Attachment and adhesion of conidia of Stagonospora nodorum to natural and artificial surfaces

Attachment and adhesion of conidia of a wheat-isolate of Stagonospora nodorum to leaf and artificial surfaces was studied. Attachment of conidia was a non-viable process, separate from adhesion, that occurred rapidly and irreversibly. Attachment involved conidial-surface carbohydrates and was partially influenced by surface hydrophobicity. The subsequent adhesion, via the secretion of extracellular matrix from conidia, was a viable process that induced the complete cover of conidia in response to wheat leaf surface components containing epi-cuticular wax and to a lesser extent to barley but inducing only partial covering on glass. Results suggest that specific surface components from the compatible host promote rapid attachment and adhesion of S. nodorum conidia.     

Micromorphology of the petals of the invasive weed,Oxalis pes‐caprae

The alien, seedless Oxalis pes‐caprae has spread and colonized many areas of the Mediterranean Basin, relying on vegetative reproduction. The flowering of O. pes‐caprae is greatly accelerated by its exposure to sunlight. When the sun is shining, both sides of the petals of the funnel‐shaped, open flowers of O. pes‐caprae are exposed to the ambient conditions. In cloudy weather, only some portions of the abaxial petal surfaces of the trumpet‐shaped, closed flowers of O. pes‐caprae are exposed to the ambient conditions. The micromorphology of the petals of O. pes‐caprae was imaged by using light, scanning and atomic force microscopy. In O. pes‐caprae, conical cells are found only on the adaxial epidermis of the petals, which also consist of a narrow mesophyll with a loosely arranged parenchyma and convex cells on their abaxial epidermis. High‐resolution imaging of the petal surfaces, using atomic force microscopy, revealed that the epidermal cells are further ornamented by submicron sculptures, indicating a different roughness, density, and arrangement of the folds between the adaxial and abaxial sides of the petals. Submicron sculpturing increases the surface area of the adaxial epidermal cells of the petals and the distances between the folds are almost equal to the visible waveband. On the abaxial epidermal cells, the distances between the folds are smaller than the subwavelength spectrum. The high and the negligible values of roughness that were obtained on the adaxial and the abaxial surfaces might facilitate the capture and the reflection of light, respectively.     

‘Universal’ spray droplet adhesion model – accounting for hairy leaves

The adhesion of a spray droplet upon initial contact with a leaf surface is extremely important to spray efficacy and is dependent on dynamic interactions between droplets (formulation, size, velocity) and leaf (micro‐topography, surface chemistry, veininess, hairiness and orientation). A ‘universal’ spray droplet adhesion model has previously been developed, using 50% aqueous acetone contact angles as a measure of leaf surface properties; this model satisfactorily predicts initial adhesion over a range of formulation surface tensions, droplet sizes and velocities. However, it failed to fit data from hairier leaves. This study investigates initial spray droplet adhesion on hairy leaves. Two categories of hairy leaves were identified by how the droplets penetrate the leaf hairs, Wenzel (hairy) and Cassie–Baxter (super hairy). For the Wenzel‐type, a simple constant accounted for the increased droplet shatter caused by the hairs. For the Cassie–Baxter‐type, a cushioning factor was introduced to account for the absorption of kinetic energy at impact by the hair mat. The cushioning factor was estimated by measuring the relative height of the hair mat. By including these two parameters, the new model successfully predicted the mean adhesion of non‐hairy, hairy and super‐hairy plants (R² = 0.96). This model and the underlying principles determining hairy leaf adhesion developed in this article will help develop spray formulations effective at targeting hairy‐leaved weed and crop species.     

Quantification of physical (roughness) and chemical (dielectric constant) leaf surface properties relevant to wettability and adhesion

BACKGROUND: Spray droplet adhesion is dependent not only on formulation and droplet parameters but also on the surface properties (physical and chemical) of the leaf. Quantifying these leaf surface properties would aid understanding and modelling of adhesion, helping to optimise spray formulations. Fractal dimensions (FDs) were used to quantify the relative leaf surface roughness of ten plant species. Static droplet contact angles were measured on each leaf surface, and wetting tension was calculated. Chemical profiles of the leaf surfaces were developed by evaluating contact angle behaviour relative to solution dielectric constants. RESULTS: The FDs of Cryo‐SEM micrographs taken at 300× magnification gave the best correlation with adhesion. The wetting tension intercept had a strong relationship with mean adhesion, and successfully accounted for the wettability of the outlier species. CONCLUSIONS: The microroughness of the leaf surface, as revealed by Cryo‐SEM, can be quantified by fractal dimension analysis. However, the wetting tension intercept is a more useful universal measure of the surface properties of the leaf (including roughness) as they pertain to adhesion. The slope of the wetting tension versus dielectric constant plot allowed preliminary quantification of the chemical contribution of leaf surface dielectric behaviour to adhesion. Copyright © 2011 Society of Chemical Industry     

The effect of environmental factors on infection of blueberry fruit by Colletotrichum acutatum

Anthracnose fruit rot of blueberries caused by Colletotrichum acutatum is a serious problem in humid blueberry‐growing regions of North America. In order to develop a disease prediction model, environmental factors that affect mycelial growth, conidial germination, appressorium formation and fruit infection by C. acutatum were investigated. Variables included temperature, wetness duration, wetness interruption and relative humidity. The optimal temperature for mycelial growth was 26°C, and little or no growth was observed at 5 and 35°C. The development of melanized appressoria was studied on Parafilm‐covered glass slides and infection was evaluated in immature and mature blueberry fruits. In all three assays, the optimal temperature for infection was identified as 25°C, and infections increased up to a wetness duration of 48 h. Three‐dimensional Gaussian equations were used to assess the effect of temperature and wetness duration on the development of melanized appressoria (R² = 0·89) on Parafilm‐covered glass slides and on infection incidence in immature (R² = 0·86) and mature (R² = 0·90) blueberry fruits. Interrupted wetness periods of different durations were investigated and models were fitted to the response of melanized appressoria (R² = 0·95) and infection incidence in immature (R² = 0·90) and mature (R² = 0·78) blueberry fruits. Additionally, the development of melanized appressoria and fruit infection incidence were modelled in relation to relative humidity (R² = 0·99 and 0·97, respectively). Three comprehensive equations were then developed that incorporate the aforementioned variables. The results lay the groundwork for a disease prediction model for anthracnose fruit rot in blueberries.     

The spore coat of the bean anthracnose fungus Colletotrichum lindemuthianum is required for adhesion, appressorium development and pathogenicity

The spores (conidia) of the bean anthracnose fungal pathogen, Colletotrichum lindemuthianum, adhere to the aerial parts of plants to initiate the infection process. In previous studies we have shown that the Colletotrichum spores are surrounded by a fibrillar spore coat, comprising several major glycoproteins. Previous evidence showed that a monoclonal antibody (UB20) that recognised these glycoproteins was able to inhibit adhesion of spores to a hydrophobic surface. In this paper we have further studied the role of the spore coat in adhesion, germination and fungal development by studying the effects of UB20 and protease treatment of spores. The latter treatment has previously been shown to remove the spore coat. Spores germinate on glass, polystyrene and water agar, however, appressoria only develop on glass or polystyrene, showing a requirement for a hard surface. Removal of the spore coat with protease inhibits adhesion at 30 min, before the secretion of ECM glycoproteins. Protease treatment also inhibits the development of appressoria and reduces pathogenicity on leaves. Incubation of spores with the MAb UB20 inhibits adhesion at 30 min, but does not affect appressorium formation or pathogenicity. The results suggest that an intact spore coat has two functions; it is required for adhesion to a hydrophobic surface and for the detection of a hard surface necessary for appressorium formation. We suggest that contact with a hard surface, rather than adhesion, is the key event leading to appressorium formation.     

Effect of cold‐induced changes in physical and chemical leaf properties on the resistance of winter triticale (×Triticosecale) to the fungal pathogen Microdochium nivale

This study showed that several mechanisms of the basal resistance of winter triticale to Microdochium nivale are cultivar‐dependent and can be induced specifically during plant hardening. Experiments and microscopic observations were conducted on triticale cvs Hewo (able to develop resistance after cold treatment) and Magnat (susceptible to infection despite hardening). In cv. Hewo, cold hardening altered the physical and chemical properties of the leaf surface and prevented both adhesion of M. nivale hyphae to the leaves and direct penetration of the epidermis. Cold‐induced submicron‐ and micron‐scale roughness on the leaf epidermis resulted in superhydrophobicity, restricting fungal adhesion and growth, while the lower permeability and altered chemical composition of the host cell wall protected against tissue digestion by the fungus. The fungal strategy to access the nutrient resources of resistant hosts is the penetration of leaf tissues through stomata, followed by biotrophic intercellular growth of individual hyphae and the formation of haustoria‐like structures within mesophyll cells. In contrast, a destructive necrotrophic fungal lifestyle occurs in susceptible seedlings, despite cold hardening of the plants, with the host epidermis, mesophyll and vascular tissues being digested and becoming disorganized as a result of the low chemical and mechanical stability of the cell wall matrix. This work indicates that specific genetically encoded physical and mechanical properties of the cell wall and leaf tissues that depend on cold hardening are factors that can determine plant resistance against fungal diseases.     

Quantifying cropping practices in relation to inoculum levels of Fusarium graminearum on crop stubble

This study was conducted in 58 producer‐field locations in Manitoba from 2003 to 2006 to understand how cropping practices influence Fusarium graminearum inoculum levels on stubble of various crops, including wheat, collected from the soil surface. Colonies per m² (CN) were determined and converted to base‐10 logarithm values (log₁₀CN). Mean log₁₀CN of the sampled field for various crops and groups of crops grown in the 3 years prior to sampling were tested to find significant differences. Average log₁₀CN values were also used to determine significant differences between tillage systems and the effect of number of years. Average log₁₀CN values for zero and minimum tillage systems were not different but were significantly higher than values for conventional tillage. A series of three crop rotation scenarios were tested using weighted log₁₀CN values for crop, tillage, their interaction and their squared terms in step‐wise regression models to identify which model was the best predictor of log₁₀CN. This was selected as the cropping practice index (CPI) model and was expressed as: CPI = 1·98423 + 0·55975 (C₂ × C₁ × T)² + 0·4390 (C₂ × T)², where C₁, C₂ and T represent the weighted log₁₀CN values for crops grown 1 and 2 years previously and tillage system, respectively. R² value for this model was 0·933 (P < 0·0001). The reliability of the CPI model was tested using jack‐knife full cross‐validation regression. The resulting R² was 0·899. The CPI model was tested using data collected from seven wheat fields in 2006 (R² = 0·567). The relationship between CPI and FHB index (R² = 0·715) was significant.     

A field tracer study of attenuation of atrazine,hexazinone and procymidone in a pumice sand aquifer

A field tracer experiment, simulating point source contamination, was conducted to investigate attenuation and transport of atrazine, hexazinone and procymidone in a volcanic pumice sand aquifer. Preliminary laboratory incubation tests were also carried out to determine degradation rates. Field transport of the pesticides was observed to be significant under non‐equilibrium conditions. Therefore, a two‐region/two‐site non‐equilibrium transport model, N3DADE, was used for analysis of the field data. A lump reduction rate constant was used in this paper to encompass all the irreversible reduction processes (eg degradation, irreversible adsorption, complexation and filtration for the pesticides adsorbed into particles and colloids) which are assumed to follow a first‐order rate law. Results from the field experiment suggest that (a) hexazinone was the most mobile (retardation factor R = 1.4) and underwent least mass reduction; (b) procymidone was the least mobile (R = 9.26) and underwent the greatest mass reduction; (c) the mobility of atrazine (R = 4.45) was similar to that of rhodamine WT (R = 4.10). Hence, rhodamine WT can be used to delimit the appearance of atrazine in pumice sand groundwater. Results from the incubation tests suggest that (a) hexazinone was degraded only in the mixture of groundwater and aquifer material (degradation rate constant = 4.36 × 10^?3 day^?1); (b) procymidone was degraded not only in the mixture of groundwater and aquifer material (rate constant = 1.12 × 10^?2 day^?1) but also in the groundwater alone (rate constant = 2.79 × 10^?2 day^?1); (c) atrazine was not degraded over 57 days incubation in either the mixture of aquifer material and groundwater or the groundwater alone. Degradation rates measured in the batch tests were much lower than the total reduction rates. This suggests that not only degradation but also other irreversible processes are important in attenuating pesticides under field conditions. Hence, the use of laboratory‐determined degradation rates could underestimate reduction of pesticides in field conditions. © 2001 Society of Chemical Industry     

Molecular characterization of a Stagonospora nodorum lipase gene LIP1

A triglyceride lipase gene (LIP1) was cloned from Stagonospora nodorum, the causal agent of wheat glume blotch. LIP1 encodes a 561 amino acid preproprotein with a predicted N‐terminal signal peptide. Its expression was up‐regulated during plant infection and in culture media supplemented with saturated fatty glycerides. The recombinant Lip1 protein possessed lipolytic activity against a broad range of lipid substrates. When applied to wheat leaves, recombinant Lip1 decreased the hydrophobicity of the leaf surface, probably by liquefaction of epicuticular wax. Pretreatment of wheat leaves with Lip1 decreased the rate of conidial adhesion from 69·5% to 22·7% and from 58·9% to 28·4% in two independent assays based on different protocols. LIP1 replacement strains showed decreased lipolytic activity on culture media relative to the wild‐type strain, and adhesion of the conidia to the wheat leaf surface was impaired in the gene replacement strains. In two experiments, adhesion rates were 54·3% and 41·6% in the LIP1 replacement strains, as opposed to 77·7% and 66·6%, respectively, in the wild‐type. Collectively, the data demonstrate that the secreted lipase Lip1 is important for the adhesion of S. nodorum infection to wheat leaves.     

Spreading of spray mixtures on leaf surfaces: II. cohesive interaction parameters as predictors of spreading and the effect of molar volume and roughness on spreading

Partial cohesive interaction parameters as predictors of the spreading of spray mixtures were investigated. Multiple linear regression of the inverse of the area of spread, 1/A, for leaf surfaces on the partial cohesive interaction parameters of pure liquids gave a good fit (r² > 0–90) on virtually all tested surfaces. The coefficients obtained from the regression analysis could be used to describe the spreading properties of each surface uniquely. Regression of 1/Afor an unknown liquid on the coefficients of known surfaces could, conversely, be used to obtain the cohesive interaction parameters of the unknown liquid. A linear relationship was observed between the calculated energy of adsorption and the inverse of the area of spread. An increase in molar volume caused a decrease in the slope of the regression line and an increase in roughness caused a similar, but more pronounced, decrease.     

Photolysis of Diniconazole-M under Sunlight

The photodegradation of diniconazole-M [(E)-(R)-1-(2,4-dichlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)-1-pentene-3-ol] was studied as thin film on glass surface under sunlight. Photoproducts were separated and identified by NMR, IR, UV and mass spectroscopy. They were characterised as the (Z)-isomer of diniconazole-M, a cyclic alcohol and its corresponding ketone and an isoquinoline derivative. © 1997 SCI.     

Investigation of rice blast development in susceptible and resistant rice cultivars using a gfp‐expressing Magnaporthe oryzae isolate

In this study, an isolate of Magnaporthe oryzae expressing the green fluorescent protein gene (gfp) was used to monitor early events in the interaction of M. oryzae with resistant rice cultivars harbouring a blast resistance (R) gene. In the resistant cultivars Saber and TeQing (Pib gene), M. oryzae spores germinated normally on the leaf surface but produced morphologically abnormal germ tubes. Germling growth and development were markedly and adversely affected in leaves of these resistant cultivars. Penetration of host cells was never seen, supporting the idea that disruption of germling development on the leaf surface might be one of the resistance mechanisms associated with Pib function. Thus, this particular R gene appeared to function in the absence of host penetration by the fungal pathogen. Confocal laser scanning microscopy of M. oryzae‐infected susceptible rice cultivars showed the dimorphic growth pattern that is typically observed during the biotrophic and necrotrophic stages of leaf colonization in susceptible cultivars. The suitability of the gfp‐expressing M. oryzae isolate for further research on R‐gene function and identification of resistant genotypes in rice germplasm collections is discussed.     

不同耕作措施下旱作春小麦农田二氧化碳排放模拟及敏感性分析

为探讨不同耕作措施下旱作春小麦农田CO2排放以及气候条件、土壤性质和作物管理措施对CO2排放影响的敏感性差异,本文基于DNDC模型,结合定西试验区旱作春小麦农田定点连续监测的CO2排放通量等数据,检验该模型模拟不同耕作措施下CO2排放的适宜性,并对其敏感性进行研究.结果表明:DNDC模型对不同处理下CO2排放的模拟结果...     

Wax matters: absence of very‐long‐chain aldehydes from the leaf cuticular wax of the glossy11 mutant of maize compromises the prepenetration processes of Blumeria graminis

Conidial germination and differentiation, the so‐called prepenetration processes, of the barley powdery mildew fungus (Blumeria graminis f.sp. hordei) are triggered in vitro by very‐long‐chain aldehydes, minor constituents of barley leaf wax. However, until now it has not been demonstrated that these cuticle‐derived molecules also play a significant role in the initiation and promotion of the fungal prepenetration processes in vivo, on the surface of a living plant leaf. In the maize (Zea mays) wax mutant glossy11, which is completely devoid of cuticular very‐long‐chain aldehydes, germination and appressorial differentiation of B. graminis were strongly impeded. Spraying the mutant leaf surface with aldehyde‐containing wild‐type wax or pure n‐hexacosanal (C₂₆‐aldehyde) fully restored fungal prepenetration, whereas maize wild‐type leaf surfaces coated with n‐docosanoic acid exhibited reduced conidial germination rates of 23%, and only 5% of the conidia differentiated infection structures. In vitro studies were performed to further corroborate the extensive prevention of fungal germination and differentiation in response to artificial surfaces coated with aldehyde‐deficient maize wax. Because of its phenotype affecting the B. graminis prepenetration processes, the glossy11 mutation of maize may become a valuable molecular target and genetic tool that could provide a means of developing basal powdery mildew resistance in the globally important crops wheat and barley.     

Monooxygenase mediated pyrethroid detoxification in sea lice (Lepeophtheirus salmonis)

The role of monooxygenases in detoxification of the pyrethroids cypermethrin and deltamethrin was examined. Four strains of sea lice (Lepeophtheirus salmonis Krøyer) with normal or moderately reduced sensitivity towards the pyrethroids were tested in bioassays by exposure to the pyrethroid alone and in combination with an oxygenase inhibitor, piperonyl butoxide (PBO). The normal (baseline) sensitivity was considered as the sensitivity range for the two most sensitive strains. Pre‐treatment with PBO elevated the sensitivity (P < 0.01) compared with groups exposed to the pyrethroid only. A positive, but not statistically significant, correlation between the activity of haem peroxidases and the pyrethroid concentration immobilizing 50% of the parasites was demonstrated (ρ = 0.500 for deltamethrin and ρ = 0.310 for cypermethrin). The results indicate that cytochrome P450 monooxygenases are involved in detoxification of pyrethroids in sea lice. ¹⁴C‐Deltamethrin was absorbed in a lesser amount in a group of sea lice exposed to a mixture of the compound and PBO than in a group exposed to ¹⁴C‐deltamethrin alone. A significant difference could be demonstrated both immediately after exposure (P < 0.01) and 24 h after exposure (P < 0.05). No significant differences were found between groups pre‐treated with PBO and groups exposed to ¹⁴C‐deltamethrin only. ¹⁴C‐Deltamethrin was taken up mainly through the cuticle, especially the cuticle on the extremities of the ventral surface, and subsequently distributed throughout the body of the parasite. Copyright © 2005 Society of Chemical Industry     

Cell-substratum adhesive protein involved in surface contact responses of the bean rust fungus

Uredospore germlings of the bean rust fungus Uromyces appendiculatus display two contact-sensitive responses on leaves or certain synthetic surfaces: a specific orientation of germ tubes and an induction of appressoria at leaf stomates or scratched surfaces. Germ tube orientation, nuclear division (used as a marker for appressorium formation) and germling adhesion on scratched “Parafilm” was reduced by the proteolytic enzyme, pronasc E (ED₅₀ = 20–50 μg ml⁻¹), but not by the heat denatured proteolytic enzyme. Forty micrograms of pronase E per millilitre significantly reduced nuclear division of germlings incubated on either a hydrophilic or a hydrophobic surface. Pronase E reduced adhesion regardless of whether the enzyme was applied during growth or after attachment had occurred, although higher concentrations were required after attachment had occurred. The data suggest that extracellular protein is required for germling adhesion to a substratum and that adhesion is required for germ tube orientation and thigmodifferentiation.We partially characterized the composition of the bean rust extracellular material and analyzed for proteins on SDS-polyacrylamide gels. Intact germlings have six predominant extracellular peptides detected by ¹²⁵I labelling. Spent culture fluid contains several different peptides; four of the five major peptides in the spent culture fluid are glycosylated.     

Effects of isolate virulence and host susceptibility on development of early blight (Alternaria solani) on tomato*

Early blight of tomato (Lycopersicon esculentum) caused by Alternaria solani has the potential to become one of the most serious diseases throughout the tomato‐producing regions of Greece. Controlled environment experiments were conducted to study the virulence of A. solani isolates and the susceptibility of commercial tomato cultivars and hybrids to early blight. The isolates used, derived from naturally infected tomato plants during the period 1997/1998, differed significantly (P>0.05) in the rate of mycelial growth as well as in their ability to sporulate in vitro. No correlation (R²= 0.33) was found between mycelial growth and conidia production. Isolates of A. solani were virulent to young tomato plants (cv. Ace 55VF), although they differed significantly (P>0.05) in the intensity of symptoms produced on leaves, stems, petioles and flowers. Defoliation was linearly related (R²= 0.87) to the percentage of leaf area with symptoms. Twenty‐three tomato cvs. or F₁ hybrids were evaluated for their susceptibility to early blight. The cultivars or hybrids were arbitrarily categorized as immune, highly tolerant, tolerant, moderately tolerant, susceptible and highly susceptible based on a percent disease index range: 0%, 1‐9%, 10‐24%, 25‐49%, 50‐74% and 75% or more respectively. None of the cultivars or hybrids tested was immune or tolerant to A. solani infection.