The effects of post-inoculation wet and dry periods, and inoculum concentration, on lesion numbers of Septoria nodorum in spring wheat seedlings

Lesion numbers in seedlings of spring wheat cvs Kolibri and Maris Butler, inoculated with Septoria nodorum in a growth cabinet, varied with the length of wet or dry period following inoculation. Increasing the length of dry period led to a curvilinear decrease in lesion numbers at 15° C (60% r.h.) and 20° C (40% r.h.), but some infection still occurred with a dry period of 72 h. Increasing the length of wet period gave a linear increase in lesion numbers in cv. Maris Butler, but lesion numbers levelled off at 12 h at 15° C (60% r.h.) in cv. Kolibri. Increasing the inoculum concentration gave an increase in lesion numbers in both cultivars irrespective of whether wet or dry periods followed inoculation. On occasion, Kolibri and Maris Butler differed significantly in lesion numbers, but differences were not consistent between experiments, and there were interactions with the length of wet period.     

Sporulation of Septoria nodorurn (and S. tritici) on spring wheat cvs Kolibri and Maris Butler in relation to growth stage, plant part and time of season

Sporulation of Septoria nodorum was assessed on inoculated spring wheat cvs Kolibri and Maris Butler. The time from inoculation to first production of spores (latent period) was similar over a range of constant temperatures (12–18°C) but was shorter for Kolibri at 6°C and at 24°C. Spore production was greatest during the period of stem extension and, for equal amounts of disease, was twice as great on Kolibri as on Maris Butler. Spore production on leaves was much more intense with Septoria tritici than S. nodorum for equal amounts of disease, and was less intense on heads than on leaves for both pathogens. Inoculation of plants resulted in significant losses in yield. Mans Butler consistently out-performed Kolibri on each component of yield. Kolibri was particular affected: 1000-grain weights were reduced at each growth stage tested especially by S. nodorum, grain numbers and yield/head were reduced particularly following inoculation at heading. Spore production of S. nodorum during the period of stem extension increased to a peak in early July and then declined but no reliable relationship with monitored weather accounted for this seasonal trend.     

Disease spread of non-specialised fungal pathogens from inoculated point sources in intraspecific mixed stands of cereal cultivars

Spread of Septoria nodorum from inoculated point sources was examined in pure stands and mixtures of two spring wheat cultivars Kolibri and Maris Butler. Gradients in disease were observed soon after inoculation; the presence of the more resistant cultivar (Maris Butler) in the mixtures retarded the outward spread of disease compared with the susceptible pure stand (Kolibri). Regression analysis suggested that gradients in incidence were influenced by nearness to the source whereas gradients in severity were not. Spread of disease was also examined in pure stands and mixtures for the host/pathogen combinations, winter wheat (cvs Maris Huntsman and Maris Ranger)/S. nodorum and winter barley (cvs Maris Otter and Hoppel)/Rhynchosporium secalis. In contrast to the spring wheat experiment, no gradients were observed; explanations for the uniform distribution of disease were (a) extensive spread prior to the period of assessment, (b) no physical barrier to dispersal due to the juvenile growth stage of the crop and (c) exhaustion of the point-source.     

Spatial root distribution of plants growing in vertical media for use in living walls

Background and Aims

For plants growing in living walls, the growth potential is correlated to the roots ability to utilize resources in all parts of the growing medium and thereby to the spatial root distribution. The aim of the study was to test how spatial root distribution was affected by growing medium, planting position and competition from other plants.

Methods

Five species (Campanula poscharskyana cv. ‘Stella’, Fragaria vesca cv. ‘Småland’, Geranium sanguineum cv. ‘Max Frei’, Sesleria heufleriana and Veronica officinalis cv. ‘Allgrün’) were grown in three growing media (coir and two of rockwool) in transparent boxes under greenhouse conditions. Root frequency was registered and the activity of individual root systems was studied via ¹⁵N uptake and plant dry weight was measured.

Results

Plants in coir had stronger root growth in all parts of the medium than plants in rockwool. Upwards root growth was limited for plants in the middle or lower parts of the medium and ¹⁵N measurements confirmed that only plants in the bottom of the box had active roots in the bottom of the medium. The species differed in root architecture and spatial root distribution.

Conclusions

The choice of growing medium, plant species and planting position is important for a living wall as it affects the spatial root growth of the plants.     

Selection for immunity in swede (Brassica napus) to infection by turnip mosaic virus

Fully heritable immunity to turnip mosaic virus (TuMV) was found in swede (Brassica napus) cvs Ruta Otofte and Bangholm. Attempts to ‘break’ this immunity were unsuccessful. The reactions of two immune lines of cv. Ruta Otofte to cauliflower mosaic virus, clubroot (Plasmodiophora brassicae) and powdery mildew (Erysiphe cruciferarum) were unchanged compared with the unselected parent line. A symptomless response to TuMV inoculation (possibly immunity) was also found in cv. Acme but not in the clubroot resistant cv. Marian. The percentage of plants which gave a symptomless response to TuMV inoculation differed greatly between cultivars and some seed stocks.     

Prediction of citrus canker epidemics generated through different inoculation methods

Citrus canker epidemics were generated with 10^8?cfu/ml of Xanthomonas axonopodis pv. citri (ex Hasse) on Citrus limonia cv. China lemon, Citrus reticulate cv. kinnow, Citrus jambhiri, Citrus reticulate cv. Feutral’s early and Citrus limettioides using four inoculation techniques. Natural inoculum was also relied upon for infection. Overall, the injection infiltration method led to maximum disease generation followed by spray, pinprick and smear inoculation methods. Citrus canker incidence along with environmental data were recorded and subjected to stepwise regression analysis. Except relative humidity, the relationship of weekly air temperature (maximum and minimum), rainfall and wind speed with citrus canker disease development in all citrus cultivars was positively correlated and best explained by linear regression. Overall, two environmental variable model containing maximum and minimum air temperature fit the data well explaining 93% variability in disease development. The observed citrus canker incidence values and those predicted by the model were close in most of citrus cultivars. This two environmental variable model can be used to issue advance warning forecasts for the timely management of the citrus canker in Pakistan.     

Mowing during rainfall enhances the control of Cirsium arvense

Pastoral farmers in New Zealand have described dramatic demises in populations of the weed Cirsium arvense (L.) Scop., a perennial herb indigenous to Eurasia, following its mowing during rainfall. To test the hypotheses (1) that the mowing of C. arvense during rainfall increases the control of this weed and (2) that the causal organism in this ‘mowing-in-the-rain’ effect is the vascular wilt fungus Verticillium dahliae, two series of field experiments were carried out in C. arvense-infested pastures in New Zealand, one in autumn 2008 (Experiment series 1, 9 farms), and another in spring/summer 2008–2009 (Experiment series 2, 12 farms). The effect of mowing in the rain as compared to mowing in the dry was to reduce the % ground cover of the thistle in the spring following treatment by 21 and 32% in Experiment series 1 and 2, respectively. Correlations of this ‘rain versus dry’ effect with the incidence of V. dahliae in the subterranean parts of C. arvense shoots sampled in each of the two field experiment series provided no statistical evidence that the effect increased with V. dahliae incidence. Thus these experiments provide no support for the hypothesis that V. dahliae is the biological mechanism for the ‘mowing-in-the-rain’ effect. Nevertheless, they do support mowing during rainfall as a simple and effective management tactic for C. arvense.     

Feeding behavior of Diaphorina citri and its transmission of ‘Candidatus Liberibacter asiaticus’ to citrus

Huanglongbing (HLB), also known as citrus greening, is currently the most destructive disease of citrus, responsible for huge economic losses in the world's major citrus production areas. The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), transmits ‘Candidatus Liberibacter asiaticus’ (Clas), the pathogen responsible to cause HLB. Understanding of vector, pathogen, and host plant interactions is important for the management of this vector‐disease complex. We used the direct‐current electrical penetration graph (DC‐EPG) system to evaluate feeding behavior of Clas‐infected D. citri adults, and their potential to transmit the pathogen to healthy citrus, Citrus reticulata Blanco cv. Sunki (Rutaceae), following a 24‐h inoculation access period. Plants were tested for the presence of Clas by qPCR 6 months after inoculation. Findings suggest that inoculation was associated with salivation into the phloem sieve elements (waveform E1). The minimum feeding time for successful transmission by a single adult was 88.8 min, with a minimum E1 duration of 5.1 min. Regression analysis indicated a significant relationship between E1 duration and transmission efficiency. The adults successful in transmitting Clas to healthy citrus were able to penetrate and feed in the phloem much earlier than those which did not transmit. The minimum duration of E1 for a female was shorter than that of a male, but transmission was higher. However, durations of other EPG parameters were not significantly different between male and female. Feeding by single Clas‐infected D. citri adults on 6‐month‐old plants (Sunki) resulted in 23% HLB‐positive plants 6 months after inoculation. Multiple nymphs or adults could transmit the pathogen more efficiently than individual adults in the field, and further enhance the severity of the disease. Effective tactics are warranted to control D. citri and disrupt transmission of Clas.     

Effects of continuous leaf wetness on photosynthesis: adverse aspects of rainfall

Above-ground parts of Phaseolus vulgaris L. plants were treated with artificial misty rain (‘rain’) in a growth chamber to investigate the effects of leaf wetness on photosynthetic performance. The following results were obtained. (1) Stomata closed completely within 2 min of the onset of continuous ‘rain’ application and gradually opened to half the original aperture by 60 min. The rate of CO₂ exchange measured on such wet leaves changed in parallel with the changes in stomatal aperture and attained 60 to 70% of the control level by 1h. (2) The dependence of the rate of leaf photosynthesis, A, on the intercellular CO₂ concentration, c_i [A(c_i) relationship], examined in thoroughly dried leaves which had been treated with ‘rain’ did not change until after 4 h of treatment. However, leaves treated for 6h showed discernible decreases in A at high c_i (c_i>500μmolmol ^?1). The photosynthetic rate of leaves treated with ‘rain’ for 24 h was reduced at all c_i, and A at the ambient CO₂ concentration of 350μmolmol^?1 was 60 to 70% of that of the control level. The rate of photosynthesis did not recover even after 3 d of treatment of the plants in a dry environment. These results clearly indicate that leaf wetness causes not only instantaneous suppression of photosynthesis but also chronic damage to the photosynthetic apparatus. Potential effects of leaf wetness on photosynthetic performance in nature are also discussed.     

Free Water Can Leach Mycotoxins from Fusarium‐infected Wheat Heads

The objective of this study was to examine the impact of free environmental moisture, such as from rainfall, on disease development and mycotoxin production and accumulation in planta. In greenhouse experiments in 2009, two single Fusarium graminearum isolates were used to inoculate spikes of three wheat cultivars: ‘Alsen’, ‘2375’ and ‘Wheaton’ at anthesis. On each wetting event/sampling day (7, 14, 21 or 28 days after inoculation), FHB severity was assessed and five pots of each of the two cultivar/isolate treatments were subjected to a wetting event. At the end of the wetting event, the spikes were sampled both from the plants that received the wetting treatment and those that did not and analysed for mycotoxins. Run‐off water samples were also taken 3 h after the start of irrigation and immediately after the wetting treatment concluded and analysed for mycotoxins. The results showed despite statistically similar FHB severity, the levels of DON and other mycotoxins detected were significantly lower in the plants receiving a single wetting event compared to the control. The levels of DON in wetted plants were lower up to 36% in ‘Alsen’, 52% in ‘2375’ and 41% in ‘Wheaton’ compared to that of corresponding controls. DON and 15‐ADON were also detected in run‐off water from the inoculated heads of all cultivars examined. Based on the results of this study, it can be concluded that DON and its derivatives produced in planta can be leached out from the host tissues by free water on contact with plant surfaces.     

Effects of virus infection on plant growth,root development and phytohormone levels in in vitro-cultured pear plants

Virus infection is common in pear (Pyrus spp.) trees commercially cultivated in China. In this study, the growth speed, root development and phytohormone levels of virus-free and virus-infected pear cultures were comparatively investigated. Results showed that the co-infection of Apple stem grooving virus (ASGV) and Apple stem pitting virus (ASPV) significantly decreased the growth and proliferation of in vitro plants of P. communis cv. ‘Confenence’, but showed relatively less effect for P. pyrifolia cv. ‘Jinshui no. 2’, P. communis cv. ‘Red Clapp Favonite’ and P. sinkiangensis cv. ‘Korla’. Strong inhibition of ASGV infection to root development was observed for in vitro plants of ‘Jinshui no. 2’, but not for ‘Confenence’. Furthermore, during root induction, ASGV infection significantly increased CTKs/IAA ratio and decreased IAA/ABA ratio in the rooting region of in vitro plants of ‘Jinshui no. 2’. Nevertheless, for in vitro plants of ‘Confenence’, these values were rarely influenced by ASGV and ASPV infection. The result indicated that the changes of CTKs/IAA and IAA/ABA ratios might be responsible for rooting inhibition of in vitro plants of ‘Jinshui no. 2’.     

Mechanical Transmission of Barley Mild Mosaic Virus (BaMMV) to Rye (Secale cereale L.)

After mechanical spraygun inoculation barley mild mosaic virus (BaMMV) was detected in barley cv. ‘Gerbel’ (control) as well as in rye cv. ‘Somro’, but not in wheat cv. ‘Kanzler’ and oat cv. ‘Alfred’. ELISA values of infected barley and rye were similar. Furthermore, infected rye plants developed symptoms typical for barley yellow mosaic virus infection.     

Bacillus thuringiensis - A New Perspective for an Induced Protection to Coffee Leaf Rust*

Susceptible coffee plants (Coffea arabica cv. ‘Mundo Novo’) treated with a suspension of Bacillus thuringiensis or with three of commercial products (Thuricide HD, Bactimos and Bactospeine PM) of B. thurngiensis were protected against a later inoculation with Hemileia vastatrix, the causal agent of coffee leaf rust. Thuricide HD was effective at concentrations from 5 to 20 mg/ml; at this concentration, protection reached 90 %, lasted for at least 5 weeks, and was systemic. Induced resistance was determined by the reduction of the average number of lesions per leaf in treated leaves when compared with non-treated controls. A decrease in lesion size as well as a delay in sporulation time were also observed in protected plants treated with Thuricide HD.     

Effect of Nitrogen and Powdery Mildew on the Yield Formation of Two Winter Barley Cultivars1

In field experiments the effect of powdery mildew on the yield formation of two barley cultivars ‘Catinka’ (partially resistant) and ‘Tapir’ (susceptible) was investigated in relation to three levels of nitrogen fertilization (40, 90, 150 kg N/ha). Nitrogen increased especially the single ear yield of cv.‘Catinka’, whereas cv.‘Tapir’ showed symptoms of N oversupply. Infection density of Erysiphe graminis hordei also increased with increased nitrogen supply, although it decreased transiently, at the highest N level for cv.‘Tapir’. In general, cv.‘Catinka’ was less tolerant to infections with E. graminis than cv.‘Tapir’, but yield response of both cultivars to powdery mildew (17, 11 and 8 % and 12, 16, and 12 % yield losses for cv.,‘Catinka’ and ‘Tapir’. respectively) showed cultivar × nitrogen interrelationships. Starch and protein contents of grains indicated differences in the intensity of detrimental effects caused by powdery mildew limiting the formation of carbohydrates and/or the translocation processes. Regression analysis demonstrated a closer correlation of yield losses with the photosynthetically active leaf area (r =+0.96 and +0.93 for ‘Catinka’ and ‘Tapir’) than with disease intensity (r =−0.32 and −0.85). Theinte,rpretation of disease/yield loss relations and the detrimental effect of powdery mildew are discussed with reference to plant resistance, remaining green leaf area, the availability of nutrients, to ‘sink and source’ conditions and to the occurrence of,additional environmental stresses.     

Disease progress of non-specialised fungal pathogens in intraspecific mixed stands of cereal cultivars. II. Field experiments

Epidemic development of Septoria nodorum was examined in pure stands and mixtures of two spring wheat cultivars, Kolibri and Maris Butler. In mixtures, disease development was reduced almost to that of the more resistant pure stand (Maris Butler). Disease levels, however, were low and no significant differences in grain yield per head were demonstrated. Disease development was further investigated for the host-pathogen combinations winter wheat/S. nodorum and winter barley/Rhynchosporium secalis. With both combinations the mixed stands again reduced disease levels to almost that of the more resistant pure stands. In general, mixed stands were more effective against R. secalis, although, with S. nodorum, disease levels were low. The yield response in mixed stands differed for the host cultivars and was not significantly changed by the presence of disease. The complexity of analysing such situations and the implications of these findings for mixtures of cultivars differing in resistance to non-specialised pathogens are discussed.     

Phenolic compounds of Sorghum vulgare in response to Sclerotium rolfsii infection

Abstract

Identification of individual phenolic acids of Sorghum vulgare Pers. cv. M.P. after interaction with Sclerotium rolfsii Sacc. using high performance liquid chromatograph (HPLC) showed the presence of phenolics namely tannic, gallic, ferulic, chlorogenic and cinnamic acids in varying amounts. After 72 h inoculation with S. rolfsii, a maximum amount of ferulic acid (166.6 µg g^?1 fresh wt) was present in the collar of inoculated plants, followed by leaves and roots and its level decreased gradually with time. Similarly, the presence of chlorogenic acid was traced after 48 h, while that of cinnamic acid was traced after 72 h of inoculation. Reddish-brown pigmentation at the collar region of inoculated plants was also observed along with the high content of tannic acid. Among other phenolics, the presence of gallic acid was recorded consistently and maximum accumulation (139.3 µg g^?1 fresh wt) was noticed at the zone of interaction (collar region) after 72 h of inoculation. In contrast, maximum lignin deposition was observed at collar region after 96 h of inoculation. Induction of phenolic acids in S. vulgare along with the lignin deposition and red pigmentation at collar region is considered a key biomarker in the non-host-pathogen interaction in the S. valgare–S. rolfsii pathosystem.     

Roditis Leaf Discoloration - A New Virus Disease of Grapevine: Symptomatology and Transmission to Indicator Plants

The symptoms and some characteristics of an unreported disease of grapevine, which was observed during the last years on the cv. ‘Roditis’ in central Greece, are described. The disease is transmissible to V. vinifera‘Mission’ and to some herbaceous test plants. The constant association of a virus to naturally infected plants cv. ‘Roditis’ and chip-budding infected indicator plants cv. ‘Mission’ supports the evidence that the disease is caused by a virus.     

Systemic Infection of Tobacco by Pepper Veinal Mottle Potyvirus (PVMV) Depends on the Presence of Potato Virus Y (PVY)

In single inoculations, both PVY and PVMV replicated in inoculated leaves of Nicotiana tabacum cv. ‘Xanthi nc’ plants, but only PVY infected the tobacco plants systemically, whereas PVMV caused localized infection. A mixed infection by the PVY-To72 and PVMV-type strains was experimentally realized in ‘Xanthi nc’ plants. In the presence of PVY, PVMV migrated systemically into the upper leaves of the tobacco plant, as was proved by back inoculation. It would appear that in tobacco, PVY acts as a “helper” virus, providing PVMV with the necessary component factor for migration. In extracts from the co–infected leaves. Immune Electron Microscopy (IEM) revealed phenotypic mixed particles which contained a mixture of coat proteins of PVY and PVMV. The role of the structural and functional interactions between the two viruses, which enable PVMV to migrate systemically in tobacco plants, is discussed.     

Eine verbesserte Extraktionsmethode für den elektronenmikroskopischen Nachweis von Pflanzenviren: (Kurze Mitteilung)

Spring barley cultivar ‘Carina’ was classed as tolerant, cv. ‘Ultra’ as vulnerable towards infestation with Rhopalosiphum padi. These differences in tolerance did, however, not exsist towards infestation with Metopolophium dirhodum. Faba bean cv. ‘Apollo’ was classed as tolerant and ‘Albatross’ as vulnerable towards Aphis fabae.

Tolerance was correlated with lower assimilate deprivation by Rhopalosiphum padi and less decreased CO₂‐assimilation rates. After infestation with Metopolophium dirhodum CO₂‐assimilation rates were not influenced in cultivar specific ways at equal rates of assimilate deprivations.

Also, Aphis fabae deprivated more assimilates on the vulnerable cv. ‘Albatross’ in comparison to the tolerant cv. ‘Apollo’. Simultaneously, for the tolerant cultivar the supply of inflorescences with assimilates after the infestation was even better than for none infested one. This surely contributed to a stabilization of yield in the tolerant cultivar.     

Chitinase Activity in Stylosanthes guianensis Systemically Protected Against Colletotricbum gloeosporioides

Three- and four-fold increases in chitinase activity were detected in the youngest, fully expanded leaf (L₁) of Stylosanthes guianensis cv. Endeavour following inoculation of the second youngest, fully expanded leaf (L₂) with virulent Type B and Type A isolates of Colletotrichum gloeosporioides compared with chitinase activity in L₁ leaves of uninoculated plants. Only small increases in β-1,3-glucanase were detected in L₁ leaves of systemically protected plants. Chitinase activity was maximal in leaf L₁ 36 to 48 h after inoculation of leaf L₂, and this was coincident with the onset of resistance to anthracnose in L₁ leaves. Chitinase activity also increased in L₁ leaves inoculated with a weakly pathogenic isolate of C. gloeosporioides. Resistance developed in these L₁ leaves to subsequent infection by a virulent isolate of the pathogen approximately 36 h after protective-inoculation with the weakly pathogenic isolate. Two chitinase isozymes, with molecular weights of 65,000 daltons (pI 3.1) and 54,000 daltons (pI 4.0), were separated from extracts of C. gloeosporioides-challenged S. guianensis cv. Endeavour leaves. S. guianensis chitinase caused death of C. gloeosporioides hyphae, particularly in the presence of β-1,3-glucanase. Mycelial viability declined as activity of chitinase was increased in mixtures containing a fixed activity of β-l,3-glucanase.