Aspects of narcissus smoulder epidemiology

The range of symptoms caused by the narcissus smoulder pathogen Botrytis narcissicola is described. Healthy bulbs inoculated with B. narcissicola , or grown in soil containing sclerotia, showed lesions in the shoot on emergence (primary symptoms). In commercial fields, B. narcissicola was commonly isolated from the bulb neck and leaf sheath of plants with primary symptoms. Plants infected by B. narcissicola one season, either with natural primary symptoms or following artificial inoculation, frequently emerged with smoulder symptoms the following season. About 40% of B. narcissicola sclerotia were viable after burial in soil for 9 months. It is suggested that infected bulbs and sclerotia present in soil are the major sources of smoulder outbreaks. Secondary infection by conidia was enhanced by damaging leaves, and open stalk ends left after flower picking were found to provide an important site for infection development. B. narcissicola was isolated from bulb necks when plants with symptoms of secondary infection had died down, A disease cycle is postulated and suggestions for controlling smoulder are discussed with reference to the cycle.     

Mixtures of plant growth-promoting rhizobacteria enhance biological control of multiple cucumber pathogens

ABSTRACT Plant growth-promoting rhizobacteria (PGPR) strains INR7 (Bacillus pumilus), GB03 (Bacillus subtilis), and ME1 (Curtobacterium flaccumfaciens) were tested singly and in combinations for biological control against multiple cucumber pathogens. Investigations under greenhouse conditions were conducted with three cucumber pathogens-Colletotrichum orbiculare (causing anthracnose), Pseudomonas syringae pv. lachrymans (causing angular leaf spot), and Erwinia tracheiphila(causing cucurbit wilt disease)-inoculated singly and in all possible combinations. There was a general trend across all experiments toward greater suppression and enhanced consistency against multiple cucumber pathogens using strain mixtures. The same three PGPR strains were evaluated as seed treatments in two field trials over two seasons, and two strains, IN26 (Burkholderia gladioli) and INR7 also were tested as foliar sprays in one of the trials. In the field trials, the efficacy of induced systemic resistance activity was determined against introduced cucumber pathogens naturally spread within plots through placement of infected plants into the field to provide the pathogen inoculum. PGPR-mediated disease suppression was observed against angular leaf spot in 1996 and against a mixed infection of angular leaf spot and anthracnose in 1997. The three-way mixture of PGPR strains (INR7 plus ME1 plus GB03) as a seed treatment showed intensive plant growth promotion and disease reduction to a level statistically equivalent to the synthetic elicitor Actigard applied as a spray.     

Biosuppression of Botrytis cinerea in grapes

There is increasing interest in the use of biological control agents (BCAs) and plant resistance stimulants to suppress botrytis bunch rot in grapes, caused by Botrytis cinerea . Numerous different filamentous fungi, bacteria and yeasts have been selected as potential BCAs for control of grey mould based upon demonstrated antagonism towards B. cinerea. Biological suppression of the pathogen arises via competition for nutrients and space, the production of inhibitory metabolites and/or parasitism. Preformed and inducible grapevine defence mechanisms also contribute to disease suppression by preventing or delaying pathogenic infection. Furthermore, various biotic and abiotic agents can stimulate grapevine defence mechanisms and so elevate resistance to B. cinerea infection. Biosuppression of B. cinerea in vineyards, using BCAs and resistance stimulants, has been inconsistent when compared with that observed in controlled glasshouse or laboratory conditions. This may be attributable, in part, to the innate variability of the field environment. Research to improve field efficacy has focused on formulation improvement, the use of BCA mixtures and combinational approaches involving BCAs and plant resistance stimulants with complementary modes of action.     

Studies on the biological control of plant pathogens in Finland1

Studies on biological control of plant pathogens in Finland were started by Pohjakallio in the 1950s, concentrating especially on the microbes parasitizing fungal sclerotia. At the turn of the 70s, Kallio studied the protective effect of Finnish isolates of Phlebia gigantea against the most important forest pathogen Heterobasidion annosum with positive results. The preparation was put on the market. Later in the 1970s, the observation by Tahvonen that Finnish light-coloured peat had an inhibitory effect on the infection of plants by some seed and soil-borne fungal pathogens led to further investigations. Of the microbes isolated from peat, Streptomyces spp. proved the most effective antagonists in both in vitro and in vivo tests. Studies are in progress with S. griseoviridis and several other fungal and bacterial antagonists as potential biocontrol agents against some seed and soil-borne pathogens and storage diseases.     

Effect of coinoculation by Verticillium dahliae and Colletotrichum coccodes on disease symptoms and fungal colonization in four potato cultivars

Interactions between Verticillium dahliae and Colletotrichum coccodes , major causal agents of potato early dying (PED) syndrome, were studied in four potato cultivars that differ in their susceptibility to these pathogens . Aseptic plantlets of Nicola, Desiree, Alpha and Cara were inoculated with identical concentrations of each pathogen or with a mixture of the pathogens, and grown for 4 weeks in a monitored growth chamber. Coinoculation of Nicola with both pathogens caused more severe foliar disease symptoms and crown rot and greater C. coccodes colonization, than inoculation with each pathogen separately. Significant reductions in weight and height were also observed in plants coinoculated with both pathogens, as compared with plants inoculated with each pathogen separately or noninoculated plants. In Desiree, more roots were covered with C. coccodes sclerotia and disease symptoms were significantly more severe in plants inoculated with both pathogens together. However, plant weight and height were similar to those of plants inoculated with C. coccodes only. In Alpha, disease symptoms and levels of sclerotia in the roots were not affected by simultaneous inoculation with both pathogens. Weight and height of all plants were similar, whether inoculated with each pathogen separately or with both pathogens together. In Cara, plants inoculated with the mixture or either pathogen alone were smaller than the noninoculated control. Disease symptoms and occurrence of sclerotia were similar in plants inoculated with the combination and with a single pathogen. Compared with the effects of inoculation with either pathogen, simultaneous inoculation with both pathogens can, in some cultivars, increase the incidence of PED syndrome and thus severely decrease yields.     

Quantitative detection and monitoring of Colletotrichum acutatum in strawberry leaves using real-time PCR

Real-time PCR assays for Colletotrichum acutatum , one of the most important pathogens of strawberry worldwide, were developed using primers designed to the ribosomal DNA internal transcribed spacer 1 (rDNA ITS1) and the β-tubulin 2 gene. Using TaqMan technology, the ITS-based assay could reliably detect as little as 50 fg genomic DNA, 100 copies of target DNA, or 25 conidia. The β-tubulin-based assay was c . 66 times less sensitive, and therefore less suitable for detection purposes. The TaqMan-ITS assay recognized all C. acutatum isolates tested from various intraspecific molecular groups, while no amplification was observed with several other Colletotrichum species or other strawberry pathogens, indicating the specificity of this assay. Detection and quantification of C. acutatum was demonstrated in artificially and naturally infected strawberry leaves. First, C. acutatum was detected in plant mixes of which only 0·001% of the tissue was infected by C. acutatum . Secondly, real-time PCR analysis of leaf samples taken at various times after inoculation indicated that the assay allowed monitoring of growth progression of C. acutatum . This real-time PCR-mediated monitoring of the pathogen was well-correlated with microscopic data, and confirmed that leaf age may play a role in the extent of C. acutatum infection. Finally, the assay allowed detection of C. acutatum in naturally infected and symptomless strawberry leaves collected from production fields and planting material.     

Cytology of infection of apple leaves by Diplocarpon mali

Diplocarpon mali, the causal agent of Marssonina leaf blotch of apple, causes severe defoliation during the growing season. Little information is available on the mode of infection and the infection process. In this study, the infection strategies of D. mali in apple leaves were investigated using fluorescence and electron microscopy. Conidia attached to leaf surface apparently by mucilage and germinated on both sides of leaves 6 h post-inoculation (hpi). The pathogen penetrated the cuticle by infection pegs formed either in germ tubes or appressoria in 6 hpi, and then formed haustoria in host epidermal and mesophyll cells accompanied by extension of subcuticular and intercellular hyphae. Five days post-inoculation (dpi), the intracellular hyphae were observed. At the same time, the subcuticular hyphal strands (SHS) were produced as a means for fast expansion and reproduction. About 7 dpi, acervuli formed on inoculated leaves. This was the first observation that D. mali formed haustoria and SHS as infection strategies. Our results suggest that D. mali may behave like a hemibiotroph, which can use both biotrophic and necrotrophic strategies to establish infections on apple leaves.     

Forest type influences transmission of Phytophthora ramorum in California oak woodlands

The transmission ecology of Phytophthora ramorum from bay laurel (Umbellularia californica) leaves was compared between mixed-evergreen and redwood forest types throughout winter and summer disease cycles in central, coastal California. In a preliminary multisite study, we found that abscission rates of infected leaves were higher at mixed-evergreen sites. In addition, final infection counts were slightly higher at mixed-evergreen sites or not significantly different than at redwood sites, in part due to competition from other foliar pathogens at redwood sites. In a subsequent, detailed study of paired sites where P. ramorum was the main foliar pathogen, summer survival of P. ramorum in bay laurel leaves was lower in mixed-evergreen forest due to lower recovery from infected attached leaves and higher abscission rates of infected leaves. Onset of inoculum production and new infections of bay laurel leaves occurred later in mixed-evergreen forest. Mean inoculum levels in rainwater and final infection counts on leaves were higher in redwood forest. Based on these two studies, lower summer survival of reservoir inoculum in bay laurel leaves in mixed-evergreen forest may result in delayed onset of both inoculum production and new infections, leading to slower disease progress in the early rainy season compared with redwood forest. Although final infection counts also will depend on other foliar pathogens and disease history, in sites where P. ramorum is the main foliar pathogen, these transmission patterns suggest higher rates of disease spread in redwood forests during rainy seasons of short or average length.     

生菜软腐和菌核病拮抗菌贝莱斯芽胞杆菌BPC6鉴定与防效

软腐病和菌核病是生菜生产中两大毁灭性病害。为有效控制这2种病害，利用平板稀释和对峙法，从生菜根际土壤中分离到一株拮抗菌BPC6，其菌悬液、无菌滤液对软腐病菌Pectobacterium carotovorum和菌核病菌Sclerotinia sclerotiorum的生长均具有抑制效果，该菌株也能抑制其他14种植物病原菌的生长。形态学、16S rDNA及基因组序列分析将其鉴定为贝莱斯芽胞杆菌Bacillus velezensis。BPC6能产生纤维素酶和蛋白酶，不能产生几丁质酶和嗜铁素。比浊法分析的生长曲线显示，BPC6能在1%～10% NaCl及pH 5.0～9.0条件下生长。通过分析不同浓度BPC6对软腐病菌和菌核病菌侵染生菜离体叶片的影响，明确了菌株BPC6适宜用量为1.4×10⁹～2.8×10⁹ CFU/mL。菌株BPC6对生菜软腐病和菌核病盆栽防效分别为44.09%和53.58%，对菌核病大田防效为77.41%。本研究表明BPC6是一株对生菜软腐病和菌核病具有拮抗作用的潜在生防菌。     

An emerging system management approach for biological weed control in crops: Senecio vulgaris as a research model

A ‘system management’ approach of biological weed control in crops is proposed and compared with other method's of biological weed control. It is based on the management of a weed pathosystem in order to maximize the natural spread and disease severity of a native or naturalized pathogen. This approach may be well-suited to situations where it is necessary to control single weed species in crops, and where no immediate and complete control is required, the production of large amounts of the agent is rather limiting (e.g. when using biotrophic fungi), and/or the importation of an exotic agent is not possible. This strategy provides fundamental knowledge of underlying mechanisms of crop production systems and is aligned with the view of modern agro-ecology, in which complete eradication of weeds is not desirable. The fundamental research required for a successful application of the ‘system management’ approach will be illustrated with the biological control project of Senecio vulgaris L. using the naturalized rust fungus Puccinia lagenophorae Cooke. A five-step procedure, together with selected results, will be presented. Main emphasis is given to the infection window, the study of the genetic structure of the plant and pathogen population, and the management of the infection conditions (a) to maximize the spread of the disease and the impact on the plants, and (b) to minimize the development of resistant plant populations. Joint application of herbicides at low doses, additional necrotrophic pathogens, and of biochemicals interfering with the weed's defence also will be envisaged, as well as their integration into general pest control practices. In this regard, biological weed control agents have to be seen as stress factors, not as weedkillers, and biological weed control as an integral part of a well-designed pest management strategy, not as a sole cure.     

Impact of <Emphasis Type="Italic">Erysiphe alphitoides</Emphasis> on transpiration and photosynthesis in <Emphasis Type="Italic">Quercus robur</Emphasis> leaves

Oak powdery mildew, (Erysiphe alphitoides) causes one of the most common diseases of oaks. We assessed the impact of this pathogen on photosynthesis and water relations of infected leaves using greenhouse-grown oak seedlings. Transpiration of seedlings infected by oak powdery mildew was also investigated. Altogether, E. alphitoides had a low impact on host gas exchange whether at the leaf or whole plant scale. Maximal stomatal conductance of infected leaves was reduced by 20–30% compared to healthy controls. Severely infected seedlings did not experience any detectable change of whole plant transpiration. The reduction in net CO₂ assimilation, A_n, was less than proportional to the fraction of leaf area infected. Powdery mildew reduced both the maximal light-driven electron flux (J_max) and the apparent maximal carboxylation velocity (Vc_max) although Vc_max was slightly more impacted than J_max. No compensation for the infection occurred in healthy leaves of partly infected seedlings as the reduced photosynthesis in the infected leaves was not paralleled by increased A_n levels in the healthy leaves of the seedlings. However, E. alphitoides had a strong impact on the leaf life-span of infected leaves. It is concluded that the moderate effect of E. alphitoides on oak might be related to the small impact on net CO₂ assimilation rates and on tree transpiration; nevertheless, the severe reduction in leaf life-span of heavily infected leaves may lead to decreased carbon uptake over the growth season.     

The underground attacks onCrocus andColchicum by the rustsUromyces croci andUromyces colchici respectively

In was proved by inoculation experiments thatUromyces croci taken from infectied leaves ofCrocus albiflorus in Switzerland can attack the underground parts ofCrocus cultivars. The symptoms obtained by inoculating tunics and corms with this material agree with the characteristics of the underground rust onCrocus observed in Dutch nurseries since 1955. Conversely the Dutch material ofU. croci from tunic infections was shown to be capable of attacking leaves also. The methods of cultivation adopted inCrocus growing in Dutch nurseries apparently makes above-ground infection almost impossible, but favors instead the underground infections. Furthermore, because of the anatomy ofCrocus, there seems to be a barrier to infection of the leaves coming from infected tunics or corms. InColchicum, on the other hand, the tunic is the basal part of the leaf sheath of the lowest leaf. This explains why in commercial growing ofColchicum, infection byUromyces colchici is not always restricted to tunic infections and systemic leaf infections and secondary above-ground infections occasionally occur.     

Influence of drought, salt stress and abscisic acid on the resistance of tomato to Botrytis cinerea and Oidium neolycopersici

Abiotic stress may affect plant response to pathogen attack through induced alterations in growth regulator and gene expression. Abscisic acid (ABA) mediates several plant responses to abiotic stress. The effects of drought, salt stress and ABA on the interaction of tomato ( Lycopersicon esculentum ) with the biotrophic fungus Oidium neolycopersici and the necrotrophic fungus Botrytis cinerea were investigated. Drought stress resulted in a twofold increase in endogenous ABA as well as a 50% reduction in B. cinerea infection and a significant suppression of O. neolycopersici on tomato cv. Moneymaker. Salt stress did not affect B. cinerea infection, but significantly reduced infection by O. neolycopersici , with no obvious increase in endogenous ABA. Compared with the wild type, the ABA-deficient sitiens mutant was more resistant to O. neolycopersici and B. cinerea . Exogenous ABA resulted in increased susceptibility of sitiens to both pathogens, but did not increase the basal susceptibility of wild-type plants. It is concluded that, in tomato, drought and salt stress stimulate different, but possibly overlapping, pathogen-defence pathways which may not necessarily involve ABA. Meanwhile, basal endogenous ABA levels suppress the resistance of tomato to O. neolycopersici and B. cinerea , but an ABA increase above the basal level, resulting from exogenous application, does not increase susceptibility to these pathogens.     

观赏百合叶枯病症状类型与病原菌鉴定

引起甘肃省百合叶枯病的主要致病菌为椭圆葡萄孢[Botrytis elliptica(Berk.)Cooker],分离率为70.5%;其次是灰葡萄孢(Botrytis cinerea Peers),分离率为23.0%。该病原菌在不同品种百合叶片上造成的症状有4种类型。不同症状类型病斑分离的病原菌种类不同,其中紫缘黄褐斑型、褐色环纹型和黄缘褐斑型主要致病菌为椭圆葡萄孢,平均分离率为82.5%;叶缘干枯型的主要致病菌为灰葡萄孢,分离率达93.7%。温度为15℃时,易产生菌核;菌核萌发最适温度为15~20℃。     

Increased rhizosphere populations of Trichoderma asperellum strain T34 caused by secretion pattern of root exudates in tomato plants inoculated with Botrytis cinerea

Root exudates secreted from plants can modify rhizosphere microbiota by enhancing or inhibiting the growth of biological control agents (BCAs) and/or pathogens. Similarly, microorganisms can modify the secretion of plant root exudates. The aim of this study was to analyse the effect of a Botrytis cinerea leaf infection on the secretion of tomato root exudates and on the populations of the BCA Trichoderma asperellum strain T34 (T34). This study found that the secretion pattern of root exudates in tomato plants was influenced by B. cinerea infection in plant leaves. An increase in the levels of gluconic acid was observed, while levels of sucrose and inositol decreased. A decrease in the severity of B. cinerea by the induction of systemic resistance triggered by T34 was also observed. Tomato plants infected with B. cinerea maintained the populations of T34 in the roots, while populations of T34 decreased in plants not inoculated with the pathogen. Samples exposed to media containing gluconic acid (as the only carbon source or at the same concentration found in roots exudates) saw an increase in the in vitro growth of T34 compared to media without gluconic acid. In conclusion, a change in the secretion pattern of root exudates caused by B. cinerea, together with the enhanced growth of T34 in the presence of gluconic acid, indicates the existence of leaf to root communication. The result of this is enhanced populations of T34, and in turn induced disease resistance and a consequential reduction in disease severity.     

核盘菌侵入油菜超微结构及侵染机制的研究

 通过电子显微镜观察核盘菌在油菜叶片上侵染过程,发现该菌首先在叶片上形成复合附着器。每个分枝末端一般生出一个侵染钉。侵染钉侵入叶表面腊质、角质层和表皮细胞壁时.不仅靠附着器产生的压力,而且供助于酶对寄主表面的软化、消解作用。该菌通过角质层和表皮细胞壁侵入油菜叶片,尚未发现通过气孔侵入的现象。侵入叶片后,该菌的继续生长,导致了油菜组织的溃烂。然后菌丝在腐烂的叶片上集结形成菌核。     

The Effect of Fungicides on the Microbial Balance in the Phyllosphere1

The sensitivity to fungicides of saprophytic fungi colonizing cereal leaves (mainly ←pink→ yeasts, Sporobolomyces spp.; ← white → yeasts, Cryptococcus spp.; Aureobasidium pullulans and Cladosporium spp.) is discussed. Fungicides like dithiocarbamates, captafol, benzimidazoles and tridemorph reduce the saprophytic microflora, while others have little or no effect. The fungicides are mostly effective for 2–3 weeks after the treatment. When evaluating the consequences of a disturbance of the microbial balance we should realize that the saprophytes are able to antagonize a number of necrotrophic pathogens of cereal leaves and ears (Cochliobolus sativus, Septoria nodorum, Fusarium culmorum, Pseudocercosporella herpotrichoides, Colletotrichum graminicola). This antagonism is based on nutrient competition, which reduces the superficial development of the pathogen before penetration, resulting in a reduction of infection of about 50 % or more. This interaction seems particularly important when saprophytes compete successfully for extraneous nutrients from pollen grains or aphid honeydew, which will otherwise stimulate infection.     

Epidemiology of Northern leaf blight on sweet corn

The epidemiology of northern leaf blight of corn, caused byExserohilum turcicum (Pass.) Leonard and Suggs, is reviewed. The minimal dew period required for infection is temperature-dependent. At 25°C, 1 h of dew is sufficient to cause infection and at this temperature the minimal dew period for sporulation is 14 h. Under natural conditions when one dew night is not long enough for conidia to develop, the dew period on the following night enables the completion of conidial formation. The amount of conidia formed is dependent on temperature, light, plant age, leaf position and plant susceptibility. Both qualitative and quantitative types of resistance were identified in several hybrids. Subsequently, there developed additional biotypes ofE. turcicum which are aggressive to plants containing qualitative monogenic resistance. Within the same physiological race, a significant variation in aggressiveness between isolates from various locations is observed. The pathogen overwinters as mycelia and conidia in infected leaves, husks and other plant parts, or onSorghum halepense L. Reduction in yield due to northern leaf blight is associated with the level of resistance of the host plant, with disease severity, plant age during infection, and position of infected leaves.