Abstract: An improved and rapid method for selection of bacteria primarily based on its inhibitory effect on pathogenesis of Phytophthora capsici on black pepper (Piper nigrum) was developed. Reductions in mycelial growth of P. capsici in dual plating assay and inhibition of lesion on excised black pepper shoots in planta assay were calculated. The inhibition ranged from 0.0% to 92.8% among the 19 bacterial isolates tested against P.
capsici in the in planta assay III where the bacterized shoots were challenge inoculated with P. capsici. The in planta assay III
could select only two bacterial strains among the 19 isolates as effective against P. capsici whereas, four and six bacterial strains in the in planta assays I and II, respectively. The dual plate method, where only two-way interaction between the pathogen and antagonist was studied, could select four bacterial strains as effective ones. The in planta selected bacterial trains could protect the black pepper plants from foot rot infection when tested in the nursery. The in planta assay based on this three-way interaction among black pepper (host)- P. capsici (pathogen)-candidate bacterial strains (antagonist) was found to be an effective method to select antagonistic bacteria against the foot rot pathogen.
Abstract: Aims: To investigate the in vitro antifungal and antioomycete activities of some aminoglycosides against true fungi and Phytophthora and Pythium species and to evaluate the potential of the antibiotics against Phytophthora late blight on plants.
Methods and Results: Antifungal and antioomycete activities of aminoglycoside antibiotics (neomycin, paromomycin, ribostamycin and streptomycin) and a paromomycin-producing strain (Streptomyces sp. AMG-P1) against Phytophthora and Pythium species and 10 common fungi were measured in potato dextrose broth (PDB) and on seedlings in pots. Paromomycin was the most active against Phytophthora and Pythium species with a minimal inhibitory concentration of 1–10 ug ml-1 in PDB, but displayed low to moderate activities towards other common fungi at the same concentration. Paromomycin also showed potent in vivo activity against red pepper and tomato late blight diseases with 80 and 99% control value, respectively, at 100 ug ml-1. In addition, culture broth of Streptomyces sp. AMG-P1 as a paromomycin producer exhibited high in vivo activity against late blight at 500 ug freeze-dried weight per millilitre.
Conclusions: Among tested aminoglycoside antibiotics, paromomycin was the most active against oomycetes both in vitro and in vivo.
Significance and Impact of the Study: Data from this study show that aminoglycoside antibiotics have in vitro and in vivo activities against oomycetes, suggesting that Streptomyces sp. AMG-P1 may be used as a biocontrol agent against oomycete diseases.
Abstract: 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.
Abstract: Aims:To examine the biological activity of Streptoverticillium albireticuli.
Methods: Isolation of S. albireticuli was carried out using the dry-heat technique. Nematicidal and pathogenic activity on Caenorhabditis elegans was measured by mortality in metabolites and colonization rate on fishmeal extract agar. Antifungal and enzymatic activities of S. albireticuli were measured by the agar plate method and the semidefined solid media method, respectively.
Results: S. albireticuli showed strong nematicidal activity against C. elegans. Pathogenic activity was also evident with the colonized nematode by the isolate on fishmeal extract agar. It also showed antifungal activity against certain fungal pathogens such as Rhizoctonia solani , Phytophthora cinnamomi and Fusarium oxysporum.
Significance and Impact of the Study: The discovery of an actinomycete showing pathogenic activity against the nematode may indicate the potential for it to be used as a biocontrol agent of parasitic nematodes, in addition to its ability to suppress fungal pathogens.
Abstract: • Pseudomonas strains have shown promising results in biological control of late blight caused by Phytophthora infestans. However, the mechanism(s) and metabolites involved are in many cases poorly understood. Here, the role of the cyclic lipopeptide massetolide A of Pseudomonas fluorescens SS101 in biocontrol of tomato late blight was examined.
• Pseudomonas fluorescens SS101 was effective in preventing infection of tomato (Lycopersicon esculentum) leaves by P. infestans and significantly reduced the expansion of existing late blight lesions. Massetolide A was an important component of the activity of P. fluorescens SS101, since the massA-mutant was significantly less effective in biocontrol, and purified massetolide A provided significant control of P. infestans, both locally and systemically via induced resistance.
• Assays with nahG transgenic plants indicated that the systemic resistance response induced by SS101 or massetolide A was independent of salicylic acid signalling. Strain SS101 colonized the roots of tomato seedlings significantly better than its massA-mutant, indicating that massetolide A was an important trait in plant colonization.
• This study shows that the cyclic lipopeptide surfactant massetolide A is a metabolite with versatile functions in the ecology of P. fluorescens SS101 and in interactions with tomato plants and the late blight pathogen P. infestans.
Abstract: Introduction of US-8 genotypes of Phytophthora infestans has coincided with an increase in severity of potato late blight in North America. As alternatives to chemical fungicides, 18 bacterial strains patented as biological control agents (BCA) of both sprouting and Fusarium dry rot were cultivated in three liquid media and screened in wounded potato bioassays for their ability to suppress late blight incited by P. infestans (US-8, mating type A2). Washed or unwashed stationary-phase bacteria were mixed with fungal zoospores to inoculate potato wounds with 5µL containing ~108 bacterial CFU/mL and 2×104 zoospore count/mL. Disease suppressiveness was evaluated after tubers were stored a week at 15°C, 90% relative humidity. One-fifth of the 108 BCA treatments screened, reduced late blight by 25-60%, including among other strains Pseudomonas fluorescens S22:T:04 (showing most consistency), P22:Y:05, S11:P:12 and Enterobacter cloacae S11:T:07. Small-scale pilot testing of these four strains, alone and in combination, was conducted under conditions simulating a commercial application. Suspensions of 4×104P. infestans sporangia/mL were sprayed at a rate of 1.6 mL followed by 0.8 mL of bacteria treatment at ~5×109 CFU/mL per each of 90 unwounded potatoes. Three replicate boxes per treatment (30 tubers per box) were randomized in storage and maintained 4 weeks at 7.2°C, 95% relative humidity. All BCA treatments significantly reduced disease; and unwashed bacteria outperformed those washed free of culture broth. Disease suppression ranged from 35% up to 86% the first test year and from 35 to 91% the second year. Highest overall performance rankings significantly above the control were achieved by the following strains in culture broth: four-strain mix > P. fluorescens S22:T:04> P. fluorescens S11:P:12. Combined with previous demonstrations of dry rot and sprout suppression, the consistent late blight control by these strains and strain mixtures suggests the commercial feasibility of a single treatment for broad spectrum suppression of post-harvest potato diseases and sprouting.
Abstract: The culture broth of Streptomyces halstedii AJ-7 suppressed the growth of Phytophthora capsici, which causes phytophthora blight in red-peppers, with less than 1% survival of the pathogen after 12 h of treatment. The low molecular fraction (610 kDa) of the culture broth retained anti-fungal activity against P. capsici after being held at 1000C for 6 h.
Abstract: The ability of Trichoderma harzianum to control the rotting of pepper (Capsicum annuum) plant roots caused by Phytophthora capsici was studied. Interactions between the fungi were assessed in vitro on three culture media (V8c, Czapek and 2% water agar) and in vivo in plants grown in a substrate inoculated with P. capsici and T. harzianum. Studies on mutual antagonism in vitro showed that P. capsici was inhibited by T. harzianum; however, the intensity of inhibition differed according to the medium used, being greatest on Czapek. Analysis of the fungal populations in the plant growth substrate showed that T. harzianum consistently reduced that of P. capsici over time. This reduction in the pathogen population was associated with a reduction in root rot of between 24 and 76%, although plant growth (dry weight) was still reduced by 21.2–24.7%, compared with the uninoculated control. In the absence of T. harzianum with the same pathogen inoculum levels, the reduction in dry weight was 59.8–68.6%, suggesting that T. harzianum reduced the damage.
Abstract: Aims: To isolate and identify antioomycete substances from Fusarium oxysporum EF119 against Phytophthora infestans and to investigate their antimicrobial activities against various plant pathogenic bacteria, oomycetes and true fungi.
Methods and Results: Two antioomycete substances were isolated from liquid cultures of F. oxysporum EF119, which shows a potent disease control efficacy against tomato late blight caused by P. infestans. They were identified as bikaverin and fusaric acid by mass and nuclear magnetic resonance spectral analyses. They inhibited the mycelial growth of plant pathogenic oomycetes and fungi. Fusaric acid also effectively suppressed the cell growth of various plant pathogenic bacteria, but bikaverin was virtually inactive. Treatment with bikaverin at 300 ug ml-1 suppressed the development of tomato late blight by 71%. Fusaric acid provided effective control against tomato late blight and wheat leaf rust over 67% at concentrations more than 100 ug ml-1.
Conclusions: Both bikaverin and fusaric acid showed in vitro and in vivo antioomycete activity against P. infestans.
Significance and Impact of the Study: Fusarium oxysporum EF119 producing both bikaverin and fusaric acid may be used as a biocontrol agent against tomato late blight caused by P. infestans.
Abstract: We isolated bacteria and fungi from composted chicken, sheep, cow and horse manure and screened each isolate for their ability to suppress P. cinnamomi in dual-culture in vitro assays. Of the 180 isolates, including 31 actinomycetes, 64 fungi, 44 fluorescent pseudomonads and 41 endospore-forming bacteria, 45 isolates significantly inhibited the growth of P. cinnamomi on plates. The inhibitory microbes included 24 fungi (including Trichoderma sp., Gliocladium penicillioides and Fusarium sp.), 0 actinomycetes (all Streptomyces sp.), 7 fluorescent pseudomonads (Pseudomonas sp.) and 4 endospore-forming bacteria (Bacillus sp.). The most common mode of action observed was antibiosis, although mycoparasitism, indicated by parallel hyphal growth, hyphal coiling, appressorium formation and direct penetration, was also observed with one isolate of Trichoderma. These results help to explain the role of microbes in the suppression and biological control of P. cinnamomi by composted manures.
Abstract