The process of antagonism of Sclerotium cepivorum in white rot affected onion roots by Trichoderma koningii

Trichoderma koningii (strain Tr5) grew in the epidermal mucilage of onion roots without entering healthy epidermal tissue. When placed on the epidermis of Sclerotium cepivorum -infected roots, T. koningii colonized epidermal passage cells, with little colonization of other epidermal tissues, then branched and spread throughout the root cortical tissues damaged by enzymes and toxins which diffused ahead of S. cepivorum hyphae, and impeded the path of the infection. When T. koningii colonized infected tissue, many S. cepivorum hyphae became detached at septa, cell walls dissolved and many hyphal apices burst. Contact between hyphae was not necessary for lysis to occur. T. koningii produced two endochitinases ( R _f 0·15 and 0·24) and two exo-acting chitinolytic enzymes ( R _f 0·46 and 0·62) during degradation of crabshell chitin and S. cepivorum cell walls. The R f 0·24 and 0·46 proteins were detected when T. koningii colonized S. cepivorum -infected roots and are likely to be a component of the antagonism process.     

Histological characterization of the early-stage infection events of Setosphaeria turcica in maize

Northern corn leaf blight (NCLB) caused by Setosphaeria turcica is a major foliar disease of maize. The early-stage infection events of this pathogen on maize leaves are unclear. We investigated the optimum temperature for conidial germination and appressorium formation, and characterized penetration and growth of S. turcica in maize leaf sheath and onion epidermis cells, including use of histological staining to assess plant cell viability. The results showed that the optimum temperature for conidial germination and appressorium formation was 20°C. On the maize leaf sheath, the appressoria were formed by germinated conidia, and penetration on the epidermal cells occurred at 8 h postinoculation (hpi). Round vesicles developed beneath the appressoria. Between 16 and 24 hpi, the branched invasive hyphae invaded three to five adjacent cells at most infection sites. The invasive hyphae tended to move along the cell wall and crossed from one cell to another. In the onion epidermis cells, the appressoria formed at 8 hpi, and in most cases the epidermal cells were penetrated through the juncture of the cell walls. At 16–24 hpi, the primary hyphal terminus swelled to a vesicle. The maize leaf sheath cells died at 8 hpi, whereas the onion cells did not. Our findings documented in detail the penetration and invasive hyphal growth in maize leaf sheath and onion epidermis, as well as viability of plant cells, at the early stages of infection, and provide a foundation for elucidating the underlying mechanism of S. turcica–maize interactions.     

Interactions between Sclerotium cepivorum and cultivars of onion, leek, garlic and Allium fistulosum

Laboratory and field experiments showed that germination of sclerotia of Sclerotium cepivorum was slower in the presence of cultivars of leek than in the presence of those of onion, garlic and Allium fistulosum . Spread of infection amongst closely spaced plants was also less in leeks than in the other three species. No evidence was found for any differences in the resistance of root tissues of the four Allium species. It is considered that differential stimulation of sclerotium germination is likely to have been the principal reason for the low incidence of white rot infection in leeks which was reported previously.     

Histopathology of Infection and Colonization of Susceptible and Resistant Port-Orford-Cedar by Phytophthora lateralis

ABSTRACT Port-Orford-cedar (POC) root disease, caused by Phytophthora lateralis, continues to kill POC in landscape plantings and natural forests in western North America. POC trees resistant to P. lateralis have been identified and propagated. Cytological observations of P. lateralis in susceptible and resistant roots and stems were made with light and transmission electron microscopy to identify resistance mechanisms. No differences in infection pathway and initial colonization were observed between susceptible and resistant roots, although there were differences in the rate and extent of development. Germ tubes formed appressoria, and penetration hyphae grew either between or directly through epidermal cell walls; inter- and intracellular hyphae colonized the root cortex. In susceptible roots, hyphae penetrated into the vascular system within 48 h of inoculation. In contrast, hyphae in roots of resistant seedlings grew more slowly in cortical cells and were not observed to penetrate to the vascular tissues. In resistant roots, infection was marked by general thickening of cortical cell walls, wall appositions around penetrating hyphae, collapse of cortical cells, and accumulation of osmophillic granules around hyphae. In susceptible stems, hyphae grew inter- and intracellularly in all cells of the secondary phloem except fiber cells, but were concentrated in sieve and parenchyma cells in the functional phloem. The pattern of penetration and colonization of hyphae was similar in the resistant stems, except that hyphae were found in the fiber cells of the xylem. In resistant stems, there were fewer hyphae in the functional phloem, and cytological changes such as damaged nuclei and disintegrated cytoplasm were evident. Structural changes in resistant stems included collapsed cells, wall thickening, secretory bodies, apposition of electron dense materials, and crystals in cell walls.     

禾顶囊壳小麦变种对小麦种子根的侵染过程

 光镜和电镜观察表明,禾顶囊壳小麦变种(Gaeumannomyces graminis var.tritici,小麦全蚀病菌)对小麦种子根的侵染过程可分为侵入前、侵入表皮层、进入皮层和进入中柱等4个连续阶段。麦根接菌后在15℃下培养,48 h后侵入表皮层细胞,60 h后进入皮层,120 h后进入中柱。病原菌主要以侵染菌丝直接侵入表皮层,表皮细胞间隙和根毛基细胞是主要侵入部位,少数由附着枝侵入。菌丝穿透细胞壁有明显的酶解作用特征,菌丝先端前方胞壁上还产生电子密物质。皮层细胞是病原菌定殖和发展的主要场所,病原菌还能离解胞间层,形成胞外空间,特别有利于菌丝和菌丝束的扩展。在侵入位点的寄主细胞壁和质膜之间,形成多种形状的木质管,其数量与侵入菌丝的数目相对应,但木质管不能阻止菌丝进入细胞。菌丝进入中柱后,可阻塞导管和筛管。小麦细胞发生退行性病变,尤以细胞壁膨大崩坏和早期质壁分离最明显,细胞间隙还产生性质不明的黄色物质。     

栗疫病菌侵染板栗枝条的显微观察

栗疫病是一种严重危害栗属植物的病害。为了明确栗疫病菌侵染板栗枝条的过程及侵染的关键时间点,本研究利用病理组织切片技术、显微镜和扫描电镜技术对栗疫病菌侵染板栗枝条的过程进行了观察。结果表明:接种栗疫病菌后0～5 h,菌丝先降解枝条表皮,进行横向营养生长的同时沿着伤口纵向侵染,为进入皮层做准备;接种后6 h病菌开始在表皮定殖,并侵入皮层;接种后9 h在皮层可观察到侵染性菌丝沿着细胞间隙向相邻细胞延伸;接种后12 h栗疫病菌侵入韧皮部,在皮层的侵染面积扩大。随着侵染程度加深,皮层、韧皮部等处细胞被菌丝降解,最终在形成层附近聚集。接菌后9 h为栗疫病菌侵染板栗枝条的关键时间点。     

Selection of fungal biological control agents of Sclerotium cepivorum for control of white rot by sclerotial degradation in a UK soil

A range of fungal isolates was tested in a three-stage screening system for their ability to degrade sclerotia of Sclerotium cepivorum on agar and in soil, and to reduce white rot disease on onion seedlings. Biological control agents (BCAs) were identified that could degrade up to 60% of sclerotia in soil and significantly reduce white rot disease on onion seedlings. The efficacy of the BCAs was enhanced when applied as wheat bran cultures compared with spore suspensions, and two of the best BCAs from the screening procedures were both identified as Trichoderma viride (L4, S17A). When L4 and S17A were fluid-drilled in guar gum with bulb onion seed in the field white rot symptoms were significantly reduced, but stem base applications applied mid-season had little effect. The strategy of selecting and using BCAs that degrade sclerotia of S. cepivorum and integration with other control methods is discussed.     

球毛壳ND35菌株在宿主植物上的侵染定殖

为了解球毛壳Chaetomium globosum ND35菌株在宿主植物上的侵染定殖方式和途径,以毛白杨组培苗为宿主植物,借助光学显微镜、扫描电镜、透射电镜,结合免疫荧光标记技术,研究了球毛壳ND35菌株子囊孢子萌发后在毛白杨上的侵染行为及其菌丝在组培苗根部的定殖。结果显示,子囊孢子萌发后形成的菌丝,能从杨树苗根、茎部表面细胞间的缝隙侵入或在根表面形成附着胞,进而形成侵染钉直接从表皮细胞侵入,在叶部主要从气孔侵入叶片内部。侵入根部的菌丝主要定殖于表皮细胞、外皮层细胞和细胞间隙,未进入内皮层和维管束组织。     

Infection of cultivars of onion, leek, garlic and A llium fistulosum by Sclerotium cepivorum

In two field experiments on land severely infested with Sclerotium cepivorum largo differences in the incidence of white rot in four Allium species were recorded. There was inuch less disease ° Cultivars of leek than in those of onion, garlic or A. fistulosum. Differences between cultivars within species were either small or absent.     

Cytological Analysis of Defense-Related Mechanisms Induced in Pea Root Tissues in Response to Colonization by Nonpathogenic Fusarium oxysporum Fo47

The ability of nonpathogenic Fusarium oxysporum, strain Fo47, to trigger plant defense reactions was investigated using Ri T-DNA-transformed pea roots. Cytological investigations of strain Fo47-inoculated roots showed that the fungus grew actively at the root surface and colonized a number of epidermal and cortical cells, inducing marked host cell metabolic changes. In roots inoculated with pathogenic F. oxysporum f. sp. pisi, the pathogen multiplied abundantly through much of the tissues, whereas in Fo47-inoculated roots, fungal growth was restricted to the epidermis and the outer cortex. Invading cells of strain Fo47 suffered from serious alterations, a phenomenon that was not observed in control roots in which F. oxysporum f. sp. pisi grew so actively that the vascular stele was invaded within a few days. Strain Fo47 establishment in the root tissues resulted in a massive elaboration of hemispherical wall appositions and in the deposition of an electron-opaque material frequently encircling pathogen hyphae and accumulating in the noninfected xylem vessels. This suggests that the host roots were signaled to defend themselves through the rapid stimulation of a general cascade of nonspecific defense responses. The specific relationship established between strain Fo47 and the root tissues is discussed in relation to other types of plant-fungus interactions, including pathogenic and symbiotic associations.     

Mycelial compatibility groups in Sclerotium cepivorum

Isolates of Sclerotium cepivorum from the Holland–Bradford Marsh, Ontario, Canada ( n  = 146) and other locations ( n  =23) were evaluated for mycelial compatibility in agar culture and by transmitted light microscopy. In compatible interactions, hyphae of paired colonies anastomosed without cell deterioration or death at the point of fusion. In some incompatible interactions, hyphae of paired colonies were initially similar to those of compatible interactions but were later followed by deterioration or death of the fused cells. Incompatible interactions were associated with a red–brown colour that, in mass, produced a dark line at the junction of the two colonies. In other incompatible interactions, hyphae were observed to bypass or repel each other without fusing. Compatible and incompatible reactions were used to define nine mycelial compatibility groups (MCGs) among the isolates examined. Among isolates from Ontario, no incompatibility responses were observed among isolates from individual fields (e.g. no intrafield variability) but two MCGs were found among isolates from different fields (e.g. interfield variability). Among isolates from other regions, seven additional MCGs were detected that consisted of one to eight isolates. Two isolates were variable in reaction and were not assigned to any MCG. Three isolates from Switzerland were compatible with MCG-1 isolates from Ontario; and one isolate from Switzerland, two from England, two from New Zealand, and two from Australia were all intercompatible. The results establish that mycelial compatibility can be used to reflect genetic heterogeneity among isolates of S. cepivorum .     

Resistance to Sclerotium cepivorum in Allium and other genera

Stimulatory and non-stimulatory species of Allium and four other members of the Liliaceae were susceptible to infection by germinating sclerotia of Sclerotium cepivorum in unsterile soil. Plants belonging to other families were not infected in soil even when germinating sclerotia were in contact with their roots. Under aseptic conditions S. cepivorum infected plant species not closely related to Allium , particularly when large amounts of inoculum were used. Infection was slight in the presence of germinating sclerotia. It is suggested that within the genus Allium and a few related genera resistance results from the lack of ability to stimulate germination of sclerotia. Non- Allium plants also lack this ability but in addition possess tissue resistance to S. cepivorum.     

Allium White Rot Suppression with Composts and Trichoderma viride in Relation to Sclerotia Viability

ABSTRACT Allium white rot (AWR) is a serious disease of Allium spp. caused by the sclerotium-forming fungus Sclerotium cepivorum. This work has examined the effects of onion waste compost (OWC) and spent mushroom compost (SMC), with and without Trichoderma viride S17A, on sclerotia viability and AWR in glasshouse and field experiments. Incorporation of OWC into soil reduced the viability of sclerotia and the incidence of AWR on onion plants in glasshouse pot bioassays, whereas SMC or T. viride S17A only reduced incidence of AWR. In two field trials, OWC reduced sclerotia viability and was as effective in reducing AWR as a fungicide (Folicur, a.i. tebuconazole). Field application of SMC had no effect on sclerotia viability and did not control AWR. However, the addition of T. viride S17A to SMC facilitated proliferation of T. viride S17A in the soil and increased the healthy onion bulb yield. The results indicate two mechanisms for the suppression of AWR: (i) reduction in the soil population of viable sclerotia, which may be due to volatile sulfur compounds detected in OWC but absent in SMC, and (ii) prevention of infection of onion plants from sclerotia following amendment of soil with OWC, SMC, or T. viride S17A.     

Effect of environmental factors and Sclerotium cepivorum isolate on sclerotial degradation and biological control of white rot by Trichoderma

Laboratory assays demonstrated that two isolates of Trichoderma viride and one isolate of Trichoderma pseudokoningii degraded up to 80% of sclerotia of four isolates of Sclerotium cepivorum in a silty clay soil, and also degraded up to 60% of sclerotia in three other soil types. Relationships were defined between the degree of sclerotial degradation by the two T. viride isolates in the silty clay soil and both temperature and soil water potential. Sclerotia were degraded between 10 and 25°C at −0·00012 MPa, but there was little activity of T. viride at 5°C or at −4 MPa. Degradation of S. cepivorum sclerotia also occurred in the absence of Trichoderma at soil water potentials approaching saturation . Experiments using onion seedling bioassays showed that the efficacy of Trichoderma isolates for the control of white rot using the same selection of soils and S. cepivorum isolates was variable, but that there was significant disease control overall. The importance of environmental factors and pathogen isolate in relation to effective biological control of white rot is discussed.     

Integrated control of Allium white rot with Trichoderma viride, tebuconazole and composted onion waste

Two isolates of Trichoderma viride (L4 and S17A) were assessed for biological control of Allium white rot (AWR) with different onion accessions and cultivars, alone and in combination with a tebuconazole-based seed treatment or composted onion waste. In glasshouse tests, 23 new bulb-onion accessions from previous work to detect resistance to Sclerotium cepivorum showed no differences in susceptibility to AWR but, when combined with S17A, disease was reduced by up to two-thirds over all accessions. Trichoderma viride L4 and S17A also reduced the proportion of infected plants for five commercial bulb-onion cultivars and one advanced breeding line by at least one-third. Further glasshouse tests using a salad-onion cultivar showed that L4, S17A, tebuconazole or composted onion waste controlled AWR and at least halved the proportion of diseased plants. Combination treatments of T. viride with either tebuconazole or compost enhanced control and, in some treatments, disease was almost eliminated. In field trials, control of AWR by S17A was significant for 17 out of 18 individual or mixed bulb-onion accessions, with disease reduced overall by more than half. In another field experiment, S17A failed significantly to reduce AWR for two out of three commercial bulb-onion cultivars, while tebuconazole reduced the final proportion of AWR-infected plants over all cultivars from 0·47 to 0·09. Combining S17A and tebuconazole resulted in a similar level of AWR to using tebuconazole alone. The use of T. viride in an integrated strategy with other treatments to enhance control of S. cepivorum is discussed.     

赖草属(Leymus Hochst.)3种牧草营养器官耐旱结构的研究

本文对赖草属 3种牧草进行了营养器官抗旱结构的研究 ,发现了许多耐旱特性 :如营养器官维管束数目增多、后生导管的直径增粗 ;根、根茎的皮层薄壁组织环加宽 ;根茎、茎的薄壁贮藏组织面积增大 ;叶表皮泡状细胞列数增多、叶肉细胞呈短柱状 ,大、中型维管束两端形成较宽的维管束鞘延伸区 ,维管束为由两层维管束鞘围成 ,为 C3 植物 ,与外表皮之间形成较宽的维管束鞘延伸区 ,内表皮下为较宽的薄壁细胞层 ,。 3种牧草中 ,大赖草的抗旱特性强于其它两种明显。     

大豆疫霉菌对大豆下胚轴侵染过程的细胞学研究

 接种后1.5~24h,用光镜和电镜研究了2个大豆品种与大豆疫霉菌Ps411的亲和性和非亲和性互作。观察结果表明,大豆疫霉菌对大豆下胚轴的侵染过程可分为侵入前、侵入、皮层组织中的扩展和进入维管束组织4个连续阶段。大豆下胚轴接种后在25℃保湿培养,1.5h后游动孢子即形成休止孢并萌发产生附着孢,3h后侵入表皮细胞,6h后进入皮层组织,24h后进入维管束组织。病原菌主要以侵染菌丝直接侵入表皮,表皮细胞间隙是主要侵入部位。皮层细胞是病原菌定殖和发展的主要场所,胞间菌丝侵入皮层细胞并形成吸器。在菌丝与寄主细胞接触部位的寄主细胞壁与质膜之间常有胞壁沉积物的形成。在抗病品种上病菌的侵染事件与感病品种基本一致,但不能形成正常的吸器,胞壁沉积物明显多于感病品种,菌丝在寄主组织内的扩展明显受到抑制。利用β-1,3-葡聚糖免疫金标记单克隆抗体进行的免疫细胞化学的研究表明,胞壁沉积物内含有大量的β-1,3-葡聚糖,在大豆疫霉菌菌丝壁中也存在β-1,3-葡聚糖。以上结果表明,病原菌的侵染可诱导抗病寄主细胞内β-1,3-葡聚糖迅速的合成与积累、并形成胞壁沉积物,以抵御病菌的侵染与扩展。