhydroxybenzenes and related compounds against potato common scab

Forty quinones, polyhydroxybenzenes and related compounds were tested for possible control of potato common scab, caused by Streptomyces scabies. They were incorporated, at 50 μg/g or less, into scab-infected soil in which potato plants were then grown in the glasshouse. Quintozene at 50/μ g/g was used as standard. The chemicals varied very widely in their effects on scab, and in their phytotoxicity, as measured by decrease in yield of tubers. Corresponding 1,4-benzoquinones and hydroquinones behaved similarly. The unsubstituted and the phenyl-, chloro-,2,5- and 2,6-dichloro-com-pounds were as effective as quintozene; with the exception of chloro-hydroquinone, they did not decrease yield. Chloranil and chloroneb (the dimethylether of 2,5-dichlorohydroquinone) were much less effective, and chloroneb decreased yield as well. 4-terf-butyl catechol was more effective than any other compound tested, including quintozene, and at suitable rates (25 or 12-5 μg/g) did not decrease yield. Other hydroxybenzenes and some 1,4-naphthaquinones (including dichlone) and anthraquinones were almost ineffective.     

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

In 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.     

Major Streptomyces species associated with fissure scab of potato in South Africa including description of Streptomyces solaniscabiei sp. nov

Streptomyces species are the causal agents of several scab diseases on potato tubers. A new type of scab symptom, caused by Streptomyces species, was observed in South Africa from 2010 onwards. The disease was initially thought to be caused by a single Streptomyces species, however, subsequent isolations from similar symptoms on other potato tubers revealed diversity of the Streptomyces isolates. The objective of this study was to characterise these isolates in order to determine what are the major species involved in the disease. This was done by sequencing and phylogenetic analyses of the 16S rDNA as well as five housekeeping genes, investigation of growth on different culture media, standard phenotypic tests and scanning electron microscopy of culture morphology. The presence of the pathogenicity island (PAI) present in plant pathogenic Streptomyces species was also investigated. The genomes of eight isolates, selected from the three main clades identified, were sequenced and annotated to further clarify species boundaries. Three isolates of each of the three main clades were also inoculated onto susceptible potato cultivars in order to establish the pathogenicity of the species. The results of the phylogenetic and genome analyses revealed that there are three main species involved, namely, Streptomyces werraensis, Streptomyces pseudogriseolus and a novel Streptomyces species that is described here as Streptomyces solaniscabiei sp. nov., with strain FS70^T (=?PPPPB BD 2226^T?=?LMG 32103^T) as the type strain. The glasshouse trial results showed that all three of the Streptomyces species are capable of producing fissure scab symptoms. None of the Streptomyces isolates from fissure scab contained the full PAI and the mechanism of disease initiation still needs to be determined. Genomic comparisons also indicated that S. gancidicus Suzuki 1957 (Approved Lists 1980) is a later heterotypic synonym of S. pseudogriseolus Okami and Umezawa 1955 (Approved Lists 1980).

     

Effects of Elicitors on the Accumulation of Proteinase Inhibitors in Injured Potato Tubers

The time course of accumulation and the composition of proteinase-inhibiting proteins in diffusates from potato tubers treated with elicitors such as salicylic, jasmonic, and arachidonic acids were studied. The 40-kDa reserve protein patatin and the chymotrypsin inhibitors, among which proteins of 24.6, 22.0, and 16.0 kDa were prevalent, accumulated in diffusates from potato tubers. Jasmonic and arachidonic acids activated the accumulation of the chymotrypsin inhibitors in tubers in response to the injury stress, whereas salicylic acid inhibited this process. The effects of jasmonic and arachidonic acids increased when their concentrations decreased to 10^–6M. Salicylic acid inhibited this process. The data suggest an important role of the lipoxygenase metabolism in signal transduction of the anti-injury defense system in dormant potato tubers.     

Effects of gibberellin and auxin on the synthesis of abscisic acid and ethylene in buds of dormant and sprouting potato tubers

Gibberellic and β-indolylacetic acids at concentrations of 10⁻⁷-10⁻⁵ M were shown to change the hormonal status and duration of true dormancy in potato tubers. Gibberellic acid shortened the true dormancy and decreased the contents of abscisic acid and ethylene in the apical meristem. β-Indolylacetic acid elongated the true dormancy and decreased abscisic acid production, but caused a more than tenfold increase in the production of ethylene by apical tissues. The data suggest that β-indolylacetic acid and ethylene, as well as gibberellic and abscisic acids, are involved in the regulation of true dormancy in potato 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).     

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.