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1.
ABSTRACT The effect of seed meals derived from Brassica juncea, B. napus, or Sinapis alba on suppression of soilborne pathogens inciting replant disease of apple was evaluated in greenhouse trials. Regardless of plant source, seed meal amendment significantly improved apple growth in all orchard soils; however, relative differences in pathogen suppression were observed. All seed meals suppressed root infection by native Rhizoctonia spp. and an introduced isolate of Rhizoctonia solani AG-5, though B. juncea seed meal often generated a lower level of disease control relative to other seed meal types. When introduction of the pathogen was delayed until 4 to 8 weeks post seed meal amendment, disease suppression was associated with proliferation of resident Streptomyces spp. and not qualitative or quantitative attributes of seed meal glucosinolate content. Using the same experimental system, when soils were pasteurized prior to pathogen infestation, control of R. solani was eliminated regardless of seed meal type. In the case of B. juncea seed meal amendment, the mechanism of R. solani suppression varied in a temporal manner, which initially was associated with the generation of allylisothiocyanate and was not affected by soil pasteurization. Among those tested, only B. juncea seed meal did not stimulate orchard soil populations of Pythium spp. and infection of apple roots by these oomycetes. Although application of B. napus seed meal alone consistently induced an increase in Pythium spp. populations, no significant increase in Pythium spp. populations was observed in response to a composite B. juncea and B. napus seed meal amendment. Suppression of soil populations and root infestation by Pratylenchus spp. was dependent upon seed meal type, with only B. juncea providing sustained nematode control. Collectively, these studies suggest that use of a composite B. juncea and B. napus seed meal mixture can provide superior control of the pathogen complex inciting apple replant disease relative to either seed meal used alone.  相似文献   

2.
ABSTRACT Apple replant disease typically is managed through pre-plant application of broad-spectrum soil fumigants including methyl bromide. The impending loss or restricted use of soil fumigants and the needs of an expanding organic tree fruit industry necessitate the development of alternative control measures. The microbial community resident in a wheat field soil was shown to suppress components of the microbial complex that incites apple replant disease. Pseudomonas putida was the primary fluorescent pseudomonad recovered from suppressive soil, whereas Pseudomonas fluorescens bv. III was dominant in a conducive soil; the latter developed within 3 years of orchard establishment at the same site. In greenhouse studies, cultivation of wheat in replant orchard soils prior to planting apple suppressed disease development. Disease suppression was induced in a wheat cultivar-specific manner. Wheat cultivars that enhanced apple seedling growth altered the dominant fluorescent pseudo-monad from Pseudomonas fluorescens bv. III to Pseudomonas putida. The microbial community resident in replant orchard soils after growing wheat also was suppressive to an introduced isolate of Rhizoctonia solani anastomosis group 5, which causes root rot of apple. Incorporation of high glucosinolate containing rapeseed ('Dwarf Essex') meal also enhanced growth of apple in replant soils through suppression of Rhizoc-tonia spp., Cylindrocarpon spp., and Pratylenchus penetrans. Integration of these methods will require knowledge of the impact of the biofumigant component on the wheat-induced disease-suppressive microbial community. Implementation of these control strategies for management of apple replant disease awaits confirmation from ongoing field validation trials.  相似文献   

3.
Mazzola M 《Phytopathology》1998,88(9):930-938
ABSTRACT Systematic studies were conducted to elucidate the role of different soil microbial groups in the development of apple replant disease. Populations of targeted microorganisms were reduced by the application of semiselective biocides and soil pasteurization. Bacteria were not implicated in the disease, because application of the antibiotic chloramphenicol reduced soil populations of bacteria but failed to improve growth of apple transplants, while enhanced growth was achieved at pasteurization temperatures that did not alter attributes of the bacterial community recovered from apple roots. Populations of Pratylenchus penetrans were below the damage threshold level in eight of nine orchards surveyed, and nematicide applications failed to enhance apple growth in four of five replant soils tested, indicating that plant parasitic nematodes have a minor role or no role in disease development. Application of the fungicide difenconazole or metalaxyl enhanced growth of apple in all five replant soils, as did fludioxinil in the two soils tested. Soil pasteurization enhanced growth of apple and resulted in specific changes in the composition of the fungal community isolated from the roots of apple seedlings grown in these treated soils. Cylindrocarpon destructans, Phytophthora cactorum, Pythium spp., and Rhizoctonia solani were consistently isolated from symptomatic trees in the field and were pathogenic to apple. However, the composition of the Pythium and Rhizoctonia component and the relative contribution of any one component of this fungal complex to disease development varied among the study orchards. These findings clearly demonstrate that fungi are the dominant causal agents of apple replant disease in Washington state.  相似文献   

4.
Mazzola M  Gu YH 《Phytopathology》2000,90(2):114-119
ABSTRACT Studies were conducted to assess the impact of short-term rotations of wheat on microbial community composition and growth of apple in soils from replant orchard sites. Soils from two orchards were cultivated with three successive 28-day growth cycles of 'Eltan', 'Penewawa', or 'Rely' wheat in the greenhouse and subsequently planted to 'Gala' apple seedlings. Cultivation of orchard replant soils with any of the three wheat cultivars enhanced growth of apple relative to that achieved in untreated soils. Improved growth was associated with a marked reduction in apple root infection by species of Rhizoctonia and Pythium. Populations of plant-parasitic nematodes were below damage threshold levels in these orchard soils; however, apple seedlings grown in wheat-cultivated soils had significantly lower root populations of Pratylenchus spp. than did seedlings grown in untreated soils. Growth of apple in 'Penewawa'-cultivated soils often was superior to that observed in soils planted with 'Eltan' or 'Rely'. In untreated orchard soils, fluorescent pseudomonad populations isolated from soil and the apple rhizosphere were dominated by Pseudomonas fluorescens biotype C and Pseudomonas syringae. Cultivation of replant soils with wheat induced a characteristic transformation of the fluorescent pseudomonad population, and Pseudomonas putida dominated the population of this bacterial group recovered from wheat-cultivated replant orchard soils. Results from this study suggest that use of short-term wheat cropping sequences during orchard renovation could be useful in management of replant disease and that this disease-control option may operate, in part, through modification of the fluorescent pseudomonad community.  相似文献   

5.
Mazzola M 《Phytopathology》1997,87(6):582-587
ABSTRACT Rhizoctonia spp. were isolated from the roots of apple trees and associated soil collected in orchards located near Moxee, Quincy, East Wenatchee, and Wenatchee, WA. The anastomosis groups (AGs) of Rhizoctonia spp. isolated from apple were determined by hyphal anastomosis with tester strains on 2% water agar and, where warranted, sequence analysis of the rDNA internal transcribed spacer region and restriction analysis of an amplified fragment from the 28S ribosomal RNA gene were used to corroborate these identifications. The dominant AG of R. solani isolated from the Moxee and East Wenatchee orchards were AG 5 and AG 6, respectively. Binucleate Rhizoctonia spp. were recovered from apple roots at three of four orchards surveyed and included isolates of AG-A, -G, -I, -J, and -Q. In artificial inoculations, isolates of R. solani AG 5 and AG 6 caused extensive root rot and death of 2- to 20-week-old apple transplants, providing evidence that isolates of R. solani AG 6 can be highly virulent and do not merely exist as saprophytes. The effect of binucleate Rhizoctonia spp. on growth of apple seedlings was isolate-dependent and ranged from growth enhancement to severe root rot. R. solani AG 5 and AG 6 were isolated from stunted trees, but not healthy trees, in an orchard near Moxee, WA, that exhibited severe symptoms of apple replant disease, suggesting that R. solani may have a role in this disease complex.  相似文献   

6.
ABSTRACT Brassica tissues are potentially useful in the control of Aphanomyces root rot of peas (Pisum sativum), but identity of the responsible compounds and specific impacts of those compounds on the pathogen's infection potential remain uncertain. Brassica napus seed meals and water extracts from these meals were used to determine the effect of glucosinolate hydrolysis products on Aphanomyces euteiches f. sp. pisi. B. napus meal ('Dwarf Essex') containing glucosinolates and intact myrosinase, the enzyme responsible for glucosinolate hydrolysis, completely inhibited infection by A. euteiches f. sp. pisi oospores. Water extracts from this meal, likewise, severely inhibited infection by oospores, as well as mycelial growth. Extracts from autoclaved 'Dwarf Essex' meal, in which myrosinase was denatured, and a low glucosinolate B. napus variety ('Stonewall') produced little disease reduction and had less impact on mycelial growth. Gas chromatographic analysis of Brassica tissues and water extracts confirmed that glucosinolates remained in autoclaved 'Dwarf Essex' meal and that 'Stonewall' meal contained low glucosinolate concentrations. 5-Vinyloxazolidine-2-thione was identified by mass spectrometry as a dominant glucosinolate hydrolysis product in aqueous extracts of the inhibitory meal. Bioassays conducted with aqueous solutions of this compound reduced mycelial growth, but not to the extent of those from intact 'Dwarf Essex' meal. Water-soluble compounds produced from the hydrolysis of glucosinolates in B. napus tissues reduced A. euteiches oospore infection and inhibited mycelial growth, thus, demonstrating potential utility of Brassica species in the control of A. euteiches.  相似文献   

7.
Burkholderia cepacia (syn. Pseudomonas cepacia) strain PHQM100 applied as a seed coating was tested in growth chamber experiments for its ability to suppress preemergence damping-off, and postemergence damping-off in corn induced by Pythium and Fusarium spp. The symptoms observed in bioassays with soils naturally infested with the fungal pathogens were seed rot with Pythium spp. and mesocotyl and root tissue necrosis in the presence of Fusarium spp. Three corn cultivars that differed in their susceptibility to damping-off pathogens were used. Cultivar L was susceptible to pre- and postemergence damping-off, whereas cv. LPDP and cv. LG11 were moderately resistant and resistant to the damping-off diseases respectively. In the presence of Pythium spp., seed treatment with B. cepacia reduced seed rot, as compared to the untreated seeds, and this reduction was more consistent in the cv. LPDP than in the resistant cv. LG11 or the susceptible cv. L. In soils infested with Fusarium spp., seed treatment significantly reduced root and mesocotyl necrosis as compared to the untreated seeds, and this reduction was more consistent in the resistant cultivars LG11 and LPDP than in the susceptible cv. L. Root colonization levels by B. cepacia were similar in the three corn cultivars tested. Biocontrol efficiency of B. cepacia varied among cultivars mainly due to the differences in their susceptibility to the fungal pathogens. In spite of variability and also irrespective of the soil characteristics, B. cepacia increased seedling emergence and decreased mesocotyl and root necrosis when used as a seed coating.  相似文献   

8.
苹果再植病及病原线虫种的研究   总被引：10，自引：0，他引：10  
 再植苹果苗表现典型的再植病,其症状为:根系分生能力差,生长缓慢,根组织尤其输导根上发生许多黑色病斑,地上部长势差,长梢抽生困难,定植成活率低。从再植苹果苗的根内分离到了线虫,经鉴定为草地根腐线虫(Pratylenchus pratensis(de Man, 1880) Filipjev,1936),并首次阐明为苹果再植病病因之一。不同栽培措施下,幼苗根内线虫密度不同:再植前土壤消毒比不消毒低,接种VA菌根比不接种低,清耕比覆草低,种植三叶草或万寿菊后线虫密度低。这4种栽培措施配套使用可能成为控制苹果再植病的潜在途径之一。  相似文献   

9.
Anaerobic soil disinfestation (ASD) has been shown to be effective in the control of a wide range of soil–borne plant pathogens but has not been examined as a means for disease control in perennial fruit crops such as apple. Since ASD has demonstrated a broad spectrum of biological activity, it may be well suited as an alternative to current fumigation–based control of apple replant disease (ARD) which is caused by a diverse pathogen complex. The efficacy of ASD for control of ARD pathogens was evaluated in growth chamber experiments using soils from two orchard sites having a history of the disease. Suppression of Pratylenchus penetrans apple root densities was dependent upon carbon source utilized during the ASD process. Volatiles emitted during the anaerobic phase from soils treated with ethanol, grass residues, or Brassica juncea seed meal as the carbon input effectively retarded growth of Rhizoctonia solani AG–5, Pythium ultimum and Fusarium oxysporum. Each carbon amendment generated a unique volatile profile produced in the treated orchard soil during ASD. Allyl isothiocyanate (AITC) and dimethyl trisulphide (DMTS) were emitted from B. juncea SM treated soils whereas the latter and 2–ethyl–1–hexanol were detected in soils treated with grass residues. When assayed individually using pure standards, Decanal, DMTS, and AITC retarded in vitro growth of all three fungal/oomycete pathogens. Nonanal was inhibitory toward only P. ultimum and R. solani AG–5, whereas 2–ethyl–1–hexanol only suppressed growth of P. ultimum. AITC and DMTS caused significantly higher mortality of P. penetrans compared to other tested volatiles. These findings demonstrate that carbon source–dependent volatile chemistries contribute significantly but not exclusively to suppression of certain ARD pathogens during the ASD process.  相似文献   

10.
ABSTRACT Good quality seeds of cotton cultivars often escaped pre-emergence damping-off incited by Pythium spp. and Rhizopus oryzae, and they were resistant to postemergence damping-off incited by Rhizoctonia solani. Poor quality seeds, however, were highly susceptible to both phases of seedling disease and required seed treatment in order to survive. Pre-emergence damping-off incited by Pythium spp. and Rhizopus oryzae could be controlled by seed treatment with biocontrol preparations of a number of Trichoderma spp., but these treatments were much less effective in controlling postemergence disease incited by Rhizoctonia solani. Postemergence seedling disease can be controlled by fungicides, but they were much less effective in controlling the pre-emergence phase of the disease. Combination seed treatments of poor quality cotton seeds with fungicides and Trichoderma spp. preparations, followed by planting in pathogen-infested soil, indicated that this technique will control both phases of seedling disease. Seed treatment with either the fungicides or the biocontrol agents alone did not achieve this goal. The optimum combination treatment for disease control was that of chloroneb plus Trichoderma spp., followed by chloroneb plus metalaxyl (Deltacoat AD) plus T. virens strain G-6.  相似文献   

11.
ABSTRACT Previously, the zoosporicidal activity and control of Pythium root rot of flower bulbs by Pseudomonas fluorescens SS101 was attributed, in part, to the production of the cyclic lipopeptide surfactant massetolide A. The capacity of strain SS101 and its surfactant-deficient massA mutant 10.24 to suppress populations and root infection by complex Pythium spp. communities resident in orchard soils was assessed on apple and wheat seedlings and on apple rootstocks. Both strains initially became established in soil and persisted in the rhizosphere at similar population densities; however, massA mutant 10.24 typically was detected at higher populations in the wheat rhizosphere and soil at the end of each experiment. Both strains effectively suppressed resident Pythium populations to an equivalent level in the presence or absence of plant roots, and ultimately suppressed Pythium root infection to the same degree on all host plants. When split-root plant assays were employed, neither strain suppressed Pythium spp. infection of the component of the root system physically separated from the bacterium, suggesting that induced systemic resistance did not play a role in Pythium control. Strain SS101 only marginally suppressed in vitro growth of Pythium spp. and growth was not inhibited in the presence of mutant 10.24. When incorporated into the growth medium, the cyclic lipopeptide massetolide A significantly slowed the rate of hyphal expansion for all Pythium spp. examined. Differences in sensitivity were observed among species, with Pythium heterothallicum, P. rostratum, and P. ultimum var. ultimum exhibiting significantly greater tolerance. Pythium spp. populations indigenous to the two soils employed were composed primarily of P. irregulare, P. sylvaticum, and P. ultimum var. ultimum. These Pythium spp. either do not or rarely produce zoospores, which could account for the observation that both SS101 and mutant 10.24 were equally effective in disease control. Collectively, the results showed that (i) Pseudomonas fluorescens SS101 is very effective in controlling diverse Pythium populations on different crops grown in different soils and (ii) production of the cyclic lipopeptide massetolide A does not play a significant role in disease suppression. Other, as yet undefined mechanisms appear to play a significant role in the interaction between P. fluorescens SS101 and soilborne Pythium spp. communities.  相似文献   

12.
从水稻旱育秧病苗上分离到67个菌株 ,经鉴定分属于镰刀菌58个、腐霉菌7个、丝核菌2个。经回接测定其致病性 ,结果表明致病的镰刀菌主要是串珠镰刀菌 (Fusarium moniliforme) ;腐霉菌中主要是盐腐霉 (Pythium salinum)、间生腐霉 (P .interedium)和顶生腐霉 (P .acrogenum) ;丝核菌为立枯丝核菌 (Rhizoctonia solani)。接种试验表明串珠镰刀菌在6～8d龄幼苗的根中部侵染发病率最高 ,腐霉菌和丝核菌在一叶一心期茎基部侵染发病率最高。药剂试验表明以浸种灵(二硫氰基甲烷)、土菌消(hymexazol)、甲霜灵(metalaxyl)等种子处理加土壤处理 ,防效优于单独种子处理或土壤处理。  相似文献   

13.
The superior growth of wheat following Brassica crops compared to that following non- Brassica crops may be due to the suppression of soilborne fungal pathogens by volatile isothiocyanates (ITCs) released in the soil during hydrolysis of glucosinolates contained in Brassica tissues. We investigated the effects of volatile compounds released from the root, shoot and seed meal tissues of canola ( Brassica napus ) and Indian mustard ( Brassica juncea ) on the mycelial growth of five soilborne pathogens of cereals— Gaeumannomyces graminis var . tritici, Rhizoctonia solani, Fusarium graminearum, Pythium irregulare and Bipolaris sorokiniana. Three isolates of each species, originally collected from the roots of wheat ( Triticum aestivum ) and barley grass ( Hordeum leporinum ) in southern Australia, were exposed to volatiles released in vitro when sterile water was added to freeze-dried Brassica tissues. The root and shoot tissues of both Brassica species were more suppressive at flowering than maturity and mustard tissues were generally more suppressive than canola. The degree of fungal suppression by the various Brassica tissues was related to the concentration and type of isothiocyanates released, which varied with Brassica species, tissue age and tissue type. There were significant differences in the sensitivity of the fungal species and among isolates of each species. Gaeumannomyces and Rhizoctonia were generally the most sensitive to the volatiles released, Pythium and Bipolaris the least. The results indicate that the effectiveness of fungal suppression by Brassica crops will depend upon the species, age and type of Brassica tissue, which influence the type and concentration of isothiocyanates evolved, and the sensitivity of the pathogen.  相似文献   

14.
Surveys of brassica seed crops in Essex and Suffolk showed that Alternaria spp. occurred in many crops of Brassica oleracea in the years 1977–1980 affecting up to 100% of pods in each year. A. brassicicola was the only species present in 1976 and was the domioant pathogen in succeeding years but A. brassicae increased in frequency from 1977, causing 24% of the pod infections on B. oleracea in 1980. The latter fungus was the dominant species in crops of oilseed rape ( B. napus ), the mean incidence of infected pods increasing from 0.5% in 1977 to 2.9% in 1980. Leptosphaeria maculans was not found in horticultural brassica seed crops in 1976 but occurred abundantly in these crops and in oilseed rape crops in each of the following years.  相似文献   

15.
Studies were conducted to assess the importance of soil-borne pathogens as impediments to the production of direct-drilled wheat following Kentucky bluegrass in rotation. Bluegrass sods were collected as cores from commercial seed fields in October (after open-field burning) and in December and March (after exposure to normal freezing and thawing), treated with glyphosate after a 2- to 3-week period of green-up in the glasshouse, and then planted to wheat 21 days later. Wheat seedlings in the 3- to 4-leaf stage were stunted and spindly in either burned or nonburned sods collected in October, but grew significantly better in burned than nonburned sods collected in December and March. Fumigation (methyl bromide), simulated tillage, and application of metalaxyl as a drench, singly or as combined treatments, improved the growth of wheat in nonburned sods. Growth of wheat in nontreated burned sods collected after frost had penetrated the upper portions was equivalent to that in fumigated sods. Experimental freezing at −20°C for up to 96 h, of burned and nonburned sods collected in October, followed by thawing also improved the growth of wheat subsequently sown directly into these sods. The stunted and spindly seedlings had roots girdled and severed by lesions typical of Rhizoctonia root rot. Isolates of Rhizoctonia cerealis were recovered from these roots and caused poor seedling emergence and stunted root and shoot growth of both wheat and bluegrass. The results suggest that both R. cerealis and Pythium spp. are potentially important soil-borne pathogens of wheat when following Kentucky bluegrass in rotation and that burning done routinely to improve the yield of bluegrass seed can also reduce the potential for damage caused by Rhizoctonia and Pythium root rots of wheat seeded directly into bluegrass sod.  相似文献   

16.
Diab HG  Hu S  Benson DM 《Phytopathology》2003,93(9):1115-1123
ABSTRACT Peat moss-based potting mix was amended with either of two composted swine wastes, CSW1 and CSW2, at rates from 4 to 20% (vol/vol) to evaluate suppression of pre-emergence damping-off of impatiens (Impatiens balsamina) caused by Rhizoctonia solani (anastomosis group-4). A cucumber bioassay was used prior to each impatiens experiment to monitor maturity of compost as the compost aged in a curing pile by evaluating disease suppression toward both Pythium ultimum and R. solani. At 16, 24, 32, and 37 weeks after composting, plug trays filled with compost-amended potting mix were seeded with impatiens and infested with R. solani to determine suppression of damping-off. Pre-emergence damping-off was lower for impatiens grown in potting mix amended with 20% CSW1 than in CSW2-amended and nonamended mixes. To identify relationships between disease suppression and microbial parameters, samples of mixes were collected to determine microbial activity, biomass carbon and nitrogen, functional diversity, and population density. Higher rates of microbial activity were observed with increasing rates of CSW1 amendment than with CSW2 amendments. Microbial biomass carbon and nitrogen also were higher in CSW1-amended mixes than in CSW2-amended potting mixes 1 day prior to seeding and 5 weeks after seeding. Principal component analysis of Biolog-GN2 profiles showed different functional diversities between CSW1- and CSW2-amended mixes. Furthermore, mixes amended with CSW1 had higher colony forming units of fungi, endospore-forming bacteria, and oligotrophic bacteria. Our results suggest that enhanced microbial activity, functional and population diversity of stable compost-amended mix were associated with suppressiveness to Rhizoctonia damping-off in impatiens.  相似文献   

17.
Howell CR 《Phytopathology》2002,92(2):177-180
ABSTRACT Planting the cotton cv. Sure-Grow 747 in cotton seedling disease plots during the 2001 growing season resulted in high levels of preemergence damping-off among the seedlings. Four cotton pathogens, Pythium aphanidermatum, P. ultimum, an unidentified Pythium sp., and Rhizopus oryzae, were isolated from diseased seed embryos and seedlings. Disease incited by the Pythium spp. could be controlled by seed treatment with Metalaxyl, but disease incited by R. oryzae could not. Seed treatment with Metalaxyl in naturally infested field soil was only partially effective; therefore, symptoms in 47% of the diseased seedlings could be attributed to R. oryzae. Susceptibility to disease appeared to be related to release in the spermosphere, by the germinating seeds, of compounds that stimulate pathogen propagule germination, because exudates from seed of the suscept Sure-Grow 747 and extracts from wheat bran induced pathogen germination and growth, whereas exudates from resistant cv. Stoneville 213 did not. However, even Stoneville 213 became susceptible when infested soil was amended with wheat bran. Seed treatment with preparations of Trichoderma virens parent, mutant, and hybrid strains gave effective biological control of preemergence damping-off. Disease control was attributable to metabolism by the biocontrol agent of pathogen germination stimulants released by the seed, because amendment of pathogen-infested soil with the propagule germination stimulants in wheat bran negated the protective effect of the seed treatment.  相似文献   

18.
ABSTRACT Suppression of seedling damping-off disease caused by Pythium spp. and Rhizoctonia solani is a potential benefit of formulating soilless container media with compost. Thirty-six compost samples from Pacific Northwest commercial composting facilities were analyzed for a number of physical, chemical, and biological properties, including suppression of damping-off caused by Pythium ultimum, P. irregulare, and R. solani. The samples were produced from diverse feedstocks and composting technol ogies; this was reflected in a large degree of variability in the measured properties. When mixed with sphagnum peat moss and inorganic aggregates, 67% of the compost samples significantly suppressed P. irregulare damping-off of cucumber, 64% suppressed P. ultimum damping-off of cucumber, and 17% suppressed damping-off of cabbage caused by R. solani. Suppression of Pythium damping-off was related to the potential of compost to support microbial activity and a qualitative index of ammonia volatilization. Suppression of Rhizoctonia damping-off was not related to any one compost factor. Currently available compost products potentially could provide commercially acceptable control of damping-off caused by Pythium spp., but it is necessary to fortify composts with microbial antagonists for the control of R. solani.  相似文献   

19.
A total of 101 Brassica napus ssp. oleifera accessions with seed differing in glucosinolate and erucic acid contents were screened for resistance to four isolates of Peronospora parasitica at the cotyledon stage. Two groups of accessions with different resistance factors were identified. Lines that were homogeneous for resistance were selected from seedling populations of accessions that exhibited a heterogeneous reaction to some isolates. The resistance of one group differs from that of cv, Cresor, the only oilseed rape cultivar reported to have an isolate-specific gene for resistance to P. parasitica. The isolate specificity of the second group was identical to that of cv, Cresor, A comparison of the response of host accessions which expressed moderate to full susceptibility at the cotyledon stage, with no clear differential response to any of the four P. parasitica isolates, indicated that those with high glucosinolate and high erucic acid contents (12 accessions) were slightly but significantly less susceptible than those with high glucosinolate and low erucic acid (19 accessions), or low glucosinolate and low erucic acid contents (28 accessions). The mean differences between accessions with low erucic acid but differing in glucosinolate content were inconsistent. The last result was further confirmed by investigating the expression of resistance to three isolates of P. parasitica at three different seedling growth stages among 11 accessions of oilseed rape with seeds low in erucic acid but differing in glucosinolate content.  相似文献   

20.
Seeds of cress and sugar-beet were coated with oospores of Pythium oligandrum using commercial seed-pelleting or film-coating procedures. Following either procedure approximately 104 oospores were recovered from both seed types, achieving 75.94% of the targeted dose. Oospore germination (9.19%) was unaffected by the coating treatments. Both types of treatment reduced damping-off of cress caused by P. ultimum in artificially infested sand and potting compost and by Rhizoctonia solani in artificially infested sand. In some cases, the level of control was equivalent to fungicide drenches. In general, pelleting of P. oligandrum on cress gave better control than film-coating treatments. P. oligandrum also reduced damping-off of sugar-beet in soil naturally infested with Aphanomyces cochlioides and Pythium spp. Control was equivalent to that achieved with hymexazol fungicide seed-coating treatments and was related to the inoculum potential of A. cochlioides in the soil; neither standard hymexazol coatings nor P. oligandrum treatments gave control at high inoculum potentials. P. oligandrum was not rhizosphere competent on cress or sugar-beet.  相似文献   

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