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.     

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 experiments on apple scab

Dilutions of various fungicides were compared on potted MM 109 clonal rootstocks in the glasshouse for protective (pre-infection) and curative (post-infection) activity against apple scab. The materials, as commercial preparations, included captan, copper (as ‘dry’ Bordeaux mixture), dichlofluanid, didecyldimethylammonium bromide (DDAB), dispersible sulphur, dithianon, dodine acetate, lime-sulphur (LS), phenylmercury chloride (PMC), and thiram; they were assessed on the basis of percentage active ingredient in the dilutions. When acting at normal rates of use, many of the fungicides were much more effective protectively than curatively, but this contrast was less applicable to DDAB, dodine acetate, LS, and dichlofluanid. PMC, however, was an especially notable exception in proving curatively superior at unusually low rates. In addition, it was particularly active in vapour form, and was effectively translocated not only from one leaf surface to the other, but, especially at 0.005%, from sprayed to younger unsprayed leaves on the same shoots. Other standard fungicides showed little or no translocated activity, but isobutyl-o-coumarate was effective at the growing shoot-tips in spring from a spray applied to the lower leaves. PMC reduced infection when applied once at any time during the scab-incubation period, and especially 24 hr. after infection, when the control was almost complete. Longer intervals before spraying reduced the curative action, but not in proportion to the intervals; the infection pattern showed multi-peak fungicidal activity and, after the later sprays, many lesions were practically infertile. The curative properties of PMC were matched by several other formulated phenylmercury salts compared at equivalent percentage mercury content, but the dimethyldithiocarbamate proved rather less effective. A preliminary attempt to supplement the curative properties of PMC with the protective properties of LS, by mixing these fungicides at low rates, showed evidence of fungicidal antagonism, and this needs further investigation. Several fungicides, notably the inorganic sulphurs, thiram, the phenylmercury compounds, dichlofluanid, and DDAB showed incidental activity against natural infection by powdery mildew.     

Thyme and suico essential oils: promising natural tools for potato common scab control

• Potato common scab is a worldwide disease mainly caused by Streptomyces scabiei. It seriously affects potato crops by decreasing tuber quality. Essential oils (EO) are natural products with recognised antimicrobial properties. In this research, the antibacterial activities of thyme, oregano, suico and mint EO against S. scabiei were analysed.
• Infected tubers and soil samples were used for bacterial isolation; the obtained isolates were genetically identified. The chemical composition of the EO was determined by GC‐MS. The broth microdilution method was used to analyse antibacterial properties of EO.
• Thirty‐one bacterial isolates were obtained. The isolate chosen for antibacterial assays was morpho‐physiologically and genetically identified as S. scabiei. Thyme EO was mainly composed of thymol and o‐cymene; suico EO of dihydrotagetone, trans‐tagetone and verbenone; oregano EO of trans‐sabinene hydrate, thymol and ?‐terpinene; and mint EO of menthone and menthol. All the EO tested were effective against S. scabiei, but thyme and suico EO were the most successful, with a minimum inhibitory concentration of 0.068 g·l^?1 and 0.147 g·l^?1, respectively, and a minimum bactericidal concentration of 0.137 g·l^?1 and 0.147 g·l^?1, respectively. Scanning electron microscopy showed similar damage caused by both thyme and suico EO to the bacterial envelope. Total phenolic content of EO was not related to their antibacterial activity.
• Thyme and suico EO are effective antibacterial agents against S. scabiei, impeding bacterial viability and disturbing the bacterial cell envelope. These EO are promising tools for control of potato common scab.

     

Glasshouse experiments on apple scab

Experiments with fungicide mixtures used protectively and curatively against apple scab showed no advantage from adding phenylmercury chloride (PMC) to certain protective fungicides. Sulphur fungicides greatly impaired the curative activity of PMC, especially in mixtures applied 24 hr. after infection, but captan showed little and dodine acetate no such effect. PMC, however, contributed appreciably to the curative effectiveness of mixtures with captan or dodine acetate. Commercially prepared mixtures of dispersible sulphur with phenylmercury dimethyldithiocarbamate and of zineb with mancozeb gave promising results, and an experimental mixture of dodine/glyodin acetates at low rates was effective protectively and especially curatively. Dodine acetate, dichlofluanid, lime-sulphur, and isobutyl-o-coumarate showed little translocated activity against scab when applied after infection, and PMC, which earlier showed strong translocated activity when applied in summer before infection, was much less effective when applied in spring 24 hr. after infection. Uncontrolled powdery mildew early in the scab-incubation period greatly reduced the establishment of scab infection on the test plants (clonal root-stocks); methods of mildew control in such experiments are suggested.     

A feruloyltyramine trimer isolated from potato common scab lesions

(1)H NMR analysis established that a potential suberin intermediate isolated from potato common scab lesions contained three O-methyl groups, a phenylcoumaran-type linkage and a conjugated trans double bond. Mass spectral data determined its molecular formula as indicative of a dehydrotrimer structure formed from three feruloyltyramine units. (1)H and (13)C NMR correlation studies supported the structure as that of a grossamide unit (3) linked through its double bond to the feruloyl phenolic of a third feruloyltyramine group. Identification of the feruloyltyramine trimer (4) expands the number of cross-linked intermediates potentially involved in the suberization process and highlights the presence of a second type of inter-unit linkage available for synthesis of the poly-phenolic domains.     

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.     

Characterization of cross-linked hydroxycinnamic acid amides isolated from potato common scab lesions

Four feruloyl amides, N-trans-feruloyloctopamine (1), N-cis-feruloyloctopamine (2), N-trans-feruloyltyramine (3), N-cis-feruloyltyramine (4), a cross-linked N-trans-feruloyltyramine dimer (5), and a cross-linked N-cis-feruloyltyramine dimer (6) were isolated from potato common scab lesions. The compounds were purified by TLC and characterized by a combination of (1)H and (13)C NMR spectroscopic techniques. The presence of an accompanying minor complex of cross-linked dimers containing both feruloyltyramines and feruloyloctopamines was also demonstrated. This is the first characterization of cross-linked hydroxycinnamic acid amides associated with wound healing in potato (Solanum tuberosum) tubers.     

Competition and antibiosis in the biological control of potato scab

Nonpathogenic, antibiotic-producing streptomycetes have been shown to reduce potato scab when added to disease-conducive soil. Spontaneous mutants of the pathogenic Streptomyces scabies RB4 that are resistant to at least one antibiotic activity produced by the nonpathogenic suppressive isolates Streptomyces diastatochromogenes strain PonSSII and S. scabies PonR have been isolated. To determine the importance of antibiosis in this biocontrol system, these mutants were investigated for their ability to cause disease in the presence of the two pathogen antagonists in a greenhouse assay. Disease caused by one of the mutant strains was reduced in the presence of both suppressive isolates, whereas disease caused by the other five mutants was not significantly reduced by either suppressive strain. In addition, a nonpathogenic mutant of S. scabies RB4 was isolated, which produced no detectable in vitro antibiotic activity and reduced disease caused by its pathogenic parent strain when the pathogen and mutant were coinoculated into soil. Population densities of the pathogen were consistently lower than those of the suppressive strains when individual strains were inoculated into soil. When a pathogen was coinoculated with a suppressive strain, the total streptomycete population density in the pot was always less than that observed when the suppressive isolate was inoculated alone. When the pathogens were inoculated individually into soil, a positive correlation was seen between population density and disease severity. In coinoculation experiments with pathogen and suppressive strains, higher total streptomycete population densities were correlated with lower amounts of disease.     

The control of common scab of potatoes by irrigation

Irrigation applied early during tuberization significantly decreased scab on tubers of Majestic, King Edward and Record, but not on Pentland Crown, a resistant cultivar, where tubers had little scab even from the driest soil. There was little benefit from keeping soils wetter than 0.6 in (15.24 mm) soil moisture deficit (S.M.D.) for more than 3 weeks after tuber initiation, and irrigation to 0.8 in (20.32 mm) S.M.D. gave economic control. Irrigating only after the ‘marble stage’ increased yield but the tubers were scabbed. Irrigation did not alter the rate eyes (nodes) separated from the apical bud but speeded the swelling of tubers.     

Organic manuring for potato scab control and its relation to soil manganese

The suggestion that organic amendment of soil controls scab by reducing insoluble soil manganese to soluble forms toxic to Streptomyces scabies was investigated in pot and field experiments by assessing numbers of manganese-reducing micro-organisms in soil. Dried grass meal (2 tons/acre) gave significant control of scab but increases in soluble manganese were insufficient to account for this control. Populations of micro-organisms in soil increased after adding grass meal but decreased again by the time tubers were initiated and scab infection began. The results suggest that manganese reduction could affect scab if soil contained easily reducible manganese, because dried grass meal increased the numbers of manganese-reducing micro-organisms.     

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.     

Bacillus altitudinis strain AMCC 101304: a novel potential biocontrol agent for potato common scab

ABSTRACT

Potato common scab, caused by Streptomyces spp., is one of the leading causes of heavy commercial losses in the potato industry and is thus one of the most serious plant diseases worldwide. This study identified and assessed potential biocontrol agents against potato common scab. In total, 110 isolates were obtained through antagonistic tests; among which, Bacillus sp. strain AMCC 101304 was found to be most effective at inhibiting the potato common scab pathogen, Streptomyces scabies. Bacillus sp. strain AMCC 101304 was finally identified as Bacillus altitudinis by morphological observation, physiological and biochemical experimentation, as well as 16S rRNA sequencing and phylogenetic analysis. Pot experiments were conducted twice (in spring and autumn) to verify the biocontrol effect of B. altitudinis AMCC 101304 against potato common scab. In spring, the control efficiency reached 76.34%. In autumn, the disease incidence was reduced from 100% to 34.19% (one treatment with strain AMCC 101304) and 38.42% (two treatments with strain AMCC 101304), and the control efficiency reached 82.50% (one application) and 78.43% (two applications). The present study demonstrated the potential of an isolate, identified as B. altitudinis AMCC 101304, as an effective biocontrol agent for future use in the field.     

Taxonomic study on nonpathogenic streptomycetes isolated from common scab lesions on potato tubers

Numerical analysis was carried out to compare sixteen nonpathogenic actinomycetes isolated from common scab lesions on potato tubers with Streptomyces scabiei type strain as well as with other streptomycete groups. These isolates were divided into two classes according to their level of similarity with S. scabiei. Isolates resembling S. scabiei were associated with S. griseoruber or with S. violaceusniger while isolates exhibiting less than 61% of similarity with S. scabiei were phenotypically related to S. albidoflavus or to S. atroolivaceus. Sequence of the 16S rRNA gene of each isolate was obtained and compared against the GenBank nucleotide database. No significant match could be established between the sequences of two potato isolates and the ones available in the GenBank database. The other isolates were closely related with S. setonii (S. griseus), S. mirabilis, S. fimbriatus, S. violaceoruber, S. melanosporofaciens and S. thermocarboxydus.     

Genetic diversity of Streptomyces spp. causing common scab of potato in eastern Canada

Common scab is an important disease of potato caused by Streptomyces scabies and other closely related species. In this study, the genetic diversity of Streptomyces spp. causing common scab of potato in eastern Canada was for the first time investigated. Forty-one Streptomyces spp. isolates were retrieved from necrotic lesions of potato tubers harvested from different regions of the Canadian provinces New-Brunswick, Nova Scotia and Prince-Edward-Island. Most isolates were closely related to known pathogenic S. scabies strains on the basis of partial 16S ribosomal (r) RNA and rpoB gene sequence analyses. Two isolates were identified as pathogenic species of Streptomyces acidiscabies. To our knowledge, this species has never been previously isolated in these areas. Genome fingerprinting studies using repetitive elements (rep) polymerase chain reactions (PCR) revealed 10 distinct genetic groups in eastern Canada. The geographical distribution of the genetic groups was region-dependant. Pathogenicity- and virulence-related genes (txtA, txtC, and tomA) were PCR-amplified from each isolate, and nucleotide sequence analysis of partial gene fragments revealed slight polymorphisms in both txtA and txtC genes. No genetic variation was noted in the partial tomA gene sequences.     

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.     

马铃薯疮痂病拮抗菌YN-2-2的分离与鉴定

【背景】近年来,疮痂病在全国马铃薯各主要产区普遍发生,危害呈逐年加重趋势,有效防控手段匮乏,给种植者造成严重的经济损失。利用拮抗微生物抑制病原菌繁殖,降低其危害程度,已成为马铃薯植保领域的研究热点。【目的】筛选对疮痂病原菌具有拮抗作用的菌株,为研制高效复合功能菌剂、有效防控马铃薯疮痂病提供菌种资源。【方法】从疮痂病发生严重的云南昭通马铃薯大田采集近根际土壤,分离、筛选代谢产物具有明显拮抗效果的菌株,通过形态学观察、生理生化特征分析和16SrRNA基因序列测定等方法进行菌种鉴定,并对其代谢产物的稳定性和抑菌效果进行分析。【结果】获得一株拮抗效果明显的细菌YN-2-2,其菌落圆形,淡黄色,边缘整齐、光滑、干燥,中间有凹陷,菌体杆状,大小为(2.51-4.09)μm×(1.09-1.68)μm,革兰氏染色阳性,16SrRNA基因序列与Bacillus thuringiensis ATCC 10792T (ACNF01000156)的相似性达到99.79%。YN-2-2的代谢产物热稳定性较好,pH耐受范围广(3.0-13.0),对蛋白酶K敏感度较低,对疮痂链霉菌的抑菌圈直径最大为22.8 mm。盆栽试验结果表明:浇施100 mL浓度为1×107 CFU/mL的培养液,可显著降低马铃薯微型薯疮痂病病情指数,防效达36.11%。【结论】菌株YN-2-2经鉴定为苏云金芽孢杆菌(Bacillus thuringiensis),可以作为高效复合功能菌剂的候选菌株。