The role of phytoalexins in resistance to fusarium wilt in pigeon pea (Cajanus cajan)

Mechanisms of resistance to fusarium wilt (Fusarium udum) were investigated in pigeon pea cultivars from Malawi. Wilt-susceptible (Malawi local) and wilt resistant (ICP 9145) plants were stem-inoculated with a spore suspension containing 2·10⁶ conidia/ml of the pathogen. Occlusion of a small proportion of infected vessels was observed, but the resistant reaction appeared to depend mainly on rapid phytoalexin synthesis. Four fungitoxic isoflavonoid phytoalexins—hydroxygenistein, genistein, cajanin and cajanol-were isolated from plants 15 days after inoculation. Cajanol was identified as the main antifungal compound. The concentration of cajanol was 329·4 μg/g in the resistant cultivar as against 88·6 μg/g in the susceptible cultivar 15 days after inoculation. Crude extract from the resistant plants sampled at 24 h after inoculation contained 34·8 μg ml of cajanol. The LD₅₀ value of cajanol for spore germination was determined as 35μg/ml. The cajanol content of fungus-infected ICP 9145 10 days after inoculation totally inhibited conidial germination of F. udum.     

Effect of different organic amendments on lettuce fusarium wilt and on selected soilborne microorganisms

This study evaluated the effect of different organic amendments on lettuce fusarium wilt caused by Fusarium oxysporum f. sp. lactucae in pots under controlled conditions. Their effects on the density of the pathogen, on the total fungi and on fluorescent Pseudomonas spp. were also evaluated after two subsequent lettuce crops. A significant reduction in the severity of the symptoms of F. oxysporum f. sp. lactucae was found after the use of Brassica carinata pellets (52–79% reduction) and compost (49–67% reduction), while Brassica green manure and cattle and chicken manure only provided partial control of fusarium wilt. However, variations in effectiveness were observed for the same treatment in repeated trials. In general, an increase was observed in Pseudomonas and a decrease in fungal populations in the growing medium, which was obtained by mixing a blonde sphagnum peat and a sandy loam soil with B. carinata pellets and compost after two consecutive cropping cycles. Prolonging the Brassica and compost treatments from 30 to 60 days did not significantly affect disease severity, plant growth or the microbial population of the total fungi or Pseudomonas. The largest lettuce biomass was obtained in the non‐inoculated growing medium amended with brassica flour, chicken manure, B. carinata pellets and compost, as a consequence of fertilization. The treatment with B. juncea green manure, B. carinata (pellets and flour) and compost applied 30 days before planting led to promising results and merits further investigation for use under field conditions.     

Suppression of fusarium wilt of radish by co-inoculation of fluorescentPseudomonas spp. and root-colonizing fungi

In an earlier study, treatment of radish seed with the bacteriumPseudomonas fluorescens WCS374 suppressed fusarium wilt of radish (Fusarium oxysporum f. sp.raphani) in a commercial greenhouse [Leemanet al., 1991b, 1995a]. In this greenhouse, the areas with fusarium wilt were localized or expanded very slowly, possibly due to disease suppressiveness of the soil. To study this phenomenon, fungi were isolated from radish roots collected from the greenhouse soil. Roots grown from seed treated with WCS374 were more abundantly colonized by fungi than were roots from nonbacterized plants. Among these were several species known for their antagonistic potential. Three of these fungi,Acremonium rutilum, Fusarium oxysporum andVerticillium lecanii, were evaluated further and found to suppress fusarium wilt of radish in a pot bioassay. In an induced resistance bioassay on rockwool,F. oxysporum andV. lecanii suppressed the disease by the apparent induction of systemic disease resistance. In pot bioassays with thePseudomonas spp. strains, the pseudobactin-minus mutant 358PSB^– did not suppress fusarium wilt, whereas its wild type strain (WCS358) suppressed disease presumably by siderophore-mediated competition for iron. The wild type strains of WCS374 and WCS417, as well as their pseudobactin-minus mutants 374PSB^– and 417PSB^– suppressed fusarium wilt. The latter is best explained by the fact that these strains are able to induce systemic resistance in radish, which operates as an additional mode of action. Co-inoculation in pot bioassays, ofA. rutilum, F. oxysporum orV. lecanii with thePseudomonas spp. WCS358, WCS374 or WCS417, or their pseudobactin-minus mutants, significantly suppressed disease (except forA. rutilum/417PSB^– and all combinations with 358PSB^–), compared with the control treatment, if the microorganisms were applied in inoculum densities which were ineffective in suppressing disease as separate inocula. If one or both of the microorganism(s) of each combination were applied as separate inocula in a density which suppressed disease, no additional suppression of disease was observed by the combination. The advantage of the co-inoculation is that combined populations significantly suppressed disease even when their individual population density was too low to do so. This may provide more consistent biological control. The co-inoculation effect obtained in the pot bioassays suggests that co-operation ofP. fluorescens WCS374 and indigenous antagonists could have been involved in the suppression of fusarium wilt of radish in the commercial greenhouse trials.Abbreviations CFU colony forming units - KB King's B - PGPR plant growth-promoting rhizobacteria - CQ colonization quotient     

The potential of nonpathogenic Fusarium oxysporum and other biological control organisms for suppressing fusarium wilt of banana

The aim of this study was to evaluate the ability of nonpathogenic F. oxysporum and Trichoderma isolates from suppressive soils in South Africa to suppress fusarium wilt of banana in the glasshouse. Several biological control agents and commercial biological control products were included in the study. The isolates were first screened in vitro on potato dextrose agar. In glasshouse evaluations, the fungal and bacterial isolates were established on banana roots before they were replanted in pathogen-infested soil, while the commercial biocontrol agents were applied as directed by the supplier. Banana plantlets were evaluated for disease development after 7 weeks. In vitro tests showed none of the nonpathogenic isolates suppressed Fusarium oxysporum f.sp. cubense ( Foc ), while slight suppression was observed with the two Trichoderma isolates. Results of the glasshouse evaluations revealed that two of the nonpathogenic F. oxysporum isolates, CAV 255 and CAV 241, reduced fusarium wilt incidence by 87·4 and 75·0%, respectively. The known biological control agent Fo47 did not suppress Foc significantly. Pseudomonas fluorescens strain WCS 417, known for its ability to suppress other fusarium wilt diseases (WCS 417), reduced disease incidence by 87·4%. These isolates should be further evaluated for potential application in the field, independently and in combination.     

Biological control of fusarium wilt of cucumber by chitinolytic bacteria

ABSTRACT Two chitinolytic bacterial strains, Paenibacillus sp. 300 and Streptomyces sp. 385, suppressed Fusarium wilt of cucumber (Cucumis sativus) caused by Fusarium oxysporum f. sp. cucumerinum in nonsterile, soilless potting medium. A mixture of the two strains in a ratio of 1:1 or 4:1 gave significantly (P < 0.05) better control of the disease than each of the strains used individually or than mixtures in other ratios. Several formulations were tested, and a zeolite-based, chitosan-amended formulation (ZAC) provided the best protection against the disease. Dose-response studies indicated that the threshold dose of 6 g of formulation per kilogram of potting medium was required for significant (P < 0.001) suppression of the disease. This dose was optimum for maintaining high rhizosphere population densities of chitinolytic bacteria (log 8.1 to log 9.3 CFU/g dry weight of potting medium), which were required for the control of Fusarium wilt. The ZAC formulation was suppressive when added to pathogen-infested medium 15 days before planting cucumber seeds. The formulation also provided good control when stored for 6 months at room temperature or at 4 degrees C. Chitinase and beta-1,3-glucanase enzymes were produced when the strains were grown in the presence of colloidal chitin as the sole carbon source. Partial purification of the chitinases, followed by sodium dodecyl sulfate polyacrylamide gel electrophoresis and activity staining, revealed the presence of five bands with molecular masses of 65, 62, 59, 55, and 52 kDa in the case of Paenibacillus sp. 300; and three bands with molecular masses of 52, 38, and 33 kDa in the case of Streptomyces sp. 385. Incubation of cell walls of F. oxysporum f. sp. cucumerinum with partially purified enzyme fractions led to the release of N-acetyl-D-glucosamine (NAGA). NAGA content was considerably greater when pooled enzyme fractions (64 to 67) from Paenibacillus sp. were used, because they contained high beta-1,3-glucanase activity in addition to chitinase activity. Suppression of Fusarium wilt of cucumber by a combination of these two bacteria may involve the action of these hydrolytic enzymes.     

Influence of chickpea genotype and Bacillus sp. on protection from Fusarium wilt by seed treatment with nonpathogenic Fusarium oxysporum

Seeds of kabuli chickpea cultivars ICCV 4 and PV 61 were treated with conidia of nonpathogenic Fusarium oxysporum isolate Fo 90105 suspended in methylcellulose (3 × 10⁶ conidia.seed^-1), or with methylcellulose alone, and sown in soil artificially infested with 500 or 1,000 chlamydospores.g^-1 of F. oxysporum f. sp. ciceris race 5. At an inoculum concentration of 500 chlamydospores.g^-1, seed treatment with Fo 90105 significantly increased the incubation period of the disease by 11 (ICCV 4) or 25 (PV 61) days, and reduced the final disease incidence, disease intensity and the standardized area under the curve of disease intensity over time. This protection from disease was higher and more consistent in PV 61 than in ICCV 4. However, it was annulled with an inoculum concentration of 1,000 chlamydospores.g^-1, except for the incubation period in PV 61 which was increased by 10 days. When ICCV 4 seeds were treated with Fo 90105 (3 × 10⁶ conidia.seed^-1) and/or Bacillus sp. isolate RGAF 51 (1 × 10⁷ cfu.seed^-1), then sown in infested soil, there was no influence by the Bacillus isolate on protection conferred by Fo 90105. However, the degree of protection by the nonpathogenic F. oxysporum was higher and more consistent when plants from treated seeds were grown in sterile sand for 6 days, then transplanted into infested soil.     

Growth inhibitors associated with fusarium wilt of tomato

The localization of vascular infections by a process of vascular occlusion is a primary basis for single dominant gene resistance of tomato to fusarium wilt. Failure of this response process to prevent systemic spread of fusarium in susceptible tomato plants is associated with an inhibition of tylose development, a key factor in the overall walling off process. This research was designed to detect, isolate and characterize inhibitory metabolites that accumulate during the course of the tomato/ Fusarium interaction which could inhibit tylose development. Isolation of inhibitors from chloroform extracts of systemically infected tissues was achieved by a series of successive chromatographic separations and bioassays. R_FValues of inhibitors in the several solvent systems employed and their respective colour reactions with chromogenic reagents were recorded. The accumulation of a highly inhibitory compound in chloroform extracts during the early stages of Fusarium infection was determined quantitatively by colourimetric assay in a subsequent experiment. This compound, tentatively identified as rishitin, was isolated from the region of initial infection in stem vascular tissues 1–11 days after inoculation. This inhibitor accumulated rapidly in near-isolines of both resistant and susceptible tomato cultivars in 1–4 days and then continued to accumulate in susceptible, but declined in resistant, plants from 5 to 11 days. The relative phytotoxicity of this compound at low concentrations was determined by assessing its effects on the viability of cultured tomato cells. These data, together with measurements of its accumulation and determinations of its distribution in infected tissues, indicates that rishitin, a host product that accumulates in response to infection, could account for the inhibition of tylose development observed in susceptible tomato plants following infection by Fusarium. Thus we are faced with a paradox in that a substance that presumably functions in defence can, under certain circumstances, interfere with the defence process. A likely resolution of this paradox is discussed.     

蛇床子提取物对黄瓜枯萎病菌抑制效果的研究

从蛇床子中提取抑制黄瓜枯萎菌的活性成分,正交优选提取条件,并在不同浓度下测定其抑制效果。结果表明,在室温下,95%乙醇连续超声提取3次,每次45 min为宜;当提取物浓度为10 mg/mL时,菌丝生长抑制率达78.65%;浓度大于100 mg/L时,孢子萌发抑制率为100%;浓度为8 mg/mL时,相对防效为40.95%。     

甘薯主栽品种对甘薯瘟和蔓割病抗性评价

用甘薯瘟 2个致病型菌株对我国南方薯区 16份主栽品种分菌系接种鉴定 ,结果表明新种花、胜利百号等 10份表现为高感～感病。瑞薯 1号等 3份为高抗Ⅰ型菌、感Ⅱ型菌 ;徐薯 18则为中抗Ⅱ型菌、感Ⅰ型菌。广薯 88-70和湘薯75-55显示出高抗Ⅰ型菌兼具中抗Ⅱ型菌。用蔓割病菌对 16份主栽品种接种鉴定结果表明 ,广薯 111、潮薯 1号等 7份表现高感～中感 ;而福薯26、岩薯5号等9份则为中抗～高抗     

Physiological races and soil population level of fusarium wilt of watermelon

Results of a comparative test oflocal isolates ofFusarium oxysporum f. sp.niveum with isolates from abroad indicate the existence of a highly virulent race of this fungus in Israel. All these isolates were found also pathogenic to resistant cultivars from the U.S.A. When inoculum density was tested, a one-hundredfold higher fungus population was required for extensive wilt of watermelon seedlings in freshly infested sterilized soil compared with the same soil kept dry for three months. In naturally infested soil, where almost 100% of the plants were infected toward the end of the season, the lowest count of the fungus population (400 propagules/g soil) was comparable to the inoculum density required for total wilt of watermelon seedlings in infested sterilized soil.     

Antifungal activity of pomegranate peel extract against fusarium wilt of tomato

Pomegranate (Punica granatum) is an important source of bioactive compounds and has been used in folk medicine for many centuries. This paper describes the in vitro antifungal activity of pomegranate peel aqueous extract (pae) on the development of Fusarium wilt of tomato caused by Fusarium oxysporum, f. sp. lycopersici. HPLC-DAD-ESI/MS analysis was performed to identify punicalagins and ellagic acid, which are the main antifungal compounds. In vivo tests established the efficacy of pae treatments in controlling Fusarium wilt by evaluating improvements in growth variables of tomato plants. At high concentrations, pae showed allelopathic activity in tomato plants. The germination and the radicle growth of tomato seeds were significantly affected by pae. Increasing the extract concentration led to a progressive decrease in germination and in the length of the radicle. The reduction of the Fusarium population in soil and the increase in number of healthy plants obtained as a result of pae treatments indicate that this plant extract could have an important role in biologically-based management strategies for the control of Fusarium wilt in tomato crops.     

Fungal polygalacturonase activity reflects susceptibility of carnation cultivars to fusarium wilt

Carnation cultivars with different levels of partial resistance were inoculated with race 2 of Fusarium oxysporum f.sp. dianthi and monitored for accumulation of host phytoalexins, fungal escape from compartmentalization, production of fungal pectin-degrading enzymes and development of external disease symptoms. Accumulation of phytoalexins, assessed after 10 days in the first 5 cm above the inoculation site, was weakly (methoxydianthramide S) or not (hydroxydianthalexin B) correlated with resistance levels after 12 weeks. Fungal escape from compartmentalization, assessed after 3 weeks as percentages colonized plants at 8 cm above the inoculation site, was highly correlated with expression of susceptibility after 12 weeks. Polygalacturonase (PG) activity, assessed after 4 weeks in the first 5 cm above the inoculation site, was highly correlated to final disease development. Linear increases in disease severity were accompanied by quadratic increases in PG activity. In contrast to water-treated plants, that lacked any PG activity, inoculated plants contained two main groups of fungal PGs, the dominant forms of which had estimated pI values of 7.0 and minimally 9.5, respectively. Compared to those of the first group, enzymes of the second group were produced only in trace amounts in liquid media containing pectin or polygalacturonate as sole source of carbon. On these media, the fungus also produced a pectin methyl esterase (PME) with an estimated pI of 9.3. Besides PMEs of host origin, inoculated plants of susceptible cultivars contained the fungal PME while no more than traces were found in resistant ones.Assessment of phytoalexin production by the host during defense responses cannot replace monitoring of external symptoms as a resistance test. Assessment of fungal growth, whether by reisolations above the compartmentalization area or by measurement of PG activity, provides a both rapid and reliable prediction of disease development.     

Influence of soil and cultural variables on fusarium wilt of melon*

An investigation was carried out into the influence of certain agronomic practices (crop rotation, deinfestation of the soil and use of resistent rootstocks) and of the texture of the soil on populations of Fusarium oxysporum f. sp. melonis found in melon fields situated in the provinces of Cremona and Mantova (northern Italy). To show the associations between the variables taken into consideration, a multiple correspondence analysis was performed. The study confirmed the role played by sandy soil in increasing inoculum density and indicated that only two consecutive years of melon cultivation reach the same result. However, use of resistant rootstocks allows melon to be grown in monoculture without an excessive rise in the level of the pathogen population.     

Foliar spray of validamycin a or validoxylamine a controls tomato fusarium wilt

ABSTRACT Tomato wilt, caused by the soilborne fungus Fusarium oxysporum f. sp. lycopersici, is effectively controlled by a foliar spray of validamycin A (VMA) or validoxylamine A (VAA) (>/=10 mug/ml); however, neither VMA nor VAA is antifungal in vitro. In pot tests, the effect of a foliar application of VMA or VAA at 100 mug/ml lasted for 64 days. Plants sprayed with VMA or VAA accumulated salicylic acid and had elevated expression of the systemic acquired resistance (SAR) marker genes P4 (PR-1), Tag (PR-2), and NP24 (PR-5). Foliar spray of VMA also controlled late blight and powdery mildew of tomato. The disease control by VMA and VAA lasted up to 64 days after treatment, was broad spectrum, and induced the expression of PR genes, all essential indicators of SAR, suggesting that VMA and VAA are plant activators. The foliar application of plant activators is a novel control method for soilborne diseases and may provide an economically feasible alternative to soil fumigants such as methyl bromide.     

香蕉枯萎病菌的培养性状和致病性研究

2001～2004年对采自海南省各市县香蕉种植区发病的1 8个香蕉和粉蕉假茎枯萎病菌分离物进行了培养性状及致病性研究。结果表明,海南省香蕉和粉蕉假茎枯萎病菌的分离物不仅在培养特性上存在明显区别,且在致病性上也存在强、弱2种不同类型:即致病性极强的4号生理小种和致病性相对较弱的1号生理小种。     

Yield loss in chickpeas in relation to development of fusarium wilt epidemics

ABSTRACT Development of 108 epidemics of Fusarium wilt of chickpea caused by Fusarium oxysporum f. sp. ciceris were studied on cvs. P-2245 and PV-61 in field microplots artificially infested with races 0 and 5 of F. oxysporum f. sp. ciceris in 1986 to 1989. Disease progression data were fitted to the Richards model using nonlinear regression. The shape parameter was influenced primarily by date of sowing and, to a lesser extent, by chick-pea cultivars and races of F. oxysporum f. sp. ciceris. Fusarium wilt reduced chickpea yield by decreasing both seed yield and seed weight. These effects were related to sowing date, chickpea cultivar, and virulence of the prevalent F. oxysporum f. sp. ciceris race. Regression models were developed to relate chickpea yield to Fusarium wilt disease intensity with the following independent variables: time to initial symptoms (t(is)), time to inflection point (t(ip)) of the disease intensity index (DII) progress curve, final DII (DII(final)), standardized area under DII progress curve (SAUDPC), and the Richards weighted mean absolute rate of disease progression (rho). Irrespective of the chickpea cultivar x pathogen race combination, the absolute and relative seed yields decreased primarily by delayed sowing. The relative seed yield increased with the delay in t(is) and t(ip) and decreased with increasing DII(final), SAUDPC, and rho. A response surface as developed in which seed yield loss decreased in a linear relationship with the delay in t(is) and increased exponentially with the increase of rho.     

Biological control of fusarium wilt of carnations with different Fusarium oxysporum strains1

Three strains of Fusarium oxysporum were tested for their antagonism to Fusarium oxysporum f.sp. dianthi, two from alluvial soils of the Durance valley (Provence, FR) known to be suppressive to F.o. f. sp. melonis (strains 47 and 20 of Alabouvette) and a third from carnation growing medium, in which it spontaneously appeared (TF3). Strains 47 and 20 were introduced into perlite before planting carnations and found to have variable efficacy against fusarium wilt, according to the formulation of the inoculum. With a talc preparation, these strains gave only 15% witted plants, compared with 49% in the control. With a bran-perlite inoculum, only strain TF3 gave better results than the control. Repeated introduction of TF3 through the season increased the incidence of disease, whereas a single treatment before planting in perlite was effective whatever the formulation (humid bran-perlite, bran-perlite powder, talc, liquid). However, the antagonistic activity of TF3 was not sufficient to control the disease in a naturally infested soil.     

Repeated applications of Penicillium oxalicum prolongs biocontrol of fusarium wilt of tomato plants

The suppression of fusarium wilt of tomato achieved by Penicillium oxalicum (PO) applied one or several times (up to four) was assessed during three glasshouse experiments. The first application of PO (10⁶ conidia g^-1 substrate) to the growing substrate (peat and vermiculite, 1 : 1, v : v) was performed prior to its infestation with Fusarium oxysporum f. sp. lycopersici (FOL) (10⁴–10⁶ chlamydospores g^-1). Repeated applications of PO prolonged the duration of control of fusarium wilt especially when disease incidence was high. The timing of repeated applications of PO did not affect the efficacy of the control. Disease reduction was not associated with a decrease in density of FOL in the rhizosphere, irrespective of the number of applications of PO. Density of PO in the tomato rhizosphere was higher when repeated applications were made. No relationship was observed between reduction of disease and high densities of PO. Reasons for a longer disease reduction in tomato plants following several applications with PO are discussed.     

非洲菊枯萎病病原菌鉴定及药剂防治

经对采集的病害标本分离培养、鉴定和致病性测定结果表明 ,危害非洲菊枯萎病的病原菌为尖孢镰刀菌(Fusarium oxysporum Schl.)。药剂防治试验结果显示 ,甲基硫菌灵、百菌清1000倍液和多菌灵800倍液对非洲菊枯萎病防治效果显著