戊唑醇对小麦赤霉菌侵染影响的细胞学研究

采用电镜技术研究了三唑类杀菌剂戊唑醇(tebuconazole)对赤霉病菌Fusarium gramineaum侵染小麦穗部过程的影响.结果表明:人工接种前2天施药,可推迟外稃内表皮、内稃及子房上分生孢子的萌发,但不能完全抑制其萌发,可引起芽管和菌丝严重畸形,不能形成侵染菌丝侵入寄主.而人工接种后2天施药,戊唑醇则严重抑制了病菌菌丝的生长,使寄主体表和寄主组织内的菌丝形态、结构发生了一系列异常变化,并最终塌陷死亡,使菌丝不能扩展到穗轴部位.接种后4天施药,病菌虽已扩展到穗轴,但戊唑醇仍对穗轴中菌丝生长具有明显的抑制作用.对赤霉毒素的免疫细胞化学标记结果表明,药剂处理与未处理的寄主和菌丝细胞中都存在有毒素,但标记密度在药剂处理的寄主和菌丝细胞中明显低于未处理对照.     

戊唑醇抑制苹果树腐烂病菌的形态毒理学研究

为揭示苹果树腐烂病防控常用药剂戊唑醇的杀菌机理,本研究通过显微技术观察了戊唑醇对苹果树腐烂病致病菌苹果黑腐皮壳菌Valsa mali 孢子萌发、菌丝形态及细胞结构的影响。结果发现:戊唑醇能够抑制病原菌孢子萌发,但不影响孢子的膨大,主要是抑制其芽管的伸长,使芽管畸形、增粗、分枝增多等,从而不能正常侵入寄主。经戊唑醇处理后,病原菌菌丝形态和细胞结构均发生了明显变化,主要表现为:菌丝顶端膨大、分枝增多,菌丝增粗等;细胞隔膜增多且不规则增厚,细胞壁不规则增厚,线粒体增多、膜增厚或不规则缢缩,细胞核增多、核仁弥散,细胞液泡化严重,形成空腔,原生质外渗,细胞最终坏死等。同时,在已坏死的菌丝内可发现子菌丝,且子菌丝也表现出异常现象,如细胞壁不规则增厚、线粒体数量增多及细胞质坏死等。研究表明,戊唑醇不仅对苹果树腐烂病菌孢子萌发具有一定影响,并可导致芽管和菌丝细胞畸形,从而显著抑制病菌的成功侵染。该结果可为采用戊唑醇淋刷树干进行病害预防提供理论依据。     

小麦穗组织中脱氧镰刀菌烯醇毒素的免疫细胞化学定位

 采用免疫细胞化学技术对禾谷镰刀菌(Fusarium graminearum)在侵染小麦穗部过程中产生的脱氧镰刀菌烯醇毒素(deoxynivalenol,DON)进行了定位分析。在接种后24h,当菌丝在外稃、内稃的内侧表面扩展而尚未侵入寄主细胞前,病菌已分泌DON,并且DON已扩散到寄主组织内。在菌丝细胞内,DON主要被定位于细胞质、线粒体及细胞壁上;在寄主细胞中DON主要分布于细胞壁、叶绿体、细胞质和内质网上。在侵染初期(接种后2 d),菌丝仅能在寄主细胞间隙扩展,随寄主组织中DON浓度的升高,寄主细胞相应发生了一系列病理变化。随寄主细胞坏死(接种后3~4d),病菌进入坏死的寄主细胞。上述结果表明,DON在禾谷镰刀菌的侵染、致病和定殖过程中起着重要的作用。毒素标记结果表明病菌产生的毒素可通过穗轴微管束组织从侵染部位向上、向下转输,毒素向上的转输量明显高于向下转输     

Efficacy of triazole-based fungicides for fusarium head blight and deoxynivalenol control in wheat: a multivariate meta-analysis

The effects of propiconazole, prothioconazole, tebuconazole, metconazole, and prothioconazole+tebuconazole (as a tank mix or a formulated premix) on the control of Fusarium head blight index (IND; field or plot-level disease severity) and deoxynivalenol (DON) in wheat were determined. A multivariate random-effects meta-analytical model was fitted to the log-transformed treatment means from over 100 uniform fungicide studies across 11 years and 14 states, and the mean log ratio (relative to the untreated check or tebuconazole mean) was determined as the overall effect size for quantifying fungicide efficacy. Mean log ratios were then transformed to estimate mean percent reduction in IND and DON relative to the untreated check (percent control: C(IND) and C(DON)) and relative to tebuconazole. All fungicides led to a significant reduction in IND and DON (P < 0.001), although there was substantial between-study variability. Prothioconazole+tebuconazole was the most effective fungicide for IND, with a C(IND) of 52%, followed by metconazole (50%), prothioconazole (48%), tebuconazole (40%), and propiconazole (32%). For DON, metconazole was the most effective treatment, with a [Formula: see text](DON) of 45%; prothioconazole+tebuconazole and prothioconazole showed similar efficacy, with C(DON) values of 42 and 43%, respectively; tebuconazole and propiconazole were the least effective, with C(DON) values of 23 and 12%, respectively. All fungicides, with the exception of propiconazole, were significantly more effective than tebuconazole for control of both IND and DON (P < 0.001). Relative to tebuconazole, prothioconazole, metconazole, and tebuconzole+prothioconzole reduced disease index a further 14 to 20% and DON a further 25 to 29%. In general, fungicide efficacy was significantly higher for spring wheat than for soft winter wheat studies; depending on the fungicide, the difference in percent control between spring and soft winter wheat was 5 to 20% for C(IND) and 7 to 16% for C(DON). Based on the mean log ratios and between-study variances, the probability that IND or DON in a treated plot from a randomly selected study was lower than that in the check by a fixed margin was determined, which confirmed the superior efficacy of prothioconazole, metconazole, and tebuconzole+prothioconzole for Fusarium head blight disease and toxin control.     

An antagonistic rhizoplane bacterium Pseudomonas sp. strain EC-S101 physiologically stresses a spinach root rot pathogen Aphanomyces cochlioides

We observed that an antagonistic rhizoplane bacterium Pseudomonas sp. strain EC-S101 induces excessive lateral and apical branching in the hyphae of a root rot phytopathogen Aphanomyces cochlioides AC-5 resulting in radial growth inhibition of hyphae in a dual culture assay. Confocal laser scanning microscopic observations using fluorescent stains indicated an increased quantity of nuclei and lipid bodies in the affected hyphae during the early stage (less affected hyphae) at day 3 of interaction. At a more advanced stage (severely affected hyphae) at day 3, nuclei became smaller and round-shaped compared with the oval shape in AC-5 control hyphae. After 7 days, nuclei disintegrated, and the nuclear materials were released into the disorganized cytoplasm. With transmission electron microscopy at 5 days of interaction, we found that the cell walls of AC-5 hyphae were considerably thicker than those of the control. Enlarged vacuoles, lipid bodies sunk into vacuoles, and vacuoles filled with electron-dense material, followed by an invagination of the AC-5 hyphal cell wall, were commonly observed. Nonmembranous electron-transparent inclusion bodies irregular in size were often distributed in the affected hyphae. By integrating our observations, we conclude that antagonistic effects evoked by strain EC-S101 resulted in the death of AC-5 hyphae, which might contribute to the suppression of A. cochlioides AC-5-linked damping-off disease in its host plants. The nucleotide sequence data reported are available in the DDBJ/EMBL/GenBank databases under the accession number AB190286     

Virulence and toxin synthesis of an azole insensitive Fusarium culmorum strain in wheat cultivars with different levels of resistance to fusarium head blight

Fusarium culmorum causes head blight, produces toxins and reduces yield and quality of cereals. To prevent damage caused by fusarium head blight (FHB), azole fungicides are mainly applied. The occurrence of insensitivity to azoles is a major problem in agriculture. The present study shows that a tebuconazole insensitive strain of F. culmorum can be readily produced in the laboratory, but that the resulting strain of the fungus is of lower fitness in vitro. Insensitivity was confirmed microscopically and by cell viability and metabolic activity. The tebuconazole insensitive strain shows cross insensitivity to nine important azoles. In addition, plants inoculated with the insensitive F. culmorum strain showed no reduction of FHB symptoms and deoxynivalenol (DON) content after tebuconazole treatment, compared to an inoculation with the sensitive strain. Use of wheat cultivars carrying a high resistance level (i.e. cv. Toras) was the most effective method for reducing symptoms and decreasing DON content, independent from the level of fungicide insensitivity of the F. culmorum strain. In conclusion, resistant cultivars and a fungicide mixture which combines different mechanisms of action in fungal metabolism should be applied to avoid fungicide insensitivity of Fusarium spp. in future.     

Relationship Between Growth and Mycotoxin Production by Fusarium species, Biocides and Environment

Fusarium head blight of cereals has, in recent years, become one of the most important pre-harvest diseases worldwide. This paper examines the in vitro efficacy of fungicides to control Fusarium species in cereals and the efficacy in the field on both Fusarium infection of ripening ears as well as their impact on mycotoxin production. Field studies suggest that fungicides such as tebuconazole and metconazole give good control of both Fusarium infection of ears and control of deoxynivalenol (DON) production. However, azoxystrobin and related fungicides are less effective, and grain from treated crops has sometimes been found to have increased concentrations of DON and nivalenol. Studies of isolates of Fusarium culmorum from different parts of Europe showed that complex interactions occur between environmental factors, fungicide type and isolate in relation to growth inhibition and DON production. These studies confirmed the ineffectiveness of azoxystrobin and suggest that environmental stress factors, particularly water availability and temperature, and low fungicide doses may stimulate mycotoxin production by Fusaria in vitro and in wheat grain.     

A rapid differential staining technique for Fusarium pseudograminearum in cereal tissues during crown rot infections

A novel fluorescence‐based differential staining procedure has been developed for the rapid visualization of Fusarium pseudograminearum hyphae in cereal tissues. Infected tissues are stained with safranin, which binds to host cell walls, while hyphal tissues are stained with the fluorescent dye solophenyl flavine 7GFE. The method is rapid, does not require clearing of culm cross‐sections, and provides high contrast informative images of fungal and plant structures during pathogenesis.     

Immunocytochemical localization of β-1,3-glucanase and chitinase in Fusarium culmorum -infected wheat spikes

Two antisera raised against acidic β-1,3-glucanase and acidic chitinase from tobacco were used to investigate the subcellular localization of the two enzymes in Fusarium culmorum -infected wheat spike by means of the immunogold labelling technique. The studies demonstrated that the distribution of β-1, 3-glucanase and chitinase were very similar in the uninoculated healthy and infected wheat spikes. The enzymes were localized mainly in the cell walls of different tissues including the lemma, ovary and rachis of the wheat spike, while the cytoplasm and organelles of cells in these tissues showed almost no labelling. However, the accumulation of β-1,3-glucanase and chitinase in the infected wheat spikes differed distinctly between resistant and susceptible wheat cultivars. The labelling densities for the two enzymes in the infected lemma, ovary and rachis of the susceptible cultivar Agent increased only slightly as compared to the corresponding uninoculated healthy tissues, whereas higher labelling densities of β-1,3-glucanase and chitinase were found in the infected tissues of wheat spikes from the resistant cultivar Arina compared to the corresponding uninoculated healthy tissues. Furthermore, the labelling of β-1,3-glucanase and chitinase also occurred over the cell walls of the hyphae in the infected wheat spike, but not over the hyphal cytoplasm. In addition, labelling for the two enzymes was often detected over the cell wall appositions and the electron-dense material located between the host cell and the hyphal cell in the infected tissues of the resistant wheat cultivar. The findings reported in the present study indicate that β-1,3-glucanase and chitinase accumulation in the F. culmorum -infected wheat spike may be involved in resistance to pathogen spread in the host tissue.     

几种杀菌剂防控小麦赤霉病穗腐及籽粒脱氧雪腐镰刀菌烯醇(DON)毒素的评价

小麦赤霉病是小麦穗期的主要病害之一。化学防控一直是小麦主产区防控赤霉病的主要措施。为明确几种新型杀菌剂对小麦赤霉病的防效和对小麦籽粒DON毒素含量的影响,于2018年进行了氰基丙烯酸酯类和三唑类杀菌剂单剂及其复配剂对赤霉病的防效试验。结果表明:30%戊唑·多菌灵悬浮剂(SC)1500 mL/hm^2处理对赤霉病病穗防效达92.40%,病指防效达93.20%,小麦籽粒DON毒素检出量较不用药对照降低80.38%;25%氰烯菌酯SC 2000 mL/hm^2处理对赤霉病的病穗防效达86.80%,病指防效达88.78%,小麦籽粒DON毒素检出量较不用药对照降低88.19%;48%氰烯·戊唑醇SC 900 mL/hm^2和40%丙硫·戊唑醇SC 600 mL/hm^2对小麦赤霉病的病穗防效分别为77.20%、78.00%,病指防效分别为80.27%和79.59%,对籽粒DON毒素检出量较不用药对照分别降低73.87%和81.42%。在小麦赤霉病较重发生的情况下,上述4种杀菌剂单剂或复配剂1次用药既能高效控制病情,又能有效控制小麦籽粒DON毒素不超标。本试验研究进一步阐明,氰烯菌酯、戊唑醇、丙硫菌唑等杀菌剂及其复配剂均能有效控制小麦赤霉病的危害,并能有效降低小麦籽粒DON毒素含量;吡唑醚菌酯单剂及其复配剂虽然对小麦赤霉病的病穗和病指防效也较高,但控制小麦籽粒DON毒素含量效果相对较差。研究结果为小麦穗期赤霉病化学防控提供了科学参考。     

Ultrastructural and immunocytochemical investigation of pathogen development and host responses in resistant and susceptible wheat spikes infected by Fusarium culmorum

Pathogen development and host responses in wheat spikes of resistant and susceptible cultivars infected by Fusarium culmorum causing Fusarium head blight (FHB), were investigated by means of electron microscopy as well as immunogold labelling techniques. The studies revealed similarities in the infection process and the initial spreading of the pathogen in wheat spikes between resistant and susceptible cultivars. However, the pathogen’s development was obviously more slow in the resistant cultivars as in comparison to a susceptible one. The structural defence reactions such as the formation of thick layered appositions and large papillae were essentially more pronounced in the infected host tissues of the resistant cultivars, than in the susceptible one. β -1,3-glucan was detected in the appositions and papillae. Furthermore, immunogold labelling of lignin demonstrated that there were no differences in the lignin contents of the wheat spikes between susceptible and resistant cultivars regarding the uninoculated healthy tissue, but densities of lignin in host cell walls of the infected wheat spikes differed distinctly between resistant and susceptible cultivars. The lignin content in the cell walls of the infected tissues of the susceptible wheat cultivar increased slightly, while the lignin accumulated intensely in the host cell walls of the infected wheat spikes of the resistant cultivars. These findings indicate that lignin accumulation in the infected wheat spikes may play an important role in resistance to the spreading of the pathogen in the host tissues. Immunogold labelling of the Fusarium toxin DON in the infected lemma showed the same labelling patterns in the host tissues of resistant and susceptible cultivars. However, there were distinct differences in the toxin concentration between the tissues of the susceptible and resistant cultivars. At the early stage of infection, the labelling densities for DON in resistant cultivars were significantly lower than those in the susceptible one. The present study indicates that the FHB resistant cultivars are able to develop active defence reactions during infection and spreading of the pathogen in the host tissues. The lower accumulation of the toxin DON in the tissues of the infected spikes of resistant cultivars which results from the host’s defence mechanisms may allow more intensive defence responses to the pathogen by the host.     

Importance of Cell Wall Degrading Enzymes Produced by Fusarium graminearum during Infection of Wheat Heads

Cytological studies were carried out to elucidate the importance of cell wall degrading enzymes (CWDE) during infection of wheat spikes by Fusarium graminearum. Scanning electron micrographs revealed that at 6–24 hours after inoculation (hai) of single spikelets with macroconidia of F. graminearum, the fungus germinated by forming several germ tubes and developed a dense hyphal network in the cavity of the spikelet. At 24–36hai, the fungus formed infection hyphae which invaded the ovary and inner surface of the lemma and palea. Transmission electron microscopical studies revealed that the fungus extended inter- and intracellularly in the ovary, lemma and rachis and caused considerable damage and alterations to the host cell walls. In different tissues of healthy and F. graminearum-infected wheat spikes the cell wall components cellulose, xylan and pectin were localized by means of enzyme-gold and immuno-gold labelling techniques. Localization of cellulose, xylan and pectin showed that host cell walls which were in direct contact with the pathogen surface had reduced gold labelling compared to considerable higher labelling densities of walls distant from the pathogen–host interface or in non-colonized tissues. The reduced gold labelling densities in the infected host cell walls indicate that these polysaccharide degrading enzymes might be important pathogenicity factors of F. graminearum during infection of wheat spikes. The results revealed that, infection and colonization of wheat spikes by F. graminearum and reactions of infected host tissue were similar to those reported for F. culmorum.     

Ultrastructural study on interaction between a sterile,white endophytic fungus and eggplant roots

Eggplant roots colonized by a sterile, white mycelial endophyte (SWM) were previously found to become highly resistant to Verticillium wilt. SWM alone, however, caused no visible, disease symptoms, such as wilting or necrosis. The mechanism of the symptomless infection by SWM was investigated in this study. Electron microscopy revealed that hyphae of SWM were abundant on and inside the root epidermal cells 2 weeks after inoculation. Many terminal appressoria formed from apical tips of hyphae, and heavy degradation of the host cell walls was evident where hyphae accumulated. By 4 weeks following inoculation, penetration pegs easily breached epidermal cells, and the infection hyphae penetrated outer cortical cells. In response to the hyphal ingress, numerous tubule-like vesicles and membrane-bound, multivesicular bodies accumulated in cortical cytoplasm near the infection sites of the outer cortical cells, but no visible signs of the host reactions were seen in the epidermal cells. Papillae developed at the spaces between cell walls and plasma membranes at the infection sites. The penetration hyphae often grew out of the papillae, but further hyphal ingress was halted in the middle cortical cell layer. By 8 weeks following inoculation, papillae that developed in these cells contained larger amounts of highly electron-dense material and were reinforced by multilamellate, fibrous elements. Hyphae that entered such papillae were confined to them, and the hyphal cytoplasm degenerated. As the result of the activated resistance reactions, root vascular cylinders remained intact, and the host plants did not wilt.     

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.     

A quantitative review of tebuconazole effect on fusarium head blight and deoxynivalenol content in wheat

ABSTRACT A meta-analysis of the effect of tebuconazole (e.g., Folicur 3.6F) on Fusarium head blight and deoxynivalenol (DON) content of wheat grain was performed using data collected from uniform fungicide trials (UFTs) conducted at multiple locations across U.S. wheat-growing regions. Response ratios (mean disease and DON levels from tebuconazole-treated plots, divided by mean disease and DON levels from untreated check plots) were calculated for each of 139 studies for tebuconazole effect on Fusarium head blight index (IND; field or plot-level disease severity, i.e., mean proportion of diseased spikelets per spike) and 101 studies for tebuconazole effect on DON contamination of harvested grain. A random-effects meta-analysis was performed on the log-transformed ratios, and the estimated mean log ratios were transformed to estimate the mean (expected) percent control for IND ( C(IND) ) and DON ( C(DON)). A mixed effects meta-analysis was then done to determine the effects of wheat type (spring versus winter wheat) and disease and DON levels in the controls on the log ratios. Tebuconazole was more effective at limiting IND than DON, with C(IND) and C(DON) values of 40.3 and 21.6%, respectively. The efficacy of tebuconazole as determined by the impact on both IND and DON was greater in spring wheat than in winter wheat (P < 0.01), with a 13.2% higher C(IND) and a 12.4% higher C(DON) in spring wheat than in winter wheat. In general, C(IND) and C(DON) were both at their lowest values (and not significantly different from 0) when mean IND and DON in the controls, respectively, were low (相似文献   

Ultrastructural localization of chitin in cell walls of Rigidoporus lignosus, the white-rot fungus of rubber tree roots

The distribution of N-acetylglucosamine residues in the cell wall of the white-rot pathogenic fungus, Rigidoporus lignosus, was studied by using gold labelled wheatgerm agglutinin bound to ovomucoid-colloidal gold. Ultrastructural investigation of R. lignosus-infected root tissues of Hevea brasiliensis showed a modification of the fungal cell wall throughout the infection process. Gold particles were found to occur on both thick- and thin-walled hyphae of R. lignosus rhizomorphs at the root surface. Walls of hyphae that had penetrated the roots were only labelled when they were out of the host cell, suggesting that modification of chitin molecules may be related to the excretion of host cell wall degrading enzymes. Variation in the distribution of gold particles was observed over hyphal walls of both colonized phellem and xylem cells. The observation that N-acetylglucosamine residues were released in the host cell cytoplasm suggests that lytic enzymes alter the fungal cell walls. Released chitin oligosaccharides may play a role in the induction of the root's defence system against fungal attack.     

丙烷脒对番茄灰霉病菌生长发育的影响

利用电子显微技术研究了丙烷脒在活体植物上对番茄灰霉病菌Botrytis cinerea Pers.生长发育的影响。结果表明:丙烷脒对番茄灰霉病菌的生长发育有明显影响,但不同时期施药,表现出一定的差异性。扫描电镜观察发现,在先施药后接菌的情况下,番茄灰霉病菌产孢量显著下降,并且所产孢子表现出凹陷、变形等不良发育状况;在先接菌后施药的情况下,丙烷脒处理后不影响番茄灰霉病菌的产孢量,但所产孢子也表现出凹陷、变形等不良发育状况;透射电镜观察发现,在先施药后接菌的情况下,番茄灰霉病菌无法有效地侵入番茄表皮以下组织,病情得以有效控制;在先接菌后施药的情况下,药剂处理的番茄灰霉病菌可以侵入番茄表皮以下组织,但生长状况不良。不同时期丙烷脒处理后均能使番茄灰霉病菌的菌丝出现塌陷、生长点缢缩等不良生长现象;病菌菌丝细胞壁表现出不规则的加厚,尤以顶端加厚明显;线粒体异常增多,并出现基质型肿胀;病菌菌丝内细胞器液泡化,最终形成大的液泡;病菌细胞内含物外渗,出现质壁分离和空腔细胞;病菌细胞器泡囊化解体和细胞组织崩解,细胞趋于死亡等。研究表明,丙烷脒对番茄灰霉病菌的生长发育具有明显的影响,除具有保护性防病效果外,还表现出较强的治疗效果。     

Infection of wheat spikes by <Emphasis Type="Italic">Fusarium avenaceum</Emphasis> and alterations of cell wall components in the infected tissue

The infection process of Fusarium avenaceum on wheat spikes and the alteration of cell wall components in the infected host tissue were examined by means of electron microscopy and cytochemical labelling techniques following spray inoculation at growth stage (GS) 65 (mid-flowering). Macroconidia of the pathogen germinated with one to several germ-tubes 6–12 h after inoculation (hai) on host surfaces. The germ-tubes did not penetrate host tissues immediately, but extended and branched on the host surfaces. Hyphal growth on abaxial surfaces of the glume, lemma and palea was scanty 3–4 days after inoculation (dai) and no direct penetration of the outer surfaces of the spikelet was observed. Dense mycelial networks formed on the inner surfaces of the glume, lemma, palea and ovary 36–48 hai. Penetration of the host tissue occurred 36 hai by infection hyphae only on the adaxial surfaces of the glume, lemma, palea and upper part of ovary. The fungus penetrated the cuticle and hyphae extended subcuticularly or between the epidermal wall layers. The subcuticular growth phase was followed by penetration of the epidermal wall, and hyphae spread rapidly inter- and intracellularly in the glume, lemma, palea and ovary. During this necrotrophic colonization phase of the wheat spike, a series of alterations occurred in the host tissues, such as degeneration of cytoplasm and cell organelles, collapse of host cells and disintegration of host cell walls. Immunogold labelling techniques showed that cell walls of spike tissues contained reduced amounts of cellulose, xylan and pectin near intercellular hyphae or infection pegs compared to walls of healthy host tissues. These studies suggest that cell wall degrading enzymes produced by F. avenaceum facilitated rapid colonization of wheat spikes. The different penetration properties of abaxial and adaxial surfaces of the spikelet tissues as well as the two distinct colonization strategies of host tissues by F. avenaceum are discussed. The penetration and colonization behaviour of F. avenaceum in wheat spikelets resembled that of F. culmorum and F. graminearum, although mycotoxins produced by F. avenaceum differed from those of the latter two Fusarium species.     

The Y123H substitution perturbs FvCYP51B function and confers prochloraz resistance in laboratory mutants of Fusarium verticillioides

Fusarium verticillioides reduces corn yield and contaminates infected kernels with the toxin fumonisin, which is harmful to humans and animals. Previous research has demonstrated that F. verticillioides can be controlled by the azole fungicide prochloraz. Currently, prochloraz is used as a foliar spray to control maize disease in China, which will increase the risk of resistance. Although F. verticillioides resistance to prochloraz has not been reported in the field, possible resistance risk and mechanisms resulting in prochloraz resistance were explored in the laboratory. Four prochloraz‐resistant strains of F. verticillioides were generated by successive selection on fungicide‐amended media. The mycelial growth rates of the mutants were inversely related to the level of resistance. All four mutants were cross‐resistant to the triazole fungicides triadimefon, tebuconazole and difenoconazole, but not to the multisite fungicide chlorothalonil or to the MAP/histidine‐kinase inhibitor fungicide fludioxonil. Based on the Y123H mutation in FvCYP51B, the four resistant mutants were subdivided into two genotypes: PCZ‐R1 mutants with wildtype FvCYP51B and PCZ‐R2 mutants with substitution Y123H in FvCYP51B. Wildtype FvCYP51B complemented the function of native ScCYP51 in Saccharomyces cerevisiae YUG37::erg11, whereas Y123H‐mutated FvCYP51B did not. For the PCZ‐R1 mutants, induced expression of FvCYP51A increased resistance to prochloraz. For the PCZ‐R2 mutants, disruption of FvCYP51B function by the Y123H substitution caused constitutive up‐regulation of FvCYP51A expression and thus resistance to prochloraz.