The effect of water deficiency in corn plants on the development of three corn diseases

The relationships between the development of disease symptoms ofPuccinia sorghi, Exserohilum turcicum andHelminthosporium maydis on susceptible sweet corn plants, and the water status of the plants, were studied. At a relative water content (RWC) of 75% the development of sori ofP. sorghi, and the leaf area infected byE. turcicum andH. maydis, were inhibited by 38, 75 and 84%, respectively, as compared with the control plants (RWC = 100%).P. sorghi andH. maydis were not inhibited by a slight decrease in the RWC (96%), but development of leaf area infected byE. turcicum was inhibited by 33%.P. sorghi seemed to be better adapted to relatively high water deficiency in the host plants thanH. maydis orE. turcicum.     

Effects of Plant Defence Activators on Anthracnose Disease of Cashew

The plant defence activators acibenzolar-S-methyl (Benzo[1,2,3]thiadiazole-7-carbothioic acid-S-methyl ester, ASM), 2,6-dichloro-isonicotinic acid (DCINA), salicylic acid (SA), and dibasic potassium phosphate (K₂HPO₄) were tested for their ability to protect cashew (Anacardium occidentale) seeds and leaves from anthracnose disease caused by Colletotrichum gloeosporioides. No inhibition of the early stages of pathogen development was caused by concentrations equal to or lower than 1.1mM a.i. ASM, 1.2mM a.i. DCINA, 5mM SA and 50mM K₂HPO₄. Maximum reduction of the disease in detached leaves, without phytotoxic effects, was obtained with 0.07mM a.i. ASM and DCINA, 5mM SA, and 50mM K₂HPO₄, with a time interval of at least 72h between application of the activator and inoculation with the pathogen. On attached leaves, foliar sprays were slightly more efficient than soil drench treatments, with 5mM SA being the most effective treatment, while 50mM SA as well as 0.3mM a.i. ASM and DCINA caused phytotoxic effects. In field-grown plants, protection was conferred by a soil drench of concentrations as low as 12.6M a.i. ASM and DCINA and 2.6mM SA. These concentrations were not phytotoxic suggesting that plant defence activators have potential for control of anthracnose disease in the field.     

Mechanisms of Phosphate-induced Disease Resistance in Cucumber

Certain phosphate salts are known inducers of systemic acquired resistance (SAR). In the present study, a local spray application of dipotassium hydrogenphosphate (K₂HPO₄) was effective in inducing a high level of systemic protection in cucumber plants against anthracnose caused by Colletotrichum lagenarium. Resistance induction by K₂HPO₄ was associated with localized cell death in cucumber leaves treated with the phosphate salt. The cell death observed, subsequently resulted in the appearance of macroscopically visible, necrotic spots. Appearing lesions resembled those provoked by tobacco necrosis virus (TNV) during a hypersensitive response (HR) that leads to pathogen-induced activation of SAR. Phosphate-mediated cell death was preceeded by a rapid generation of superoxide and hydrogen peroxide. As a further consequence of phosphate application, a local and systemic increase in free and conjugated salicylic acid (SA) levels was detected. The phosphate-induced responses were also identified with a similar time range in cucumber leaves that had been pre-inoculated with TNV. In contrast, none of these responses was triggered by application of the commercial plant activator benzo[1,2,3]thiadiazole-7-carbothioic acid-S-methyl ester (BTH), which nevertheless was highly effective in inducing SAR in cucumber against anthracnose. In conclusion, the chemical SAR inducer K₂HPO₄ and the biological inducer TNV share some common early steps in signal transduction leading to SAR in cucumber, which differ from those involved in BTH-mediated SAR.     

Involvement of a phytotoxic peptide in the development of the Northern leaf blight of corn

Non pathogenic isolates ofExserohilum turcicum successfully infect corn plants in the presence of syntheticE. turcicum toxin during inoculation. The toxin significantly increased the number of appressoria and the ramification of germinating conidia both on host leaves and on artificial media. These findings indicate that this toxin plays an important role in infection of Northern leaf blight.This article is dedicated to the memory of Yehouda Levy who passed away recently.     

Induction of Systemic Resistance Against Bacterial Wilt in <Emphasis Type="Italic">Eucalyptus urophylla</Emphasis> by Fluorescent <Emphasis Type="Italic">Pseudomonas</Emphasis> spp

The ability of selected strains of fluorescent Pseudomonas spp. to cause induced systemic resistance (ISR) in Eucalyptus urophylla against bacterial wilt caused by Ralstonia solanacearum was investigated. Four of the five strains used can produce salicylic acid (SA) in vitro and, therefore, chemical SA, that is known to induce resistance in many plant species, was used as a reference treatment. Whereas a soil drench with SA did induce systemic resistance in E. urophylla, infiltration of SA into leaves did not. None of the fluorescent Pseudomonas spp. strains caused ISR against bacterial wilt when applied to the soil, but two strains, P. putida WCS358r and P. fluorescens WCS374r triggered ISR when infiltrated into two lower leaves 3–7 days before challenge inoculation. A mutant of strain WCS358r defective in the biosynthesis of the fluorescent siderophore pseudobactin, did not cause ISR, while the purified siderophore of WCS358r did, suggesting that pseudobactin358 is the ISR determinant of WCS358. A siderophore-minus mutant of WCS374r induced the same level of disease resistance as its parental strain, but the purified siderophore induced resistance as well, indicating that both the siderophore and another, unknown, inducing determinant(s) of WCS374r can trigger ISR in Eucalyptus. A possible role of WCS374r-produced SA remains uncertain. Transformation of a siderophore-minus mutant of WCS358 with the SA biosynthetic gene cluster from WCS374 did not enable this transformant to cause ISR in E. urophylla.     

Foliar spray of phosphates induces growth increase and systemic resistance to Puccinia sorghi in maize

A single foliar spray of 0-1 M phosphate salts solution was applied to the upper side of maize (cv. Jubilee) leaves 1,2 and 3 at the five-to six-leaf stage, 2-4 h before inoculation with Puccinia sorghi. This treatment induced systemic resistance against common rust as expressed by 98% reduction in the number of pustules of P. sorghi developed on leaves 5, 6 and 7, There was no damage or chlorotic stippling on the induced leaves (1, 2 and 3) as a result of the phosphate spray, A less effective, but still significant. reduction (90%) in the number of pustules was obtained when the foliar spray was applied 6 days before inoculation. One foliar spray of K₂HPO₄ on leaves 1,2 and 3,6 days before inoculation, stimulated plant growth regardless of inoculation. The foliar spray on the day of inoculation was less effective in stimulating growth. Appropriate mixtures of phosphate solutions revealed that the level of protection was not necessarily dependent on the gradual changes in the pH of the solution. The possible dual use of phosphate salts as foliar fertilizers and as agents for induced resistance is discussed.     

A histological assessment of the infection strategy of Exserohilum turcicum in maize

Northern leaf blight is a lethal foliar disease of maize caused by the fungus Exserohilum turcicum. The aim of this study was to elucidate the infection strategy of the fungus in maize leaves using modern microscopy techniques and to understand better the hemibiotrophic lifestyle of E. turcicum. Leaf samples were collected from inoculated B73 maize plants at 1, 4, 9, 11, 14 and 18 days post-inoculation (dpi). Samples were prepared according to standard microscopy procedures and analysed using light microscopy as well as scanning (SEM) and transmission electron microscopy (TEM). Microscopic observations were preceded by macroscopic observations for each time point. The fungus penetrated the leaf epidermal cells at 1 dpi and the disease was characterized by chlorotic leaf flecks. At 4 dpi the chlorotic flecks enlarged to form spots, and at 9 dpi hyphae were seen in the epidermal cells surrounding the infection site. At 11 dpi lesions started to form on the leaves and SEM revealed the presence of hyphae in the vascular bundles. At 14 dpi the xylem was almost completely blocked by hyphal growth. Hyphae spread into the adjacent bundle sheath cells causing cellular damage, characterized by plasmolysis, at 18 dpi and conidiophores formed through the stomata. Morphologically, lesions started to enlarge and coalesce leading to wilting of leaves. This study provides an updated, detailed view of the infection strategy of E. turcicum in maize and supports previous findings that E. turcicum follows a hemibiotrophic lifestyle.     

Induction of systemic resistance byPseudomonas fluorescens Pf1 againstXanthomonas oryzae pv.Oryzae in rice leaves

When lower leaves of rice plants were inoculated with powder formulation of a saprophytic strain ofPseudomonas fluorescens, Pfl, upper leaves, in addition to the inoculated lower leaves, showed resistance to the rice bacterial blight pathogenXanthomonas oryzae pv.oryzae. When the leaves were challenge-inoculated withX. oryzae pv.oryzae 4 days afterP. fluorescens application on lower leaves, the disease intensity in upper leaves decreased from 6.7 to 1.1. When rice seeds were treated with the formulation ofP. fluorescens Pfl and sown, 30-day-old seedlings showed resistance toX. oryzae pv.oryzae and the disease intensity decreased from 6.8 to 1.2. The induced resistance was transient; leaves sprayed withP. fluorescens Pfl at 30 days after treatment and leaves of 60-day-old seedlings fromP. fluorescens-treated seeds did not show resistance to the pathogen. In field trials, seed treatment followed by foliar application of the powder formulation ofP. fluorescens Pfl effectively controlled rice bacterial blight and increased the yield. In the induced resistant leaves a sharp increase in lignification and activities of peroxidase, phenylalanine ammonia-lyase and 4-coumarate: CoA ligase was observed when the leaves were challenge-inoculated withX. oryzae pv.oryzae. An approximately threefold increase in lignin content, peroxidase activity and phenylalanine ammonia-lyase activity and a fivefold increase in 4-coumarate: CoA ligase activity were observed 5 days after challenge inoculation withX. oryzae pv.oryzae in rice leaves pretreated withP. fluorescens for 5 days. A similar increase in defense-related activities was not observed in susceptible interactions or inP. fluorescens-treated plants at later stages of interactions when no resistance to the pathogen was observed.     

Survival in the phylloplane of an introduced biocontrol agent (Trichoderma harzianum) and populations of the plant pathogenBotrytis cinerea as modified by abiotic conditions

Leaf populations ofTrichoderma were studied on tomato, pepper and geranium plants incubated under various conditions. Treatments involved high (>90%) or lower (75–85%) relative humidity (r.h.), temperatures of 15±3°C or 25±3°C, and soil fertilization with formulations of 2,2,5%, 3,3,8% or 5,3,8% NPK. The size of populations on leaves treated with the fungusTrichoderma harzianum differed according to plant species, leaf age, length of incubation, atmospheric conditions, and plant nutrition.T. harzianum populations were promoted in many cases by high r.h. and by 3,3,8% NPK. Interactions of introduced populations ofBotrytis cinerea with populations ofT. harzianum on tomato leaves under combinations of the above conditions showed that the population ofB. cinerea wasca tenfold lower in the presence ofT. harzianum than in the absence of this fungus.     

Calcium reducesBotrytis c1nerea damages to plants ofRuscus hypoglossum

Plants ofRuscus hypoglossum L. were grown in containers with perlite, an inert medium, or with soil, under similar-to-commercial conditions, for three winter seasons. Constant fertilization with soluble calcium [Ca(NO₃)₂] through the root zone resulted in a reduction of up to 78% in postharvest rot caused by artificial inoculation withBotrytis cinerea Pers. ex Fr. Control plants were treated by fertilization with N,P,K and all other elements, identical to the fertilizer containing calcium. Foliar application of Ca(NO₃)₂ to ruscus bushes or application of this compound or of CaCl₂ or CaSO₄ to mature cut branches standing in water, resulted also in a significant reduction in postharvest phylloclade rot. Application of CaSO₄ and Ca(H2PO₄)₂ + CaSO₄ to ruscus plants, from fertilizers containing 22% Ca and releasing the cation gradually, effectively reduced the susceptibility of cut branches to infection by_{B. cinerea.}     

A foliar spray of micronutrient solutions induces local and systemic protection against powdery mildew (Sphaerotheca fuliginia) in cucumber plants

A single spray of solutions of 0.005M H₃BO₃, 0.0025M CuSO₄, and 0.0025 MnCl₂, on the upper surface of the first true leaf of cucumber plants 2 h before inoculation with a conidial suspension of Sphaerotheca fuliginea, induced systemic protection against powdery mildew in leaves 2 and 3 without causing any damage on the induced leaf (first leaf). A similar level of systemic protection was observed when plants were induced by micronutrients, 2, 24 and 72 h before challenge with S. fuliginea. The level of protection induced by various concentrations varied from solution to solution. In general, the systemic protection induced by K₂HPO₄ was similar to that by the microelements. Spraying of a 1:1 mixture of phosphate and micronutrient solutions did not improve the systemic protection over that obtained with each of the solutions alone. Increasing the inoculum concentration of S. fuliginea increased the number of powdery mildew colonies produced on both induced and non-induced plants and has relatively affected the systemic protection on induced plants. A single foliar spray of micronutrient solutions, as a prophylactic treatment, on the upper surface of all the leaves of 3-leaf stage cucumber plants significantly inhibited powdery mildew development. A single spray of MnCl₂ on leaf 1 elevated peroxidase activity in the soluble fraction and caused an enhancement of -1,3-glucanase content in the ionically bound fractions of leaf 2 of non-inoculated plants. Forty-eight hours after inoculation, the level of both fractions of the enzymes increased in non-treated plants and decreased (-1,3-glucanase) or remained unchanged (peroxidase) in treated (induced) plants as compared to non-treated plants. The possible mechanism for this protection, and the use of microelements and phosphate solutions as inducers for systemic protection and as agents for disease control are discussed.     

Stage specificity of catalase isoform activity in Exserohilum turcicum

Exserohilum turcicum is the fungal agent that causes northern leaf blight disease in maize. Spores of E. turcicum can germinate in water containing up to 15 m M hydrogen peroxide. Initially the catalase isoform activities from E. turcicum cultured on artificial medium were analysed by polyacrylamide gel electrophoresis. Seven different catalase isoform activities were detected during a 72-hour time course from spore germination to young hyphal formation. During the time period studied the activity levels of each isoform varied independently. In two-week-oldE. turcicum mycelia, only the activities of catalase isoforms #2 and #3 were detected. During a compatible interaction on maize leaves, plant catalases were suppressed with E. turcicum isoform #3 being detected from 72 h after inoculation onwards. E. turcicum #2 was the predominant isoform detected in necrotic lesions. Salicylic acid was not found to effect fungal or maize catalase activities. E. turcicum isoform #3 activity was found to be strongly induced by hydrogen peroxide and by the herbicide 3-amino-1,2,4-triazole (AT), while the activity of all other isoforms was suppressed by AT.     

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.     

The effect of nitrogen on the growth and development of giant witchweed,Striga hermonthica Benth.: effect on cultured germinated seedlings in host absence

For the first time in sterile nutrient media in the absence of the host plant, different forms and rates of nitrogen compounds were screened for their effect onS. hermonthica Benth. shoot development beyond seed germination. There was no shoot formation beyond the inoculation stages whenS. hermonthica germlings were grown in media devoid of nitrogen source. In culture media containing some nitrogen sources, healthy shoots were formed. Increasing concentrations of KNO₃, NaNO₃, Ca(NO₃)₂, Mg(NO₃)₂ and asparagine resulted in a significant increase inS. hermonthica shoot development. Inversely, increasing concentrations of (NH₄)₂SO₄, NH₄H₂PO₄, NH₄Cl and urea led to increasing significant reduction ofS. hermonthica shoot development. The amino acids, glycine and asparagine supported reduced shoot development ofS. hermonthica, indicating that organic nitrogen cannot replace inorganic nitrogen forStriga growth. The ammonium nitrogen compounds, (NH₄)₂SO₄ and NH₄H₂PO₄, suppressed further shoot elongation and total dry weight of 20 and 40 days oldStriga hermonthica plants, in sterile culture media. The organic compounds urea, allylthiourea and thiourea had an effect similar to ammonium compounds. Arginine and glycine on the other hand did not suppress the further development of the parasite. The suppressive effect of nitrogen however, was greater when the parasite was 20 days old than when it was 40 days old. This work provides data to show that some nitrogen compounds reduce the severity ofS. hermonthica attack by direct suppression ofStriga growth and development at the post-germination stage and after shoots have been formed.     

Rhizobacteria-mediated Induced Systemic Resistance: Triggering, Signalling and Expression

Selected strains of rhizosphere bacteria reduce disease by activating a resistance mechanism in the plant named rhizobacteria-mediated induced systemic resistance (ISR). Rhizobacteria-mediated ISR resembles pathogen-induced systemic acquired resistance (SAR) in that both types of induced resistance render uninfected plant parts more resistant towards a broad spectrum of plant pathogens. Some rhizobacteria trigger the salicylic acid (SA)-dependent SAR pathway by producing SA at the root surface. In other cases, rhizobacteria trigger a different signalling pathway that does not require SA. The existence of a SA-independent ISR pathway has been demonstrated in Arabidopsis thaliana. In contrast to pathogen-induced SAR, ISR induced by Pseudomonas fluorescens WCS417r is independent of SA accumulation and pathogenesis-related (PR) gene activation but, instead, requires responsiveness to the plant hormones jasmonic acid (JA) and ethylene. Mutant analyses showed that ISR follows a novel signalling pathway in which components from the JA and ethylene response are successively engaged to trigger a defensive state that, like SAR, is controlled by the regulatory factor NPR1. Interestingly, simultaneous activation of both the JA/ethylene-dependent ISR pathway and the SA-dependent SAR pathway results in an enhanced level of protection. Thus combining both types of induced resistance provides an attractive tool for the improvement of disease control. This review focuses on the current status of our research on triggering, signalling, and expression of rhizobacteria-mediated ISR in Arabidopsis.     

The overwintering ofExserohilum turcicum in Israel

Exserohilum turcicum (Pass.) Leonard and Suggs, the causal agent of northern leaf blight of corn, overwinters onSorghum halepense L. plants and on corn debris (dead leaves). Spqrulating lesions ofE. turcicum were observed on sorghum plants in the winter (February). Spores from these lesions were pathogenic to susceptible sweet corn plants cv. ‘Jubilee’. Infected sporulating leaves of corn were stored for 1 year at 20°C (40-60% relative humidity), at 5°C (60% relative humidity), or buried 5 cm underground. During the storage period, 32% and 22% of the spores formed chlamydospores, at 20° and 5°C, respectively. Leaves buried 5 cm underground were totally decomposed after 6 months. After 4 months, 25% of the spores in the buried leaves had formed chlamydospores. Spores with chlamydospores were pathogenic to corn plants. The viability of spores without chlamydospores stored at 20°, 5°C or buried underground was 0, 60 and 0%, respectively. In a parallel experiment infected leaves were stored under similar conditions and allowed to sporulate. No sporulation occurred on infected leaves buried in soil. Spores produced on infected leaves stored at 20° and 5°C were highly pathogenic to corn plants. In leaves treated with 0.1N glucose, chlamy dospore formation was significantly inhibited.     

Identification of an ISR‐related metabolite produced by rhizobacterium Klebsiella oxytoca C1036 active against soft‐rot disease pathogen in tobacco

BACKGROUND: Klebsiella oxytoca C1036 (C1036) causes induced systemic resistance (ISR) activity against the soft‐rot pathogen Pectobacterium carotovorum subsp. carotovorum SCC1 (SCC1). However, microbial metabolites from C1036 involved in ISR activity remain unknown. The present study was performed to identify an ISR‐related metabolite produced by C1036. RESULTS: The supernatants of C1036 cultures grown on Luria‐Bertani medium were subjected to solvent extraction, repeated column chromatography and preparative liquid chromatography for isolation of an ISR‐related metabolite. High‐resolution mass spectrometer analysis of the isolated metabolite indicated a C₉H₁₅O₃N compound with a mass of 185.11. Low‐resolution mass spectrometer analysis of the metabolite showed a molecular ion peak at 185 and its fragment ions at 84 and 56. Nuclear magnetic resonance spectrometer analyses characterised all protons and carbons of the isolated metabolite. Based on the data, the isolated metabolite was determined to be butyl 2‐pyrrolidone‐5‐carboxylate (BPC). BPC at 12 mM significantly suppressed the disease symptoms in ISR bioassays against SCC1. CONCLUSION: This is the first report identifying BPC as an ISR‐related metabolite produced by C1036. C1036 may play a role in promoting plant growth because it produces ISR‐related metabolites against the plant pathogen SCC1. Copyright © 2009 Society of Chemical Industry     

Maize resistance to northern corn leaf blight is potentiated by nickel

Northern corn leaf blight (NCLB), caused by Exserohilum turcicum, is one of the most devastating diseases affecting maize yield worldwide. A foliar spray of nickel (Ni) to potentiate maize resistance against NCLB was investigated by examining alterations in the photosynthetic apparatus (leaf gas exchange and chlorophyll [Chl] a fluorescence parameters), production of ethylene and reactive oxygen species as well as activities of defence and antioxidant enzymes. Mycelial growth of E. turcicum was inhibited by Ni in vitro. Inoculated plants sprayed with Ni exhibited higher foliar Ni concentration, reduced NCLB symptoms, and lower concentrations of malondialdehyde and hydrogen peroxide. In inoculated leaves of plants not sprayed with Ni, concentrations of Chl a, Chl b, and carotenoids were lower and the photosynthetic apparatus was impaired at the biochemical level due to high NCLB severity. The activities of antioxidant enzymes were not affected by Ni, except an increase in glutathione reductase activity for noninoculated plants sprayed with Ni. High lipoxygenase and polyphenoloxidase activities, lower ethylene production, as well as elevated production of phenolics and lignin helped decrease NCLB severity in the leaves of Ni-sprayed plants.     

Induced resistance to<Emphasis Type="Italic">Cladosporium cucumerinum</Emphasis> in cucumber by pectinases extracted from<Emphasis Type="Italic">Penicillium oxalicum</Emphasis>

Pectinases extract (PE) from the fermentation product ofPenicillium oxalicum BZH-2002 was tested for its ability to induce protection against scab caused byCladosporium cucumerinum on cucumber (Cucumis sativus L.) plants. Seedlings with one true leaf were sprayed with various concentrations of PE (10–200 units ml⁻¹) 3 days before inoculation withC. cucumerinum. Results showed that the induced local protection against the pathogen was dose-dependent when the concentrations of PE were between 20 and 120 units ml⁻¹; systemically induced resistance against the pathogen was not observed. Boiled PE had a slight effect on disease reduction. Commercial pectinases prepared fromAspergillus niger showed lower protection against scab compared with PE when they were used at the same concentration of enzyme activity. No inhibitory activity was observed on conidial germination or germ-tube growth ofC. cucumerinum. PE was further evaluated for its enhancement of defense-related enzymes. Peroxidase (PO), polyphenol oxidase (PPO) and phenylalanine ammonia lyase (PAL) increased in cucumber seedlings after treatment with PE. PO and PPO remained at a higher level in PE-pretreated seedlings throughout the experiment period whether pathogen-inoculated or non-inoculated, whereas PAL activity began to decrease 2 days after PE treatment. http://www.phytoparasitica.org posting July 14, 2004.