Colonization of lettuce cultivars and rotation crops by Fusarium oxysporum f. sp. lactucae,the cause of fusarium wilt of lettuce

The severity of fusarium wilt is affected by inoculum density in soil, which is expected to decline during intervals when a non‐susceptible crop is grown. However, the anticipated benefits of crop rotation may not be realized if the pathogen can colonize and produce inoculum on a resistant cultivar or rotation crop. The present study documented colonization of roots of broccoli, cauliflower and spinach by Fusarium oxysporum f. sp. lactucae, the cause of fusarium wilt of lettuce. The frequency of infection was significantly lower on all three rotation crops than on a susceptible lettuce cultivar, and the pathogen was restricted to the cortex of roots of broccoli. However, F. oxysporum f. sp. lactucae was isolated from the root vascular stele of 7·4% of cauliflower plants and 50% of spinach plants that were sampled, indicating a greater potential for colonization and production of inoculum on these crops. The pathogen was also recovered from the root vascular stele of five fusarium wilt‐resistant lettuce cultivars. Thus, disease‐resistant plants may support growth of the pathogen and thereby contribute to an increase in soil inoculum density. Cultivars that were indistinguishable based on above‐ground symptoms, differed significantly in the extent to which they were colonized by F. oxysporum f. sp. lactucae. Less extensively colonized cultivars may prove to be superior sources of resistance to fusarium wilt for use in breeding programmes.     

大丽轮枝菌侵染抗感棉种的组织学过程观察

 棉花黄萎病是一种极难防治的土传性真菌病害,研究病原菌侵染棉花的组织学过程对致病机理解析和抗病资源利用具有重要意义。本研究利用绿色荧光蛋白标记的大丽轮枝菌系统研究了其对抗病棉种海岛棉7124和三裂棉、感病棉种军棉1号和戴维逊棉的侵染过程。结果表明,大丽轮枝菌对抗/感棉种的初始侵染没有明显差异,接菌5 h后,分生孢子均能吸附在感病和抗病棉种的根表面。但侵染过程存在显著差异,侵染感病棉种中病原菌3~5 d到达皮层,5~7 d达到维管束,随后迅速扩展繁殖,侵染14 d后即完成系统侵染,并开始产生黄萎病症状;而病原菌侵染抗病棉种,5~7 d才侵入皮层,7~10 d到达维管束,14 d后仍无法扩展,病原菌的定殖与发展受到限制,无法形成系统侵染,较少形成黄萎病症状。本研究通过绿色荧光蛋白标记大丽轮枝菌对抗/感棉种的侵染过程研究,为大丽轮枝菌致病机理研究和抗性资源利用提供了强有力的理论依据。     

The basis of host recognition in Fusarium oxysporum f. sp. lycopersici

Filtrates from shake-cultures of Fusarium oxysporum f. sp. lycopersici race 1, concentrated to 20% of the original volume, caused cell death in tomato leaf protoplasts from near-isogenic lines corresponding to the compatible cultivar/race reactions of whole plants. Maximum activity was found in late log phase cultures on Czapek-Dox supplemented with 2% casamino acids. Selective toxicity was associated only with the protein fraction of the culture filtrate. LD₅₀ values for susceptible Ace and Moneycross to F. oxysporum f. sp. lycopersici race 1 culture filtrates were 1·92 and 0·36 μg protein ml⁻¹. Corresponding values for cvs Royal Ace and MM161, each containing the I-gene conferring resistance to race 1, were >350. Culture filtrates from F. oxysporum f. sp. lycopersici race 2 gave LD₅₀ values of 2·34 and 2·08 μg protein ml⁻¹ on cvs Ace and Royal Ace, both susceptible to race 2. The LD⁵⁰ of cv. Ace to a non-pathogenic isolate of F. xysporum f. sp. lycopersici was > 350. Culture filtrates from non-host formae of F. oxysporum were 9–149-fold less toxic on cv. Ace. Protoplasts from Pisum sativum, Lactuca sativa, Zea mays, Gossypium barbadense and Solanum melongena, all non-hosts of F. oxysporum f. sp. lycopersici, were 6–175 times less sensitive to F. oxysporum f. sp. lycopersici filtrates than susceptible tomato. The putative toxins lycomarasmin and fusaric acid showed no differential toxicity to I⁺ and I⁻ tomato protoplasts. The results are discussed in the wider context of host-pathogen interaction in which specificity is considered as the recognition of susceptibility by a proteinaceous toxic metabolite of the pathogen. This hypothesis is further extended to include the specificity of F. oxysporum formae and races.     

Fusarium wilt‐resistant lines of Brazil banana (Musa spp., AAA) obtained by EMS‐induced mutation in a micro‐cross‐section cultural system

This study combined the micro‐cross‐section cultural system with in vitro mutagenesis induced by ethyl methanesulphonate (EMS) to screen for fusarium wilt‐resistant lines of Brazil banana (Musa spp., AAA). The results indicated that the optimum EMS concentration and duration for the treatment of micro‐cross‐sections cut from the pseudostem of tissue‐cultured plantlet were 300 mm and 60 min, respectively. Under the optimal treatment, an average of 2·2 regenerated shoots were produced from each explant. One hundred regenerated plantlets were used for screening for fusarium wilt‐resistant lines by the early screening technique. The initial disease symptom – yellowing in lower leaves of susceptible plantlets – was observed 2 weeks after inoculation. After 2 months, only six plants survived – the putative fusarium wilt‐resistant lines. The fusarium wilt pathogen Fusarium oxysporum f. sp. cubense race 4, was identified in the preliminary test field by a SCAR marker technique. Of the six putative resistant lines, five survived the preliminary field test. The regenerated plantlets from these five fusarium wilt‐resistant lines were subjected to early screening again, where they showed markedly reduced disease incidences compared with regenerated plantlets of Brazil banana (control). It was concluded that EMS‐induced mutation of banana through the micro‐cross‐section cultural system is potentially useful for banana improvement.     

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.     

Development of a rapid,accurate glasshouse bioassay for assessing fusarium wilt disease responses in cultivated Gossypium species

A rapid glasshouse‐based bioassay method to screen large numbers of cotton plants for responses to Fusarium oxysporum f. sp. vasinfectum (Fov) was developed. Different Fov inoculum concentrations and methods of inoculation were assessed using resistant and susceptible cotton cultivars. Cotton seeds were planted directly into Fov‐inoculated soil. Studies of seed germination, seedling establishment, seedling mortality and fusarium wilt symptoms (i.e. stunting, foliar symptoms and vascular browning) were performed to optimize the bioassay parameters. Growing seedlings in Fov‐inoculated soils at 5 × 10⁴ or 1 × 10⁵ CFU g^?1 soil, in individual seedling tubes with 12 h at 28–30°C and 12 h at 15–18°C, gave consistent results when assessing Fov disease responses 6 weeks after inoculation. When fusarium wilt resistance ranks (FWRRs) and vascular browning index (VBI) means of 18 Australian and other cotton cultivars from the Fov glasshouse bioassay were compared against their fusarium field performance ranks (F‐ranks), assessed on adult plants for cotton cultivar release, Pearson’s correlation was highly significant for both comparisons. The level of congruence between field and glasshouse data indicated that this protocol should be an effective tool for large‐scale screening for Fov‐resistance responses in diverse germplasm and breeding populations and for advancing genetic research to develop molecular markers for Fov resistance in cotton.     

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.     

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.     

Behaviour of Solanum spp. on inoculation with different isolates of Fusarium oxysporum f. sp. melongenae*

Twelve wild Solanum accessions were tested in a glasshouse at the seedling stage for resistance to Fusarium oxysporum f. sp. melongenae, the causal agent of fusarium wilt of aubergine. Four isolates of the fungus (three Turkish and one Italian) were used. Solanum incanum and S. linneanum were highly susceptible, whereas S. sisymbrifolium, S. torvum and S. aethiopicum Gilo group (one accession) were resistant. In Solanum aethiopicum Aculeatum (two accessions), S. aethiopicum Gilo, S. viarum and S. macrocarpon there were both resistant and susceptible individuals. The sources of resistance found in these wild Solanum spp. could be conveniently used to breed aubergine cultivars resistant to fusarium wilt.     

Changes in polygalacturonase,pectin lyase and cellulase in tomato plants,associated withFusarium inoculation

Activities of polygalacturonase, pectin lyase, and cellulase increased in susceptible, catechol-treated, and resistant tomato plants, after inoculation withFusarium oxysporum f.lycopersici (Sacc.) Snyder and Hansen race 2. The catechol-treated and the resistant plants remained symptomless, while susceptible plants developed symptoms of disease. It is therefore suggested that increased activity of cell-wall-degrading enzymes in inoculated plants does not necessarily cause the development of disease symptoms.     

芝麻枯萎病病原菌致病力室内鉴定方法

 芝麻枯萎病(Sesame Fusarium wilt, SFW)是由尖孢镰刀菌芝麻专化型(Fusarium oxysporum Schl. f. sp. sesami (Zap.), FOS)引起的一种土传真菌病害, 是世界芝麻生产上的主要病害之一。为测定FOS致病力, 本文选用郑芝98N09等4个芝麻品种, 在苗期对15个FOS菌株的致病力强弱进行了室内鉴定和评价。结果表明, 采用1×10⁶个/mL分生孢子悬浮液与无菌蛭石和无菌土壤按V1∶V2∶V6比例混合接菌(即最终接菌浓度为1.4×10⁵孢子/g土壤), 在接菌后第7 d幼苗开始出现枯萎病症状, 调查菌株致病性的最佳时间为接菌后第25 d~28 d;在供试15个FOS菌株中, 对4个品种均表现为强致病力的菌株有8个(DI>50), 均表现为弱致病力的菌株有5个(DI<20);不同芝麻品种对不同菌株的抗性有一定差异。该方法可应用于芝麻枯萎病病原菌致病性测定和芝麻种质抗枯萎病特性评价, 并为后续的机理研究提供了技术支持。     

尖孢镰刀菌侵染对马铃薯光合效率和叶绿素荧光参数影响

为研究枯萎病对马铃薯光合特性的影响,在幼苗期接种尖孢镰刀菌Fusarium oxysporum后,统计抗病品种陇薯10号和感病品种新大坪的病情指数,并测定叶绿素含量、光合及荧光参数。结果表明,尖孢镰刀菌侵染30 d后马铃薯表现出枯萎病症状,叶绿素含量显著降低,其中叶绿素a含量降幅最大,抗病品种陇薯10号比对照降低9.64%,感病品种新大坪比对照降低14.24%。尖孢镰刀菌侵染后马铃薯光合效率显著降低,侵染30 d后,抗病品种陇薯10号净光合速率比对照降低39.56%,感病品种新大坪比对照降低47.13%。病株的光响应曲线参数光补偿点、暗呼吸速率和表观量子效率都显著提高;而光饱和点和最大净光合速率都显著低于对照,表明尖孢镰刀菌侵染缩小了马铃薯对光能的利用有效范围。病株CO2响应曲线参数CO2饱和点、最大净光合速率和羧化效率分别显著低于对照;病株CO2补偿点和光呼吸速率反而升高,说明碳同化过程受到尖孢镰刀菌的限制。暗适应下的初始荧光、最小荧光、最大荧光、PSII最大光化学效率、光适应下PSII最大光化学效率及实际光化学效率、光化学猝灭系数、非光化学猝灭系数和光合电子传递速率均显著低于对照,...     

Effect of Pythium spp. and glyphosate on phytoalexin production and exudation by bean (Phaseolus vulgaris L.) roots grown in different media

Kievitone, phaseollinisoflavan and phaseollin were detected in roots of bean seedlings (Phaseolus vulgaris L.) grown in natural soil. Comparison of phytoalexin production by roots grown in different media indicated that these phytoalexins were probably induced by microorganisms in soil. The influence of common root rot pathogens of bean, Pythium spp., on phytoalexin production was determined. Pythium ultimum elicited kievitone, phaseollinisoflavan and phaseollin in roots grown in sterilized silica sand. P. sylvaticum induced only kievitone and phaseollin in the same growth medium. Glyphosate did not significantly affect the accumulation of phytoalexins within 3 days. However, by day 5, significantly more phaseollin was detected in the roots of Pythium inoculated plants treated with glyphosate than in Pythium inoculated plants not treated with glyphosate. In a hydroponic system, both Pythium spp. elicited accumulation of kievitone and phaseollin in root tissue, and both phytoalexins were exuded into the bathing solution. Glyphosate application did not significantly affect accumulation or exudation of phytoalexins by bean roots in the hydroponic system. The results from this study illustrate the nature and extent of phytoalexin production by bean roots in the absence and presence of microbes.     

A new aggressive strain of Verticillium albo-atrum in Verticillium resistant cultivars of tomato in the Netherlands

In 1991 serious losses caused byVerticillium wilt were found on two holdings in the Westland glasshouse district in the Netherlands in which theVerticillium resistant tomato cultivars Calypso and Criterium were grown in soilless systems. Isolates from diseased plants were identified asVerticillium albo-atrum.In inoculation experimentsVerticillium resistant tomato cultivars were seriously affected by the new isolates but not by a control isolate. Moneydor, a susceptible cultivar without the Ve gene, was the most seriously diseased by all isolates. The isolates from theVerticillium resistant tomato cultivars were less virulent on the susceptible cultivar than the control isolate.     

Induced resistance in tomato plants against Fusarium wilt invoked by Fusarium oxysporum f.sp. dianthi

Tomato plants, susceptible toFusarium oxysporum f. sp.lycopersici, were inoculated by immersing the roots in a conidial suspension ofF. oxysporum f. sp.lycopersici race 1,F. oxysporum f. sp.dianthi race 2 or a mixture of both fungi. Plants inoculated withF. oxysporum f. sp.lycopersici showed disease symptoms after 2 weeks, whereas plants inoculated withF. oxysporum f. sp.dianthi or a mixture of both fungi remained symptomless for over 7 weeks, the duration of the experiment. In another experiment root systems of plants were split and each half was separately inoculated. One half was firstly inoculated withF. oxysporum f. sp.dianthi or treated with water, followed after a week by a second inoculation of the other half withF. oxysporum f. sp.lycopersici or by a water treatment. The disease symptoms in the half firstly inoculated withF. oxysporum f. sp.dianthi were significantly delayed, compared to plants of which that half had been treated with water. BecauseF. oxysporum f. sp.dianthi reduced disease symptoms caused byF. oxysporum f. sp.lycopersici without any direct interaction with this pathogen, it is concluded thatF. oxysporum f. sp.dianthi is able to induce resistance againstF. oxysporum f. sp.lycopersici in tomato plants.     

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     

Cross protection provides evidence for race-specific avirulence factors inFusarium oxysporum

Simultaneous inoculation with races 1 and 2 of the vascular wilt pathogenFusarium oxysporumf.sp.lycopersiciprovided a high level of protection against race 2 in three tomato cultivars carrying resistance geneI, which confers resistance to race 1 but not race 2. However, simultaneous inoculation did not provide any protection in cultivars lacking this gene. Protection resulted in reduction and delay of wilt symptoms. Similarly, avirulent races ofF. oxysporumf.sp.melonisprotected muskmelon plants against virulent races of the sameforma specialis.A ratio 10:1 between spore concentrations of inducer and challenger organism gave the highest cross protection, but ratio 0.1:1 still provided significant disease reduction. Cross protection was also obtained when inoculation with the inducer organism was performed 6 or 12 h before inoculation with the challenger organism. Autoclaved spores of the inducer did not have any protective effect, indicating that living propagules were required to initiate protection. The results suggest the presence of a gene-for-gene interaction betweenF. oxysporumf.sp.lycopersici-tomato andF. oxysporumf.sp.melonis-muskmelon, in which cross protection against a virulent race is mediated by recognition of a specific elicitor from the avirulent race by the plant resistance gene product and by subsequent induction of the plant defense reaction.     

甘薯品种对蔓割病抗性的研究

1984—1987年采用盆苗接种法鉴定890个甘薯品种(系),结果表明:属于高抗和抗病型的41个;中抗型的71个;中感型的261个;感病型和高感型的517个。不同抗性型品种用不不同菌株和不同菌量测定,或在不同气候下重复鉴定,虽会发生不同幅度的抗性变化,但都保持相对稳定的抗病性。同时,用不同抗性型的10个品种分别在盆栽苗期和田间成株期进行抗性鉴定,结果证实了两者间抗性呈极显著正相关。     

Induction of systemic resistance byPseudomonas fluorescens in radish cultivars differing in susceptibility to fusarium wilt,using a novel bioassay

Pseudomonas fluorescens-mediated induction of systemic resistance in radish against fusarium wilt (Fusarium oxysporum f. sp.raphani) was studied in a newly developed bioassay using a rockwool system. In this bioassay the pathogen and bacterium were confirmed to be confined to spatially separate locations on the plant root, throughout the experiment. Pathogen inoculum obtained by mixing peat with microconidia and subsequent incubation for four days at 22 °C, yielded a better percentage of diseased plants than a microconidial suspension drench, an injection of a microconidial suspension into the hypocotyl, or a talcum inoculum.Pseudomonas fluorescens strain WCS374 applied in talcum or peat, but not as a suspension drench, induced systemic resistance. A minimal initial bacterial inoculum density of 10⁵ CFU WCS374 root^–1 was required to significantly reduce the percentage diseased plants. At least one day was necessary between bacterization of strain WCS374 in talcum on the root tips and inoculation of the pathogen in peat on the root base, for an optimal induction of systemic resistance. Strain WCS374 induced systemic resistance in six radish cultivars differing in their susceptibility toF. oxysporum f. sp.raphani. Significant suppression of disease by bacterial treatments was generally observed when disease incidence in the control treatment, depending on pathogen inoculum density, ranged between approximately 40 to 80%. Strains WCS374 and WCS417 ofPseudomonas fluorescens induced systemic resistance against fusarium wilt, whereasP. putida WCS358 did not. This suggests that the induction of systemic resistance byPseudomonas spp. is dependent on strain-specific traits.Abbreviations CFU colony forming units - IFC immunofluorescence colony-staining - ISR induced systemic resistance - PBS phosphate buffered saline - SAR systemic acquired resistance     

Fusarium wilt of basil in Greece: Foliar infection and cultivar evaluation for resistance

Fusarium wilt of basil (Ocimum basilicum), caused byFusarium oxysporum f.sp.basilici, is reported for the first time in Greece. Foliage inoculation of young plants resulted in a downward movement of the pathogen to the crown and roots and 20–30% plant mortality. Of 14 commercial basil cultivars evaluated for possible disease resistance using young plants, six out of eight large-leaved cultivars were found resistant, while all six small-leaved ones were susceptible. http://www.phytoparasitica.org posting Feb. 23, 2004.