Biocontrol activity and root colonization by Pseudomonas corrugata strains CCR04 and CCR80 against Phytophthora blight of pepper

Previously, we selected Pseudomonas corrugata strains CCR04 and CCR80 as rhizobacteria suppressive to Phytophthora blight of pepper caused by Phytophthora capsici. In this study, we investigated soil microbial activity in pepper plants root-drenched with strains CCR04 and CCR80 in relation to their biocontrol activity, root colonization by using bacterial population counts and scanning electron microscopy, biofilm formation and cell motility as well as cell sensitivity to hydrogen peroxide (H₂O₂). As a result, strains CCR04 and CCR80 more effectively suppressed disease expression in pepper plants through root colonization than did Paenibacillus polymyxa AC-1 (positive control), Escherichia coli DH5α (negative control) or MgSO₄ solution (untreated control). Strains CCR04 and CCR80 had efficient biofilm formation and cell motility (swimming and swarming activities) abilities and responded to certain tested compounds (amino acids, organic acids and sugars), which can be found in root exudates. Strains CCR04 and CCR80 and the positive control strain AC-1 were relatively insensitive to H₂O₂, a reactive oxidative species at concentration up to 20 mM, unlike the negative control strain DH5α. Taken together, these results suggest that P. corrugata CCR04 and CCR80 can effectively inhibit P. capsici infection of pepper plants through successful colonization of plant roots. This bacterial colonization may be facilitated by the biofilm formation ability and cell motility in addition to reduced sensitivity to H₂O₂ and probably the production of antimicrobial compounds. These findings highlight the potential of strains CCR04 and CCR80 as biocontrol agents for the management of Phytophthora blight of pepper.     

The volatile-producing Flavobacterium johnsoniae strain GSE09 shows biocontrol activity against Phytophthora capsici in pepper

Aims: Previously, we selected a bacterial strain (GSE09) antagonistic to Phytophthora capsici on pepper, which produced a volatile compound (2,4‐di‐tert‐butylphenol), inhibiting the pathogen. In this study, we identified strain GSE09 and characterized some of the biological traits of this strain in relation to its antagonistic properties against P. capsici. In addition, we examined bacterial colonization on the root surface or in rhizosphere soil and the effect of various concentrations of the volatile compound and strain GSE09 on pathogen development and radicle infection as well as radicle growth. Methods and Results: Strain GSE09 was identified as Flavobacterium johnsoniae, which forms biofilms and produces indolic compounds and biosurfactant but not hydrogen cyanide (HCN) with little or low levels of antifungal activity and swimming and swarming activities. Fl. johnsoniae GSE09 effectively colonized on pepper root, rhizosphere, and bulk (pot) soil, which reduced the pathogen colonization in the roots and disease severity in the plants. Various concentrations of 2,4‐di‐tert‐butylphenol or strain GSE09 inhibited pathogen development (mycelial growth, sporulation, and zoospore germination) in I‐plate (a plastic plate containing a center partition). In addition, germinated seeds treated with the compound (1–100 μg ml^?1) or the strain (10²–10¹⁰ cells ml^?1) significantly reduced radicle infection by P. capsici without radicle growth inhibition. Conclusions: These results indicate that colonization of pepper root and rhizosphere by the Fl. johnsoniae strain GSE09, which can form biofilms and produce indolic compounds, biosurfactant, and 2,4‐di‐tert‐butylphenol, might provide effective biocontrol activity against P. capsici. Significance and Impact of the Study: To our knowledge, this is the first study demonstrating that the Fl. johnsoniae strain GSE09, as a potential biocontrol agent, can effectively protect pepper plants against P. capsici infection by colonizing the roots.     

Functional, genetic and chemical characterization of biosurfactants produced by plant growth-promoting Pseudomonas putida 267

Aims: Plant growth‐promoting Pseudomonas putida strain 267, originally isolated from the rhizosphere of black pepper, produces biosurfactants that cause lysis of zoospores of the oomycete pathogen Phytophthora capsici. The biosurfactants were characterized, the biosynthesis gene(s) partially identified, and their role in control of Phytophthora damping‐off of cucumber evaluated. Methods and Results: The biosurfactants were shown to lyse zoospores of Phy. capsici and inhibit growth of the fungal pathogens Botrytis cinerea and Rhizoctonia solani. In vitro assays further showed that the biosurfactants of strain 267 are essential in swarming motility and biofilm formation. In spite of the zoosporicidal activity, the biosurfactants did not play a significant role in control of Phytophthora damping‐off of cucumber, since both wild type strain 267 and its biosurfactant‐deficient mutant were equally effective, and addition of the biosurfactants did not provide control. Genetic characterization revealed that surfactant biosynthesis in strain 267 is governed by homologues of PsoA and PsoB, two nonribosomal peptide synthetases involved in production of the cyclic lipopeptides (CLPs) putisolvin I and II. The structural relatedness of the biosurfactants of strain 267 to putisolvins I and II was supported by LC‐MS and MS‐MS analyses. Conclusions: The biosurfactants produced by Ps. putida 267 were identified as putisolvin‐like CLPs; they are essential in swarming motility and biofilm formation, and have zoosporicidal and antifungal activities. Strain 267 provides excellent biocontrol activity against Phytophthora damping‐off of cucumber, but the lipopeptide surfactants are not involved in disease suppression. Significance and Impact of the Study: Pseudomonas putida 267 suppresses Phy. capsici damping‐off of cucumber and provides a potential supplementary strategy to control this economically important oomycete pathogen. The putisolvin‐like biosurfactants exhibit zoosporicidal and antifungal activities, yet they do not contribute to biocontrol of Phy. capsici and colonization of cucumber roots by Ps. putida 267. These results suggest that Ps. putida 267 employs other, yet uncharacterized, mechanisms to suppress Phy. capsici.     

Inhibitory effects of xenocoumacin 1 on the different stages of <Emphasis Type="Italic">Phytophthora capsici</Emphasis> and its control effect on Phytophthora blight of pepper

Phytophthora species cause enormous economic loss every year worldwide. Xenocoumacin 1 (Xcn1), isolated from the bacterium Xenorhabdus nematophilus, is a broad-spectrum antibiotic against agricultural pathogens, especially Phytophthora. To understand the inhibitory mode of Xcn1 toward Phytophthora pathogens, we determined the inhibitory effects of Xcn1 on Phytophthora capsici both in vitro and in vivo. In vitro, Xcn1 inhibited different stages in the life cycle of P. capsici, including sporangium formation, zoospore germination, and mycelial growth, with 50% effective concentration (EC₅₀) values of 0.037, 0.81, and 2.44 μg ml^?1, respectively. Xcn1 also reduced zoospore motility. In vivo, Xcn1 efficiently controlled the Phytophthora blight of pepper with a disease reduction of 99% at a concentration of 5 μg ml^?1 assessed on the third day after incubation of wound stem plants. In addition, Xcn1-treated P. capsici mycelia exhibited increased mycelial branch spacing, evident plasmolysis, and leakage of intracellular components. In conclusion, in the presence of Xcn1, several stages in the life cycle of P. capsici were inhibited, and the hyphae exhibited obvious morphological changes.     

Effect of oxathiapiprolin on asexual life stages of Phytophthora capsici and disease development on vegetables

Phytophthora blight caused by Phytophthora capsici is a serious disease in the production of peppers and other vegetables worldwide. Application of fungicides is an important component in developing effective disease management programmes. However, resistance in P. capsici populations to some commonly used fungicides has been documented. Identification of effective new fungicides with different mode of actions is highly desirable. This study was conducted to determine baseline sensitivity of P. capsici isolates to oxathiapiprolin, the first member of a new class of isoxazoline fungicides, and efficacy of this compound for reduction of Phytophthora blight on bell pepper. A collection of 126 P. capsici isolates were evaluated and all the isolates were sensitive to oxathiapiprolin. EC₅₀ values of oxathiapiprolin in inhibiting mycelial growth, sporangium formation and zoospore germination of 25 selected isolates averaged 0.001, 0.0003 and 0.54 µg mL^?1, respectively. It appeared that asexual life stages of P. capsici were more sensitive to oxathiapiprolin than other compounds used for control of oomycete pathogens. In field studies, oxathiapiprolin applied at different rates through drip irrigation tubes, or by soil drench plus foliar sprays, reduced Phytophthora blight and increased pepper yield significantly. This is the first report of the efficacy of oxathiapiprolin in suppression of P. capsici, which indicates that oxathiapiprolin is effective in inhibiting the pathogen and has the promise to be a viable option for managing Phytophthora blight in bell pepper production.     

Growth promotion and root colonisation in pepper plants by phosphate‐solubilising Chryseobacterium sp. strain ISE14 that suppresses Phytophthora blight

Previously, we selected bacterial strain ISE14 through a sequential selection procedure that included radicle, seedling, and in planta assays and field tests. This strain not only suppressed a destructive soilborne disease, Phytophthora blight, caused by Phytophthora capsici but also increased fruit yields of pepper plants in the fields. This study was conducted to identify strain ISE14 by 16S rRNA gene sequence analysis and to characterise biocontrol and plant growth promotion activities of the strain in pepper plants. Strain ISE14, identified as Chryseobacterium sp., significantly reduced disease severity in plants inoculated with Ph. capsici and promoted plant growth (lengths and dry weights of shoots and roots) compared with those in plants treated with Escherichia coli DH5α (negative control) or MgSO₄ solution (untreated control). This strain effectively colonised pepper plant roots as assessed by bacterial population analysis and confocal laser scanning microscopy; it enhanced soil microbial activity and biofilm formation, but not the production of indole acetic acid. Strain ISE14 also solubilised organic or inorganic phosphate by production of acid and alkaline phosphatases or reduction in pH, resulting in enhanced pepper plant growth. This strain exhibited similar or greater activity in disease control and plant growth promotion tests compared with positive control strains Paenibacillus polymyxa AC‐1 (biocontrol) and Bacillus vallismortis EXTN‐1 (plant growth). Therefore, Chryseobacterium sp. ISE14 may be a phosphate‐solubilising and plant growth‐promoting rhizobacterium (PGPR) strain that suppresses Phytophthora blight of pepper. To our knowledge, this is the first report of a phosphate‐solubilising PGPR strain of Chryseobacterium sp. that suppresses the pepper disease.     

Diversity and activity of biosurfactant-producing Pseudomonas in the rhizosphere of black pepper in Vietnam

Aims: Phytophthora capsici is a major pathogen of black pepper and zoospores play an important role in the infection process. Fluorescent pseudomonads that produce biosurfactants with zoosporicidal activities were isolated from the black pepper rhizosphere in Vietnam, and their genotypic diversity and potential to control Phy. capsici root rot was determined. Methods: Biosurfactant‐producing pseudomonads were genotypically and biochemically characterized by BOX‐polymerase chain reaction (PCR), 16S‐rDNA sequencing, reverse‐phase‐high‐performance liquid chromatography and liquid chromatography‐masss spectrometry analyses. Results: Biosurfactant‐producing fluorescent pseudomonads make up c. 1.3% of the culturable Pseudomonas population in the rhizosphere of black pepper. Although BOX‐PCR revealed substantial genotypic diversity, the isolates were shown to produce the same biosurfactants and were all identified as Pseudomonas putida. When applied to black pepper stem cuttings, several of the biosurfactant‐producing strains provided significant disease control. In absence of the disease, several of the bacterial strains promoted shoot and root growth of black pepper stem cuttings. Conclusions: Biosurfactant‐producing pseudomonads indigenous to the rhizosphere of black pepper plants are genotypically diverse and provide a novel resource for the control of Phy. capsici root rot and growth promotion of black pepper stem cuttings. Significance and Impact of the Study: The results of this study provide a strong basis for further development of supplementary strategies with antagonistic bacteria to control foot and root rot of black pepper and to promote plant growth.     

Plant-Plant-Microbe Mechanisms Involved in Soil-Borne Disease Suppression on a Maize and Pepper Intercropping System

Background

Intercropping systems could increase crop diversity and avoid vulnerability to biotic stresses. Most studies have shown that intercropping can provide relief to crops against wind-dispersed pathogens. However, there was limited data on how the practice of intercropping help crops against soil-borne Phytophthora disease.

Principal Findings

Compared to pepper monoculture, a large scale intercropping study of maize grown between pepper rows reduced disease levels of the soil-borne pepper Phytophthora blight. These reduced disease levels of Phytophthora in the intercropping system were correlated with the ability of maize plants to form a “root wall” that restricted the movement of Phytophthora capsici across rows. Experimentally, it was found that maize roots attracted the zoospores of P. capsici and then inhibited their growth. When maize plants were grown in close proximity to each other, the roots produced and secreted larger quantities of 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one (DIMBOA) and 6-methoxy-2-benzoxazolinone (MBOA). Furthermore, MBOA, benzothiazole (BZO), and 2-(methylthio)-benzothiazole (MBZO) were identified in root exudates of maize and showed antimicrobial activity against P. capsici.

Conclusions

Maize could form a “root wall” to restrict the spread of P. capsici across rows in maize and pepper intercropping systems. Antimicrobe compounds secreted by maize root were one of the factors that resulted in the inhibition of P. capsici. These results provide new insights into plant-plant-microbe mechanisms involved in intercropping systems.     

Pre-plant bacterisation: a strategy for delivery of beneficial endophytic bacteria and production of disease-free plantlets of black pepper (Pipernigrum L.)

Naturally occurring endophytic bacteria from black pepper vines were found to exhibit strong antagonistic activities against Phytophthora capsici and Radopholus similis. In order to deliver these bacterial strains, as well as to produce disease-free plantlets of black pepper, a pre-plant stem and root bacterisation was standardised. Stem bacterisation with endophytic Pseudomonas spp. was found to suppress P. capsici infection (over 90% reduction in lesion length) on cut shoots. Pre-plant root bacterisation with Pseudomonas aeruginosa, Pseudomonas putida and Bacillus megaterium yielded over 60% of plantlets free from P. capsici infection on roots. Curtobacterium luteum and B. megaterium recorded over 70% reduction of nematode population in soil with concomitant production of over 65% of nematode-free plantlets. Besides protecting the plants from the pathogens, the bacteria were also found to enhance the growth of rooted cuttings. The biocontrol potential of the above endophytic bacteria and their exploitation for disease management in the black pepper nursery are discussed.     

Behaviour of Phytophthora cinnamomi Zoospores on Roots of Four Avocado Cultivars

Scanning electron microscopy was used to study the behaviour of Phytophthora cinnamomi zoospores on the roots of three tolerant avocado cultivars. Duke 7, G6 and Martin Grande, and a susceptible Edranol cultivar. Zoospores were attracted to the region of cell elongation and encysted on the roots of all cultivars studied. Adhesion of the zoospores appeared to be aided by root slime. Cysts usually produced one germ tube which penetrated the root directly, or formed an appressorium-like swelling before penetration occurred. Extensive growth of germ tubes occurred where zoospores germinated some distance behind the region of elongation. Cysts germinating behind this region often formed branched germ tubes and more than one appressorium-like swelling. There were no clear differences in the type of pre-penetration structures, formed by zoospore cysts, on the roots of the different avocado cultivars.     

Plant growth promoting rhizhobacteria (PGPR)-mediated root proliferation in black pepper (Piper nigrum L.) as evidenced through GS Root software

Abstract

Pseudomonas fluorescens strains which are proven biocontrol agents in black pepper against foot rot (caused by Phytophthora capsici ) were also found to enhance root proliferation and fibre root production. Experiments conducted in the greenhouse with five efficient strains of P. fluorescens (IISR-6, IISR-8, IISR-11, IISR-13 and IISR-51) showed that the bacterial strains could significantly increase the root biomass of the plants (30 – 135%). Parameters for total root length, root area and root tips were estimated by scanning the entire root system and analysis through GS Root® software (PP systems, Winterstreet, USA). All the strains increased the root length in the treated plants (12 – 127%), the highest being with IISR-6, which was on a par with IISR-11 and IISR-51. A similar trend was observed with the total root area after bacterization (43 – 200%). The P. fluorescens treated plants had a higher number of feeder roots as evidenced by the increased number of root tips (82 – 137%). The enhanced growth parameters upon root bacterization could be corroborated with the production of the plant growth hormones IAA & GA by the bacterial strains and their P-solubilization potential.     

Treatment of Paenibacillus illinoisensis suppresses the activities of antioxidative enzymes in pepper roots caused by Phytophthora capsici infection

Summary The activity of antioxidative enzymes after inoculation of pepper (Capsicum annuum L. Chungok) with a pathogen, Phytophthora capsici (P), the causal agent of Phytophtora blight and dual inoculation of pathogen and an antagonist, Paenibacillus illinoisensis KJA-424 (P+A), were measured and compared with that of non-inoculated (C) roots. Root mortality was significantly reduced by about 84% in P+A treatment compared with P treatment alone. When compared to the non-inoculated (C) roots, malondialdehyde (MDA) concentration gradually decreased by 52.4% in 7 days only in P-treated roots and hydrogen peroxide (H₂O₂) was not significantly affected by the treatment for 5 days but significantly decreased in the P+A-treated roots at day 7. P-treatment continuously induced peroxidase (POD) and superoxide dismutase (SOD), resulting in significant increases of 36.7% and 27.7% at day 7, respectively, compared to the control. In P+A-treated roots, the activities of POD and SOD also increased for 5 days but returned to the control level at day 7. Catalase activity fluctuated but again increased over the 7-day period following P+A inoculation. These results indicate that an antagonist P. illinoisensis KJA-424 alleviated root mortality and suppressed the elevated activities of POD and SOD in the root of pepper plant root caused by P.␣capsici infection.     

Biological Control of Oomycete Soilborne Diseases Caused by Phytophthora capsici,Phytophthora infestans,and Phytophthora nicotianae in Solanaceous Crops

Oomycete pathogens that belong to the genus Phytophthora cause devastating diseases in solanaceous crops such as pepper, potato, and tobacco, resulting in crop production losses worldwide. Although the application of fungicides efficiently controls these diseases, it has been shown to trigger negative side effects such as environmental pollution, phytotoxicity, and fungicide resistance in plant pathogens. Therefore, biological control of Phytophthora-induced diseases was proposed as an environmentally sound alternative to conventional chemical control. In this review, progress on biological control of the soilborne oomycete plant pathogens, Phytophthora capsici, Phytophthora infestans, and Phytophthora nicotianae, infecting pepper, potato, and tobacco is described. Bacterial (e.g., Acinetobacter, Bacillus, Chryseobacterium, Paenibacillus, Pseudomonas, and Streptomyces) and fungal (e.g., Trichoderma and arbuscular mycorrhizal fungi) agents, and yeasts (e.g., Aureobasidium, Curvibasidium, and Metschnikowia) have been reported as successful biocontrol agents of Phytophthora pathogens. These microorganisms antagonize Phytophthora spp. via antimicrobial compounds with inhibitory activities against mycelial growth, sporulation, and zoospore germination. They also trigger plant immunity-inducing systemic resistance via several pathways, resulting in enhanced defense responses in their hosts. Along with plant protection, some of the microorganisms promote plant growth, thereby enhancing their beneficial relations with host plants. Although the beneficial effects of the biocontrol microorganisms are acceptable, single applications of antagonistic microorganisms tend to lack consistent efficacy compared with chemical analogues. Therefore, strategies to improve the biocontrol performance of these prominent antagonists are also discussed in this review.     

Inhibitory Effect of Paenibacillus polymyxa GBR-462 on Phytophthora capsici Causing Phytophthora Blight in Chili Pepper

In the present work 25 strains of Paenibacillus polymyxa isolated from rotted ginseng roots were screened for their antimicrobial activity against Phytophthora capsici in vitro . Based on antimicrobial activity, 15 strains categorized as strongly antimicrobial, among them GBR-462 was found as the most active, and five strains each as weekly antimicrobial and no antimicrobial. Antimicrobial activity was influenced by the initial inoculum density, as strains of P. polymyxa with a strong antimicrobial activity (including P. polymyxa GBR-462) showed the antimicrobial activity against P. capsici and could form biofilm only when they were applied at the higher initial inoculums, 10⁸ cfu/ml. No inhibitory effect was noted on the mycelial growth and zoospore germination of the pathogen when applied at the lower inoculum density of 10⁶ cfu/ml of P. polymyxa GBR-462. However, sporangium formation and zoospore release was significantly inhibited at the lower inoculum density. Also light and electron microscopy revealed the structures of sporangia aberrant with no or few healthy nuclei, indicating sporangium and zoospore formation inhibited at the lower inoculum density. Application of P. polymyxa GBR-462 into potted soil suppressed disease progression as well as disease severity; disease severity was reduced by 30% as compared to untreated pots, suggesting P. polymyxa GBR-462 could be a potential biocontrol agent against Phytopthora capsici .     

Resistance to the Novel Fungicide Pyrimorph in Phytophthora capsici: Risk Assessment and Detection of Point Mutations in CesA3 That Confer Resistance

Pyrimorph is a novel fungicide with high activity against the plant pathogen Phytophthora capsici. We investigated the risk that P. capsici can develop resistance to pyrimorph. The baseline sensitivities of 226 P. capsici isolates, tested by mycelial growth inhibition, showed a unimodal distribution with a mean EC₅₀ value of 1.4261 (±0.4002) µg/ml. Twelve pyrimorph-resistant mutants were obtained by repeated exposure to pyrimorph in vitro with a frequency of approximately 1×10⁻⁴. The resistance factors of the mutants ranged from 10.67 to 56.02. Pyrimorph resistance of the mutants was stable after 10 transfers on pyrimorph-free medium. Fitness in sporulation, cystospore germination, and pathogenicity in the pyrimorph-resistant mutants was similar to or less than that in the parental wild-type isolates. On detached pepper leaves and pepper plants treated with the recommended maximum dose of pyrimorph, however, virulence was greater for mutants with a high level of pyrimorph resistance than for the wild type. The results suggest that the risk of P. capsici developing resistance to pyrimorph is low to moderate. Among mutants with a high level of pyrimorph resistance, EC₅₀ values for pyrimorph and CAA fungicides flumorph, dimethomorph, and mandipropamid were positively correlated. This indicated that point mutations in cellulose synthase 3 (CesA3) may confer resistance to pyrimorph. Comparison of CesA3 in isolates with a high level of pyrimorph resistance and parental isolates showed that an amino acid change from glutamine to lysine at position 1077 resulted in stable, high resistance in the mutants. Based on the point mutations, an allele-specific PCR method was developed to detect pyrimorph resistance in P. capsici populations.     

Inoculation of Paenibacillus illinoisensis alleviates root mortality, activates of lignification-related enzymes, and induction of the isozymes in pepper plants infected by Phytophthora capsici

To investigate the variations of the enzymes responsible for lignification, after inoculation with Phytophthora capsici and/or Paenibacillus illinoisensis KJA-424, in relation to biocontrol of Phytophthora blight in pepper, roots of two-month-old plants were inoculated with P. capsici inoculation (P), and co-inoculation of P. capsici and P. illinoisensis cell cultures (P + A). Root mortality of pepper plants induced by inoculation of P. capsici was completely recovered by co-inoculation with antagonistic KJA-424. At day 7, peroxidase (POD) activity increased by 36.7% in P-treated roots but by 7.1% only in P + A-treated, compared with control. Polyphenol oxidase (PPO) activity increased for 3 days and then drastically decreased in P-treated roots but maintained a constant level in control and P + A-treated. At day 7, PPO activity in P-treated leaves decreased but recovered to the level of control in the P + A-treated. Three major POD isozymes (45, 53, and 114 kDa) were shown in P-treated roots, while two major (53 and 114 kDa) in control and P + A-treated, suggesting that the 45 kDa of POD was actively induced in P-treated roots but not induced in P + A-treated roots. A PPO isozyme of 80 kDa was induced in P-treated roots but not induced by co-treated with KJA-424. In leaves, the POD isozyme of 45 kDa appears to be systemically induced in P-treated only. The PPO isozyme of 80 kDa in leaves was not induced by pathogen challenge but recovered by co-inoculated with P. illinoisensis. All these results suggest that the inoculation of an antagonist, P. illinoisensis alleviates root mortality, activates of lignification-related enzymes and induction of the isozymes in pepper plants infected by P. capsici.     

Activity of Ten Fungicides against Phytophthora capsici Isolates Resistant to Metalaxyl

Pepper Phytophthora blight (PPB), caused by Phytophthora capsici, is an important disease of pepper in China. The extensive application of metalaxyl has resulted in widespread resistance to this fungicide in field. This study has evaluated the activities of several fungicides against the mycelial growth and sporangium germination of metalaxyl‐sensitive and metalaxyl‐resistant P. capsici isolates by determination of EC₅₀ values. The results showed that the novel carboxylic acid amide (CAA) fungicide mandipropamid exhibited excellent inhibitory activity against PPB both in vitro and in vivo, with averagely EC₅₀ values of 0.075 and 0.004 μg/ml in mycelial growth and sporangium germination, respectively, and over 88% efficacy in controlling PPB. The other three CAA fungicides also provided over 70% efficacy in controlling PPB. The mycelial growth was less sensitive to quinone outside inhibitor (QoI) fungicides azoxystrobin and trifloxystrobin than that of sporangium germination in P. capsici isolates. However, azoxystrobin and trifloxystrobin provided over 80% efficacy in controlling PPB. It was noted that propamocarb and cymoxanil did not exhibit activity against the mycelial growth or sporangium germination of P. capsici isolates in the in vitro tests, with over 70% efficacy in controlling PPB. The new fungicide mixture 62.5 g/l fluopicolide + 625 g/l propamocarb (trade name infinito, 687.5 g/l suspension concentrate (SC)) produced over 88% efficacy in controlling PPB caused by both metalaxyl‐sensitive and metalaxyl‐resistant isolates. The data of this study also proved that there was obviously no cross‐resistance between metalaxyl and the other tested fungicides. Therefore, these fungicides should be good alternatives to metalaxyl for the control of PPB and management of metalaxyl resistance.     

Some fungal endophytes from vegetable crops and their anti-oomycete activities against tomato late blight

AIMS: To isolate endophytic fungi from vegetable plants and examine their in vivo anti-oomycete activity against Phytophthora infestans in tomato plants. METHODS AND RESULTS: Endophytic fungi were isolated from surface-sterilized plant tissues and anti-oomycete activity was measured by in vivo assay using tomato seedlings. Endophytic fungi showing potent anti-oomycete activity were identified by morphological characteristics and nuclear ribosomal ITS1-5.8S-ITS2 sequence analysis. A total of 152 isolates were obtained from 66 healthy tissue samples of cucumber, red pepper, tomato, pumpkin and Chinese cabbage and the fermentation broths of 23 isolates showed potent in vivo anti-oomycete activity against tomato late blight with control values over 90%. The Fusarium oxysporum strain EF119, which was isolated from roots of red pepper, showed the most potent disease control efficacy against tomato late blight. In dual-culture tests, it inhibited the growth of Pythium ultimum, P. infestans and Phytophthora capsici. CONCLUSIONS: Among endophytic fungi isolated from healthy tissues of vegetable plants, F. oxysporum EF119 showed the most potent in vivo anti-oomycete activity against tomato late blight and in vitro anti-oomycete activity against several oomycete pathogens. SIGNIFICANCE AND IMPACT OF THE STUDY: Endophytic fungi showing anti-oomycete activity in vitro and in vivo may be used as biocontrol agents particularly of tomato late blight.     

Control of Late Blight (Phytophthora capsici ) in Pepper Plant with a Compost Containing Multitude of Chitinase-producing Bacteria

Compost sustaining a multitude of chitinase-producing bacteria was evaluated in a greenhouse study as a soil amendment for the control of late blight (Phytophthora capsici L.) in pepper (Capsicum annuum L.). Microbial population and exogenous enzyme activity were measured in the rhizosphere and correlated to the growth and health of pepper plant. Rice straw was composted with and without a chitin source, after having been inoculated with an aliquot of coastal area soil containing a known titer of chitinase-producing bacteria. P. capsici inoculated plants cultivated in chitin compost-amended soil exhibited significantly higher root and shoot weights and lower root mortality than plants grown in pathogen-inoculated control compost. Chitinase and β-1,3-glucanase activities in rhizosphere of plants grown in chitin compost-amended soil were twice that seen in soil amended with control compost. Colony forming units of chitinase-producing bacteria isolated from rhizosphere of plants grown in chitin compost-amended soil were 10³ times as prevalent as bacteria in control compost. These results indicate that increasing the population of chitinase-producing bacteria and soil enzyme activities in rhizosphere by compost amendment could alleviate pathogenic effects of P. capsici.     

Evidence of genetically diverse virulent mating types of <Emphasis Type="Italic">Phytophthora capsici</Emphasis> from <Emphasis Type="Italic">Capsicum annum</Emphasis> L.

Chili pepper (Capsicum annum L.) is an important economic crop that is severely destroyed by the filamentous oomycete Phytophthora capsici. Little is known about this pathogen in key chili pepper farms in Punjab province, Pakistan. We investigated the genetic diversity of P. capsici strains using standard taxonomic and molecular tools, and characterized their colony growth patterns as well as their disease severity on chili pepper plants under the greenhouse conditions. Phylogenetic analysis based on ribosomal DNA (rDNA), β-tubulin and translation elongation factor 1α loci revealed divergent evolution in the population structure of P. capsici isolates. The mean oospore diameter of mating type A1 isolates was greater than that of mating type A2 isolates. We provide first evidence of an uneven distribution of highly virulent mating type A1 and A2 of P. capsici that are insensitive to mefenoxam, pyrimorph, dimethomorph, and azoxystrobin fungicides, and represent a risk factor that could ease outpacing the current P. capsici management strategies.