Etiology of asparagus replant-bound early decline

Asparagus replant-bound early decline (ARED) was characterized and its etiology was elucidated in experiments under greenhouse and field conditions. Selective soil treatments were used to differentiate between autotoxic compounds and soil-borne pathogens as causal agents. In greenhouse experiments, there were symptoms of ARED within 12—15 weeks. Asparagus plants grown in soil formerly used for asparagus (asparagus soil) showed brown lesions on primary and secondary roots, and many secondary roots had rotted. Root weights of plants grown in asparagus soil were lower than those of plants grown in fresh soil.Fusarium oxysporum f. sp.asparagi (Foa) was by far the most common species among the fungi isolated from roots with lesions. Under greenhouse and field conditions, there were similar symptoms, which indicates that the results obtained under greenhouse conditions are similar to those in the field. The vertical distribution of the ARED-causing factor(s) was studied in a greenhouse experiment in which plants were grown in soil from three layers: 0–30, 30–60, and 60–90 cm. For all four asparagus soils tested, there were ARED symptoms and similar disease severity in samples from all three depths. The causal factor persisted at least 11 years after soil was no longer used for asparagus. When asparagus soil was diluted with fresh soil to give mixtures with 100%, 80%, 50%, 20% and 0% asparagus soil, disease severity did not decrease with increasing dilution of the asparagus soil from 100% to 20%. Disease severity of all mixtures with asparagus soil was significantly higher than that for fresh soil. The results imply that ARED is caused by a pathogen colonizing the soil rather than inhibition by autotoxins released from residues of the preceding asparagus crop. This conclusion is supported by the results of greenhouse and outdoor experiments with heat and fungicide treatments of soil. ARED was nullified by heat treatments of 30 min at 55 or 60 °C but not 45 and 50 °C, eliminating autotoxins as an important cause of ARED because they are heat-stable. Foa is eliminated by a 30-min soil treatment at 55–60 °C but not 50 °C. Prochloraz, known for its toxicity toF. oxysporum, also nullified ARED. Disease severity level was related to the density of Foa in soil. The results provide conclusive evidence thatF. oxysporum f. sp.asparagi is the main cause of ARED in the Netherlands, which largely removes the need to discriminate between early decline and replant-bound early decline, because Foa is the main cause of both diseases.     

Activation of Defense Responses to Fusarium Infection in Asparagus densiflorus

Defense responses to Fusarium oxysporum f. sp. asparagi and F. proliferatum were compared after root inoculation of the asparagus fern, Asparagus densiflorus vars. Myersii and Sprengeri, and cultivated asparagus, A. officinalis cv. Guelph Millennium. Both varieties of A. densiflorus exhibited a hypersensitive response with rapid death of epidermal cells within 8–24 h and restricted the fungal growth. In A. officinalis roots, rapid cell death was not found, and necrotic lesions were observed 8–14 d after fungal inoculation. Peroxidase and phenylalanine ammonia-lyase activities increased significantly in inoculated A. densiflorus but not A. officinalis plants. Local and systemic induction of peroxidase activity was detected after pathogen inoculation in root and spear tissues, respectively, of A. densiflorus. POX activity decreased in roots of inoculated A. officinalis by 8 d post-inoculation. Germination and germ tube growth were inhibited when spores of F. oxysporum f. sp. asparagi were incubated in root exudates and on root segment surfaces of inoculated A. densiflorus plants exhibiting hypersensitive cell death. Spore germination of F. proliferatum and three fungi non-pathogenic to cultivated asparagus was inhibited as well. Rapid induction of hypersensitive cell death in A. densiflorus was associated with restriction of fungal growth, and activation of peroxidase and phenylalanine ammonia-lyase, two defense enzymes thought to be important for plant disease resistance.     

Surveys of cereal diseases and pests in the Netherlands. 3. Monographella nivalis and Fusarium spp. in winter wheat fields and seed lots

Data from surveys of winter wheat fields in the period 1974–1986 and of seed lots in the period 1962–1986 and identifications of diseases on plant samples were compiled to describe the occurrence of snow mould (Monographella nivalis) andFusarium spp. On average,M. nivalis dominated overFusarium spp. The complex ofFusarium spp. constituted mainly ofF. culmorum, followed byF. avenaceum andF. graminearum. M. nivalis was dominant in May on stem-bases and in July on leaves and leaf sheaths. On seedsM. nivalis predominated only in years with low temperatures in July and August.Average brown footrot infection in the field was 4% tillers in May and 5% culms in July. Brown footrot intensity in July was high in cropping seasons with high precipitation in October and with low temperatures in October, November and December. In July during the early eighties, an average of 8% of leaves and 6% of flag leaf sheaths were infected byM. nivalis. Average ear blight incidence was 1.2% glumes infected. Seed contamination by these pathogens averaged 16% in the years 1962–1986. The contamination was high in years with high precipitation in June, July and August. Aspects of cv. resistance and yield loss are illustrated.     

Fusarium wilt on sweet basil: Cause and sources in southeastern Spain

This study concerned a new disease detected in 1997 in southeastern Spain — Fusarium wilt in basil (Ocimum basilicum L.), caused byFusarium oxysporum f.sp.basilici. Its importance was evaluated at two locations in the Almería area, where 14% of the plants presented symptoms of the disease after 4 months of cropping. The search for sources of the disease inoculum was centered on the health of the seeds and the polypropylene trays that were reused for plant production. Analysis of four lots of seeds from Germany and Italy showed that two of them harboredF. oxysporum f.sp.basilici. This finding was confirmed by the analysis of seeds collected from diseased plants. Furthermore, analysis of three reused trays revealed the presence of the pathogen on them and it was concluded that the trays acted as the source of dispersion of the mycosis. http://www.phytoparasitica.org posting July 14, 2004.     

Effect of Fungicides on Fusarium Head Blight and Deoxynivalenol Content in Durum Wheat Grain

In 1998–99 and 1999–2000 six trials were conducted to evaluate the effect of fungicides on Fusarium head blight in the field, on infected kernels and deoxynivalenol (DON) concentration in grain. A single application of prochloraz, tebuconazole, epoxiconazole or bromuconazole, applied to durum wheat varieties at the manufacturer's recommended dose at the beginning of anthesis stage, provided good control of the disease when infective pressure in the field was low to medium, and when the main pathogens were F. graminearum and F. culmorum. Kresoxim-methyl showed a low efficacy at controlling the disease. Tebuconazole, prochloraz and bromuconazole were effective at controlling F. graminearum and F. culmorum, while kresoxim-methyl was not effective in reducing Fusarium infected kernels. DON concentration in grain of cultivars inoculated with F. graminearum and F. culmorum was high, averaging 4.2mgkg^–1 (untreated control). Tebuconazole, prochloraz and bromuconazole reduced DON concentration by 43%, while epoxiconazole was ineffective. DON concentration in kernels of naturally infected cultivars was 1.95mgkg^–1, a concentration which exceeds the 1mgkg^–1 maximum level of contamination allowed in the United States. Furthermore prochloraz, bromuconazole and tebuconazole applications, in the naturally inoculated trials, reduced DON concentration from 73% to 96%, while epoxiconazole showed the lowest effectiveness. Moreover, a positive linear correlation between Fusarium infected grains and the DON concentration was observed.     

Comparison of five stem inoculation methods with respect to phytoalexin accumulation and Fusarium wilt development in carnation

Five methods of stem inoculation of carnations with conidial suspensions ofFusarium oxysporum f.sp.dianthi were compared for uptake of the suspension, induction of phytoalexin accumulation and wilt development. Inoculation was performed by incision of the stem across droplets of inoculum placed on leaves, or by injection of droplets into the stem. With both methods, higher inoculum dosages led to higher wilting rates and higher phytoalexin concentrations. Injection was more effective than incision since a lower inoculum dosage was required to obtain the same phytoalexin levels. Injection therefore appears the most promising technique for the development of routine screening methods for resistance based on phytoalexin accumulation.Samenvatting Vijf methoden om anjers via de stengel te inoculeren met een sporensuspensie vanF. oxysporum f.sp.dianthi werden vergeleken wat betreft de opname van de suspensie, de accumulatie van fytoalexinen en het ziekteverloop. Inoculatie vond plaats door de stengel aan te snijden dwars door een druppel inoculum die op een blad was gelegd, of door druppeltjes inoculum te injecteren. Bij beide methoden leidden hogere doses inoculum tot heviger ziektesymptomen en accumulatie van grotere hoeveelheden fytoalexinen. Injectie van inoculum was effectiever dan aansnijden, daar er minder inoculum nodig was voor eenzelfde resultaat. Injectie biedt daarom meer perspectief voor de ontwikkeling van routinematige resistentietoetsen op basis van de accumulatie van fytoalexinen.     

Microbial community responses associated with the development of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">cucumerinum</Emphasis> after 24-epibrassinolide applications to shoots and roots in cucumber

Fusarium wilt, caused by Fusarium oxysporum f. sp. cucumerinum (FO), is one of the major diseases in cucumber (Cucumis sativus) production. Root and foliar applications of 24-epibrassinolide (EBL), an immobile phytohormone with antistress activity, were evaluated for their effects on the incidence of Fusarium wilt and changes in the microbial population and community in roots of cucumber plants. EBL pre-treatment to either roots or shoots significantly reduced disease severity followed by an improved plant growth regardless of the treatment methods applied. EBL applications decreased the Fusarium population on root surfaces and in nutrient solution, but increased the population of fungi and actinobacteria on root surfaces. PCR-DGGE analysis showed that FO-inoculation had significant effects on the bacterial community on root surfaces as expressed by a decreased diversity index and evenness index, but EBL applications alleviated these changes. Moreover, several kinds of decomposing bacteria and growth-promoting bacteria were identified from root surfaces of FO-inoculated plants and EBL-pre-treated plants, respectively. Overall, these results show that the microbial community on root surfaces was affected by a complex interaction between phytohormone-induced resistance and plant pathogens.     

Genetic variation among <Emphasis Type="Italic">Fusarium</Emphasis> isolates from onion,and resistance to <Emphasis Type="Italic">Fusarium</Emphasis> basal rot in related <Emphasis Type="Italic">Allium</Emphasis> species

The aim of this research was to study levels of resistance to Fusarium basal rot in onion cultivars and related Allium species, by using genetically different Fusarium isolates. In order to select genetically different isolates for disease testing, a collection of 61 Fusarium isolates, 43 of them from onion (Allium cepa), was analysed using amplified fragment length polymorphism (AFLP) markers. Onion isolates were collected in The Netherlands (15 isolates) and Uruguay (9 isolates), and received from other countries and fungal collections (19 isolates). From these isolates, 29 were identified as F. oxysporum, 10 as F. proliferatum, whereas the remaining four isolates belonged to F. avenaceum and F. culmorum. The taxonomic status of the species was confirmed by morphological examination, by DNA sequencing of the elongation factor 1-α gene, and by the use of species-specific primers for Fusarium oxysporum, F. proliferatum, and F. culmorum. Within F. oxysporum, isolates clustered in two clades suggesting different origins of F. oxysporum forms pathogenic to onion. These clades were present in each sampled region. Onion and six related Allium species were screened for resistance to Fusarium basal rot using one F. oxysporum isolate from each clade, and one F. proliferatum isolate. High levels of resistance to each isolate were found in Allium fistulosum and A. schoenoprasum accessions, whereas A. pskemense, A. roylei and A. galanthum showed intermediate levels of resistance. Among five A. cepa cultivars, ‘Rossa Savonese’ was also intermediately resistant. Regarding the current feasibility for introgression, A. fistulosum, A. roylei and A. galanthum were identified as potential sources for the transfer of resistance to Fusarium into onion.     

Pathogenic fungi involved in root rot of peas in the Netherlands and their physiological specialization

Research on root rot pathogens of peas in the Netherlands has confirmed the prevalence ofFusarium solani, F. oxysporum, Pythium spp.,Mycosphaerella pinodes andPhoma medicaginis var.pinodella. Aphanomyces euteiches andThielaviopsis basicola were identified for the first time as pea pathogens in the Netherlands. Other pathogens such asRhizoctonia solani andCylindrocarpon destructans were also found on diseased parts of roots. F. solani existed in different degrees of pathogenicity, and was sometimes highly specific to pea, dwarf bean of field bean, depending on the cropping history of the field.A. euteiches was specific to peas, whereasT. basicola showed some degree of physiological specialization.     

Physiologic races ofFusarium oxysporum f.sp.melonis in the southeastern anatolia region of turkey and varietal reactions to races of the pathogen

Thirty-four isolates ofFusarium oxysporum f.sp.melonis (F.o.m.) obtained from 205 fields in melon-producing areas in the southeastern Anatolia Region of Turkey were identified on the basis of colony morphology and pathogenicity by the root dip method. In this region the mean prevalence of wilt disease was 88.1% and the mean incidence of disease was 47.5%. Physiologic races 0, 1, 2, and 1,2 of the pathogen were determined by their reactions on differential melon cultivars ‘Charentais T,’ ‘Isoblon’, ‘Isovac’ and ‘Margot’ in the greenhouse. Race 1,2, representating 58.8% (20/34) of all isolates, was widely distributed. Of the other pathogenic isolates, eight were identified as race 0, five as race 1, and one as race 2. This is the first report of physiologic races ofF.o.m. in Turkey. Of 44 melon cultivars tested in the greenhouse for resistance toF.o.m. races, 36 were found to be moderately resistant to race 0, 17 were susceptible to race 1,2, 34.1% were highly resistant to race 1, and 52.2% had moderate resistance to race 2. http://www.phytoparasitica.org posting July 16, 2002.     

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.     

Studies on the Infection Process of Fusarium Culmorum in Wheat Spikes: Degradation of Host Cell Wall Components and Localization of Trichothecene Toxins in Infected Tissue

After single spikelet inoculation, the infection process of Fusarium culmorum and spread of fungal hyphae in the spike tissues were studied by scanning and transmission electron microscopy. While hyphal growth on outer surfaces of the spike was scanty and no successful penetration was observed, the fungus developed a dense mycelium on the inner surfaces and effectively invaded the lemma, glume, palea and ovary by penetration pegs. During the inter- and intracellular spreading of the fungus, marked alterations in the host tissues were observed, including degeneration of cytoplasm, cell organelles, and depositions of electron dense material between cell wall and plasmalemma. Ultrastructural studies revealed that host cell walls in proximity of the penetration peg and in contact with hyphae were less dense or transparent which suggested that cell wall degrading enzymes were involved in colonisation of host tissues by fungal hyphae. Enzyme- and immunogold-labelling investigations confirmed involvement of extracellular enzymes, that is cellulases, xylanases and pectinases, in degradation of cell wall components. Localization studies of trichothecenes indicated that toxins could be detected in host tissues at an early stage of infection.     

Siderophore-mediated competition for iron and induced resistance in the suppression of fusarium wilt of carnation by fluorescent Pseudomonas spp

The mechanisms of suppression of fusarium wilt of carnation by two fluorescentPseudomonas strains were studied.Treatments of carnation roots withPseudomonas sp. WCS417r significantly reduced fusarium wilt caused byFusarium oxysporum f. sp.dianthi (Fod). Mutants of WCS417r defective in siderophore biosynthesis (sid^–) were less effective in disease suppression compared with their wild-type. Treatments of carnation roots withPseudomonas putida WCS358r tended to reduce fusarium wilt, whereas a sid^– mutant of WCS358 did not.Inhibition of conidial germination of Fod in vitro by purified siderophores (pseudobactins) of bothPseudomonas strains was based on competition for iron. The ferrated pseudobactins inhibited germination significantly less than the unferrated pseudobactins. Inhibition of mycelial growth of Fod by bothPseudomonas strains on agar plates was also based on competition for iron: with increasing iron content of the medium, inhibition of Fod by thePseudomonas strains decreased. The sid^– mutant of WCS358 did not inhibit Fod on agar plates, whereas the sid^– mutants of WCS417r still did. This suggests that inhibition of Fod by WCS358r in vitro was only based on siderophore-mediated competition for iron, whereas also a non-siderophore antifungal factor was involved in the inhibition of Fod by strain WCS417r.The ability of thePseudomonas strains to induce resistance against Fod in carnation grown in soil was studied by spatially separating the bacteria (on the roots) and the pathogen (in the stem). Both WCS417r and its sid^– mutant reduced disease incidence significantly in the moderately resistant carnation cultivar Pallas, WCS358r did not.It is concluded that the effective and consistent suppression of fusarium wilt of carnation by strain WCS417r involves multiple mechanisms: induced resistance, siderophore-mediated competition for iron and possibly antibiosis. The less effective suppression of fusarium wilt by WCS358r only depends on siderophore-mediated competition for iron.     

Reinforcement of cell wall in roots of Lycopersicon esculentum through induction of phenolic compounds and lignin by elicitors

Changes in phenolic metabolism and lignin deposition have been studied in roots of tomato plants after elicitation with four elicitors which are Fusarium mycelium extract (FME), chitosan (CHT), Fusarium culture filtrate (FCF) and Trichoderma mycelium extract (TME). Most profound effect of elicitors was observed on ferulic acid among the phenolic compounds. After 24 h elicitation, the increase in ferulic acid content of root cell wall was 3.71 and 3.30 times by FME and CHT, respectively. The increase of 4-hydroxybenzoic acid was 2.71 and 2.16 times by these two elicitors. The level of 4-coumaric acid was little more than double by these two elicitors after 24 h elicitation. Most pronounced increase in lignin synthesis was also effected by FME followed by CHT. Lignin deposition in the root cell wall was increased 3.6, 5.4 and 7.1 times by FME during 12, 24 and 36 h after elicitation, respectively. Similarly, CHT increased lignin deposition by 2.8, 5.1 and 6.8 times at 12, 24 and 36 h after elicitation, respectively. FCF and TME also increased lignin deposition significantly in the cell walls of tomato roots during the above time periods of elicitation. Activity of phenylalanine ammonia lyase reached highest level at 24 h post elicitation under the influence of the elicitors. Peroxidase activity registered a sharp increase at 24 h post elicitation. Markedly increased level of polyphenol oxidase activity was found at 12 h post elicitation. Cinnamyl alcohol dehydrogenase activity was observed to reach highest level at 48 h post elicitation. Cell wall strengthening, through the deposition of lignin, preceded by the induction of the synthesizing enzymes appears to play an important role in the defense response of Lycopersicon esculentum in reaction to elicitors, including one derived from Fusarium oxysporum f. sp. lycopersici, the causal organism of Fusarium wilt of tomato.     

Effect of solarization and fumigant applications on soilborne pathogens and root-knot nematodes in greenhouse-grown tomato in Turkey

A study was conducted in two greenhouses with a history of Fusarium crown and root rot (Fusarium oxysporum f.sp.radicis-lycopersici, Forl) and root-knot nematodes (Meloidogyne javanica andM. incognita). During the 2005–06 growing season, the effectiveness of soil disinfestation by solarization in combination with low doses of metham-sodium (500, 750, 1000 and 1250l ha⁻¹) or dazomet (400 g ha⁻¹), was tested against soilborne pathogens and nematodes in an attempt to find a suitable alternative to methyl bromide, which is soon to be phased out. Solarization alone was not effective in the greenhouse with a high incidence ofForl. In the greenhouse with a low level ofForl, all the treatments tested reduced disease incidence, and were therefore considered to be applicable for soil disinfestation. In addition, root-knot nematode density decreased with all the treatments tested in both of the greenhouses.     

Influence of pH, nutrient solution disinfestation and antagonists application in a closed soilless system on severity of fusarium wilt of gerbera

Three trials were carried out during the years 2002–2005 at the Agricultural Experimental Center of Albenga (northern Italy) on gerbera plants grown in a closed soilless system. The efficacy of slow sand filtration and UV treatment in eliminatingFusarium oxysporum f.sp.chrysanthemi (Foc) propagules, naturally present or artificially added to the recirculating nutrient solution, was evaluated. These techniques were tested alone and in combination with the application into the soilless system of different antagonistic strains ofFusarium spp. andTrichoderma spp., isolated from gerbera rhizosphere, and of a commercial formulation ofStreptomyces griseoviridis. The role of the nutrient solution pH in reducingFoc infections was also evaluated. Results showed that slow sand filtration, alone and in combination with the application of biocontrol agents, and a nutrient solution pH higher than 6.0, may induce a significant reduction inFoc infections on gerbera plants grown in closed soilless systems. http://www.phytoparasitica.org posting June 8, 2008. Corresponding author     

大蓟总黄酮对尖孢镰刀菌甜瓜专化型生长、生理的影响及其田间防治效果

为探究大蓟总黄酮对尖孢镰刀菌甜瓜专化型Fusarium oxysporum f. sp. melonis的抑制效果,采用微波超声法提取大蓟种子中的总黄酮,并测定其对其尖孢镰刀菌甜瓜专化型生长、生理指标的影响及其田间防治效果。结果显示,大蓟总黄酮浓度为10 mg/mL时,对尖孢镰刀菌甜瓜专化型的菌丝生长抑制率和孢子萌发抑制率达到100.0%,菌丝畸变、断裂;尖孢镰刀菌甜瓜专化型OD650 nm和pH最小,分别为0.3和5.5;其电导率是对照组1.2倍。不同浓度大蓟总黄酮处理下尖孢镰刀菌甜瓜专化型呼吸强度、超氧化物歧化酶(superoxide dismutase,SOD)和过氧化氢酶(catalase,CAT)活性均呈现先上升后下降的趋势,羧甲基纤维素酶和β-葡萄糖苷酶活性降低,浓度为10 mg/mL大蓟总黄酮处理尖孢镰刀菌甜瓜专化型60 h后,呼吸强度、SOD和CAT活性均降至最低,分别为7.8 mgCO2·cm-2·h-1、22.9 U/mL和20.8 U/mL。田间防治结果表明,浓度为10 mg/mL大蓟总黄酮处理下,甜瓜枯萎株数最少,防治效果最佳,为93.5%。表明大蓟总黄酮能够有效地抑制尖孢镰刀菌甜瓜专化型生长,具有进一步发展为绿色新型植物源尖孢镰刀菌甜瓜专化型防治剂的能力。     

Phylogenetic relationships between the lettuce root rot pathogen Fusarium oxysporum f. sp. lactucae races 1, 2, and 3 based on the sequence of the intergenic spacer region of its ribosomal DNA

The genetic relationship between the vegetative compatibility groups (VCGs) and between physiological races of Fusarium oxysporum f. sp. lactucae (FOL), the causal pathogen of lettuce root rot, was determined by analyzing the intergenic spacer (IGS) region of its ribosomal DNA. A total of 29 isolates containing a type strain were tested: 24 Japanese isolates, 2 Californian isolates, and 3 Italian isolates. Three races (races 1, 2, and 3) were found in Japan, and race 1 was also distributed in California and Italy. Races 1, 2, and 3 each belonged to a distinct VCG: VCG-1, VCG-2, and VCG-3 (VCG-3-1, VCG-3-3), respectively. Phylogenetic (neighbor-joining) analysis of the IGS sequences revealed that races 1, 2, and 3 coincided with three phylogenetic groups (PG): PG-1, PG-2, and PG-3, respectively. These results indicate that the three races are genetically quite different and have a strong correlation with VCGs and phylogenetic groupings. The nucleotide sequence data reported are available in the DDBJ/EMBL/GenBank databases under the accession no. AB195218     

Production of phytotoxic metabolites by five species of Botryosphaeriaceae causing decline on grapevines, with special interest in the species Neofusicoccum luteum and N. parvum

In recent years an increasing number of species of Botryosphaeriaceae have been associated with grapevine decline worldwide. Five species isolated from declining grapevines in Spain (Botryosphaeria dothidea, Diplodia seriata, Dothiorella viticola, Neofusicoccum luteum and N. parvum) were checked for toxin production in liquid cultures. Cultural conditions for all fungi were adjusted to obtain optimal production of phytotoxic culture filtrates, by growing the fungi in steady liquid cultures of Czapek–Dox broth for different time intervals. Phytotoxicity of D. seriata and N. parvum reached a maximum after 14 days while the remaining species showed the highest phytotoxicity levels after 21 days in culture. All fungi produced hydrophilic high-molecular weight compounds with phytotoxic properties. In addition, N. luteum and N. parvum produced lipophilic low-molecular weight phytotoxins, not detected consistently among the remaining species. This led to a more exhaustive study on the phytotoxicity of N. luteum and N. parvum. Culture filtrates and corresponding extracts of both species were consistently highly phytotoxic in different assays. The gas-chromatography analysis of the acetylated O-methyl glycosides of the phytotoxic exopolysaccharides produced by N. parvum showed these substances to be composed mainly of glucose, mannose and galactose. Results suggest that phytotoxic metabolites could be involved in the virulence of both species in planta.     

河北省甘薯镰孢菌腐烂与溃疡病的病原鉴定

为明确河北省甘薯镰孢菌腐烂与溃疡病的症状特点和病原种类,在不同种植区采集储藏期和育秧期病样,描述其症状特征;对病原菌进行分离纯化,采用柯赫氏法则回接验证,依据病原菌的形态特征和基因序列确定病原菌种类。结果表明,在储藏期甘薯发病可导致薯块表面腐烂的占总病薯的59.09%,端部腐烂的占40.91%;薯块带有褐色轮纹病斑的占61.36%,病斑没有轮纹或者轮纹不明显的占38.64%;发病初期薯块内部病斑浅、黑褐色的占27.27%,发病后期薯块内部形成空腔、布满白色菌丝的占72.73%;病斑带有苦味的占59.09%,病斑没有苦味或苦味不明显的占40.91%;在育秧田发病导致薯秧溃疡,表现为主茎基部呈点片发生黑色或者褐色病斑,部分有开裂。分离的病原菌能够同时侵染薯块和薯秧;病原菌单瓶梗产孢,大型分生孢子稍弯,两端钝圆,多数3~7个分隔,顶细胞钝圆,基细胞足跟较明显。其rDNA-ITS、EF-1α、β-tubulin基因序列与茄镰孢菌Fusarium solani的同源性分别为97%、99%和98%。初步确定甘薯镰孢菌腐烂与溃疡病的病原菌为茄镰孢菌F.solani。