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.     

Comparison of fungicides for the control of Rhizoctonia solani causing datnping-off of mung bean (Phaseohis aureus)

Of 41 fungicides tested in the laboratory, copper carbonate, copper sulphate, mercuric chloride, Agrosan GN, quintozene, kasugamycin, carboxin, pyracar-bolid, carbendazim, chloroneb, benomyl, Ohric, RH 893 (2-n-octyl-4-isothiazole-3-one) and Terrazole were most inhibitory to the mycelial growth of Rhizoctonia solani on Czapek's agar plates and had EC₅₀ values of less than 1 μg a.i./ml, while copper oxychloride, Udonkor, zineb, ziram, F 319 (3-hydroxy-5-methyl isoxazole) and anilazine were much less toxic, ziram being least inhibitory with an EC₅₀ of 214 μg a.i./ml. Of 17 fungicides tested in the greenhouse as seed treatments, thiabendazole, carbendazim, benomyl, thiophanate-methyl, dichlozoline and Ohric gave 80–90% control of damping-off of mung bean seedlings. A single soil drench with thiophanate-methyl and two drenches with benomyl gave about 90% disease control, More seedlings with R. solani infection survived when thiophanate-methyl was used as a post-inoculation soil drench than when benomyl or chloroneb were used.     

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.     

Some factors affecting the control of Rhizoctonia solani by systemic and non-systemic fungicides

In laboratory tests, Rhizoctonia solani grew best on agar at 30 ^oC and pH 5-5. Mycelial growth was strongly inhibited by benomyl, chloroneb and quintozene, less so by thiophanate-methyl. The optimum temperature for inhibition was 25 ^oC for quintozene; the other three fungicides gave greatest inhibition at the lowest temperature tested (20 ^oC). Benomyl and thiophanate-methyl were most inhibitory at pH 7–8, chloroneb and quintozene at pH 5–6. In pot trials using mung bean, long melon, egg-plant, common pea and sugar beet, R. solani caused maximum disease at 20 ^oC and in wet and alkaline soils. As seed treatments, benomyl and thiophanate-methyl gave optimum control at 20 ^oC and pH 7-6; chloroneb and quintozene were most effective at 30 ^oC and pH 5-4. All four fungicides gave maximum control on plants growing in wet sandy loam. Comparisons of host effects showed that, on all the four hosts tested, thiophanate-methyl wettable powder at 0.25% (0.175% a.i.) gave 90% control, chloroneb w.p. at 0.3% (0.195% a-i-) gave 80% control on mung bean and sugar beet, benomyl w.p. at 0.3% (0.15% a.i.) was satisfactory on mung bean, egg-plant and sugar beet but not on long melon, quintozene w.p. at 0.3 % (0.225 % a-i-) gave effective control on sugar beet only. The senior author is grateful to the Council of Scientific and Industrial Research, New Delhi, India, for the award of a Research Fellowship for undertaking these studies. Thanks are due to Dr S. D. Gupta and Mr S. L. Verma for determining the soil characteristics.     

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.     

Decreased severity of potato common scab after foliar sprays of 3,5 -dichlorophenoxyacetic acid, a possible antipathogenic agent

Glasshouse tests on the potato cultivars Majestic and Maris Bard measured the effects of single early foliar sprays of ring-substituted phenoxyacetic acids (0.9 × 10–³ M) on the incidence of potato common scab, caused by soil-borne Streptomyces scabies. The most effective compound was 3,5-dichlorophenoxyacetic acid (3,5-D), which decreased scab by about 90%; its action was preventative, early sprays being more effective than late However, it slightly decreased yield, and increased the number of tubers per plant and the proportion of deformed tubers. The trichloro- and tetrachloro- acids containing the 3,5-dichloro- group also decreased scab, though to lesser extents; the most active of these was the 2,3,4,5-tetrachloro- acid, which had the same side effects as 3,5-D. The herbicide 2,4-D almost halved scab incidence, but seriously decreased yield. Other acids tested had little or no effect on scab. In tests against S. scabies in culture, 3,5-D was no more toxic than other disubstituted acids which were inactive, or weakly active, against the disease in vivo. 3,5-D may be an ‘antipathogenic agent’, preventing scab development by altering tuber metabolism.     

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.     

Fungicidal control of Rhizoctonia solani in relation to soil texture, organic matter and clay minerals

In pot tests, MEMC, quintozene, captafol, carboxin, thiabendazole, carbendazim, benomyl and thiophanate-methyl used as seed treatments gave much better control of cowpea seedling rot in light-textured sandy and loamy sand soils than in heavy-textured loam and silt loam soils inoculated with Rhizoctonia solani. Disease control by chloroneb was not altered by soil texture. Amendment of sandy soil with montmorillonite reduced disease control with all fungicides, except chloroneb and carboxin; similar amendments with kaolinite decreased efficacy of MEMC and captafol. Green manuring with cluster bean reduced disease control by MEMC, captafol, benomyl and thiophanate-methyl; sunnhemp reduced efficacy of MEMC. Most fungicides gave poor disease control when farm yard manure or biogas sludge was added to soil, the sludge having the more marked effect. All the fungicides tested, except carboxin, were inactivated to different extents by humic acid extracted from farm yard manure.     

Microbiological Synthesis of d-Cysteine and an Analogue

A biofertilizer, showing antagonistic activity against potato common scab in pot tests, was produced from swine feces with a newly isolated strain, CH-33, identified as Streptomyces albidoflavus. This strain characteristically grew on fresh swine feces at 20~35°C without sterili-zation or any additives, and produced an antibiotic substance against Streptomyces scabies, the common scab-pathogen, during composting. The addition of the biofertilizer at from 0.1 g to 1.6 g total nitrogen (N) per 600 g humic volcanic ash soil in a pot did not inhibit the growth of Brassica rapa var. perviridis but increased it, even at the highest nitrogen content tested. Common scab was completely inhibited when the biofertilizer was added at 0.1 g to 1.6g as nitrogen (N) per 4 kg of scab-infected soil in a pot. Thus a biofertilizer suppressing plant pathogenic microorganisms was developed.     

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.     

A comparison ofin vitro andin vivo effect of clay minerals,humic acid and micronutrients on the activity of fungicides againstRhizoctonia solani

Activity of nine fungicides against mycelial growth ofRhizoctonia solani in potato dextrose broth and in pot tests as seed treatment against cowpea seedling rot in infested soil was differentially in fluenced by clay minerals, humic acid and micronutrients. Humic acid, extracted from farmyard manure, considerably lowered the activity, bothin vitro andin vivo, of all fungicides except chloroneb. Montmorillonite caused substantial decrease in disease control by fungicides but enhanced the toxicity of 2-methoxyethyl mercury chloride (MEMC), and quintozene in culture. Kaolinite inactivated carbendazim, benomyl and thiophanatemethyl in nutrient broth but had little effect on disease control by these fugicides. The six micronutrients altered the activity of fungicides to varying levels often without definite correlation betweenin vitro andin vivo results. The results ofin vitro growth inhibition tests are largely inapplicable to dieseas control tests in infested soil mainly due to the differences in the ambient conditions of the two systems.     

Designing a crop rotation strategy to manage Streptomyces scabies causing potato scab in north India

Common scab of potato caused by various species in the genus Streptomyces has assumed serious proportions in certain potato-growing regions of North India. Although it does not have significant effect on tuber yield, it downgrades quality and reduces marketability. Being soil-borne, this disease is often difficult to manage. Therefore, the present studies were conducted to evaluate different potato-based crop rotations prevalent in north India as a part of a strategy to manage Streptomyces scabies population in scab sick fields. Our results showed that S. scabies population in soil can be effectively reduced either by keeping the field fallow after potato or by including mungbean or sunhemp as green manure, in a single year cropping sequence over a period of four years. Three crop rotations, viz. fallow–rice–potato, mungbean–rice–potato and sunhemp–rice–potato, showed maximum decrease in the population of S. scabies in soil resulting in reduced scab severity. The mungbean- and sunhemp-based crop rotations also enhanced rhizospheric soil microflora especially fluorescent pseudomonads and Trichoderma spp.     

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.     

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.     

<Emphasis Type="Italic">Fusarium verticillioides</Emphasis>: Evaluation of Fumonisin Production and Effect of Fungicides on In Vitro Inhibition of Mycelial Growth

The objectives of this study were to evaluate the fumonisin production by 16 F. verticillioides strains on corn cultures and the effect of quintozene and fludioxonil + metalaxyl-M fungicides on “in vitro” mycelial growth on agar. In addition, the effect of fludioxonil + metalaxyl-M on fumonisin production in defined liquid culture medium was analyzed. Fumonisin B₁ levels on corn cultures ranged from 2.41 to 3996.36 μg/g and the F. verticillioides 103F strain produced the highest level (3996.36 ± 390.49 μg/g, P < 0.05). F. verticillioides strains were inoculated in potato dextrose agar with the addition of quintozene (75 to 9,375 μg/ml) and fludioxonil + metalaxyl-M (1.5 + 0.6 to 187.5 + 75 μg/ml) in order to evaluate the effect of these fungicides on “in vitro” mycelial growth. The F. verticillioides strains showed great variability concerning ED₅₀ values, which were below the recommended application dose for quintozene, but above that for fludioxonil + metalaxyl-M. Moreover, fungicide addition to the culture medium increased mean FB₁ levels compared to the control, suggesting the importance of focusing on the effect of fungicides on mycotoxin production as well as on the phytopathogen control.     

Antimycin A-producing nonphytopathogenic Streptomyces turgidiscabies from potato

Aim:  To detect if substances with mammalian cell toxicity are produced by Streptomyces turgidiscabies and Streptomyces scabiei isolated from potato scab lesions. Methods and Results:  In vitro cultures of phytopathogenic and nonphytopathogenic strains of S. scabiei and S. turgidiscabies, isolated from scab lesions of potato tubers originating from nine different cultivars from Finland and Sweden, were tested for toxicity using the rapid spermatozoan motility inhibition assay, previously shown useful in the detection of many different Streptomyces toxins and antimicrobial compounds. Purified toxins were used as reference. Three nonphytopathogenic strains of S. turgidiscabies were found to produce antimycin A when cultured on solid medium. Conclusions:  Boar sperm-motility-inhibiting substances are produced by strains of S. turgidiscabies and S. scabiei. The most powerful inhibitory substance, produced by three nonphytopathogenic S. turgidiscabies strains, was identified as antimycin A. The phytotoxic compounds thaxtomin A and concanamycin A did not inhibit sperm motility even at high doses. Significance and Impact of the Study:  The presence of antimycin A-producing Streptomyces strains, nonpathogenic to potato, was unexpected but important, considering the high mammalian toxicity of this cytochrome bc-blocking antibiotic.     

Control of potato cyst nematode (Globodera rostochiensis) by nematicides and a resistant potato cultivar at four sites in Scotland

The control of potato cyst nematode (Globodera rostochiensis) by the oxime-carbamates aldicarb and oxamyl was tested in four fields in Scotland. Dazomet was tested in three of these fields and carbofuran in one. In untreated plots in the three most heavily infested fields Maris Piper (resistant) yielded better than Pentland Crown (non-resistant). All nematicides increased the yields of both potato cultivars but had a greater effect on the yield of Pentland Crown. Dazomet increased yields of tubers most. Heavy nematode infestation reduced yield of tubers more in a sandy soil than in two sandy loams. In a field with few potato cyst nematodes nematicides did not significantly affect tuber yields. Although the nematicides greatly increased yields, they were not completely effective in controlling potato cyst nematodes. In treated plots in the lightly infested field, there were more nematode eggs following a crop of Pentland Crown than before. In contrast, Maris Piper markedly decreased post-cropping populations and except at one site, where dazomet further decreased nematode numbers, combining nematicides with the resistant cultivar failed to decrease nematode numbers further. Nematicides decreased the numbers of larvae invading potato roots by up to 95%, oxamyl at 5–6 kg/ha being consistently the best treatment.     

Phenolic acid content in potato peel determines natural infection of common scab caused by <Emphasis Type="Italic">Streptomyces</Emphasis> spp.

Common scab of potato caused by the actinomycete Streptomyces scabies is a common pathogen in almost all the potato growing areas of the world. Twenty cultivars of potato were screened in naturally scab infested farmers fields at two locations Tikari and Bachhawan, Varanasi, in two successive crop seasons (2006–2007 and 2007–2008). Among the cultivars, five cultivars were recorded to be least susceptible and the others ranged from medium susceptible to very highly susceptible. Most of the cultivars showed a stable resistance reaction in both the years. Qualitative as well as quantitative estimation of phenolic acids present in peels of the potato cultivars showed their possible role in protection of the potato cultivars against common scab. All the red skinned potato cultivars that were least susceptible to common scab infection were usually found to be rich in phenolic acid contents in their peels. This showed a positive correlation between cultivar resistance to common scab and phenolic acid content in the peel.     

Fusarium crookwellense,a newly isolated fungus from wheat in Japan: Its mycotoxin production and pathogenicity to wheat and barley

Fusarium crookwellense was isolated for the first time in Japan from scabby wheat harvested in Hokkaido in 1991. Four isolates were obtained and examined for their mycological features on culture media, mycotoxin production, and pathogenicity to wheat and barley. The texture of fungal colonies and the morphology of macroconidia on a potato dextrose agar medium were similar to those ofFusarium graminearum. AllF. crookwellense isolates examined produced nivalenol, 4-acetylnivalenol, and zearalenone on rice media at levels ranging from 0.9 to 22.5 µg/g, 0.5 to 25.0 µg/g, and 1.4 to 162.5 µg/g, respectively. All were pathogenic toward the wheat and barley strains tested, and scab symptoms were found on the heads and leaves of plants. This is the first report on domestic isolates ofF. crookwellense from the crop field in Japan.     

Variation in the sensitivity to metalaxyl of Pythium spp. isolated from carrot and other sources

Over a 3-yr period 261 isolates of 17 species of Pythium were tested for sensitivity to metalaxyl at concentrations of 5, 50 or 100 μ/ml. A wide range of responses was observed, from isolates where growth ceased at 5 μg/ml to those where growth at 100 μg/ml was similar to that of the untreated controls. In further tests isolates of 11 different species had ED50's < 1 μg/ml. A lower sensitivity was detected in isolates of six Pythium spp. where values in the range 1–10 μg/ml were obtained. This lower sensitivity was not related to previous known use of metalaxyl. Three isolates of Pythium dissotocum from sites where the fungicide had been used repeatedly had ED50's > 100 μg/ml and were considered resistant. The resistance was stable over a 2-yr period and isolates were cross-resistant to furalaxyl, benalaxyl, ofurace, cyprofuram and oxadixyl. Increasing concentrations of metalaxyl reduced or prevented the production of zoospores by four species of Pythium, although when zoospores were produced, this was followed by the normal processes of encystment and germination. Culturing P. dissotocum on different sub-lethal concentrations of metalaxyl for 18 wk did not induce a high level of resistance to the fungicide.