Abstract: Aims: To evaluate the influence of environmental parameters (water activity aw, temperature, and pH) on the radial growth rate of Trichoderma asperellum (strains PR10, PR11, PR12, and 659-7), an antagonist of Phytophthora megakarya, the causal agent of cocoa black pod disease. Methods and Results: The radial growth of four strains of T. asperellum was monitored for 30 days on modified PDA medium. Six levels of aw (0·995, 0·980, 0·960, 0·930, 0·910, and 0·880) were combined with three values of pH (4·5, 6·5, and 8·5) and three incubation temperatures (20, 25, and 30°C). Whatever the strain, mycelial growth rate was optimal at aw between 0·995 and 0·980, independently of the temperature and pH. Each strain appeared to be very sensitive to aw reduction. In addition, all four strains were able to grow at all temperatures and pH values (4·5–8·5) tested, highest growth rate being observed at 30°C and at pH 4·5–6·5. The use of response surface methodology to model the combined effects of aw, temperature, and pH on the radial growth rate of the T. asperellum strains confirmed the observed results. In our model, growth of the T. asperellum strains showed a greater dependence on aw than on temperature or pH under in vitro conditions. Conclusion: aw is a crucial environmental factor. Low aw can prevent growth of T. asperellum strains under some conditions. The observed and predicted radial growth rate of strain PR11 showed its greater capacity to support low aw (0·93) as compared with other tested strains at 20°C. This is in agreement with its better protective level when applied in medium-scale trials on cocoa plantations. Significance and Impact of the Study: This study should contribute towards improving the biocontrol efficacy of T. asperellum strains used against P. megakarya. Integrated into a broader study of the impact of environmental factors on the biocontrol agent–pathogen system, this work should help to build a more rational control strategy, possibly involving the use of a compatible adjuvant protecting T. asperellum against desiccation.
Abstract: A study was conducted to investigate the possibility of involvement of chitinase and β-1,3-glucanase of an antagonistic fluorescent Pseudomonas in growth suppression of phytopathogenic fungi, Phytophthora capsici and Rhizoctonia solani. Fluorescent Pseudomonas isolates GRC3 and GRC4 were screened for their antifungal potential against phytopathogenic fungi by using dual culture technique both on solid and liquid media. The percent inhibition was calculated. Various parameters were monitored for optimization of enzyme activities by fluorescent Pseudomonas GRC3. The involvement of chitinases, β-1,3-glucanases, and antifungal metabolites of nonenzymatic nature was correlated with the inhibition of P. capsici and R. solani. The results provide evidence for antibiosis as a mechanism for antagonism. The study also confirms that multiple mechanisms are involved in suppressing phytopathogens as evidenced by the involvement of chitinase and β-1,3-glucanase in inhibition of R. solani but not P. capsici by isolate GRC3.
Abstract: A survey was undertaken in southern California in 1999–2000 to search for an avocado soil that exhibits natural suppression to avocado root rot caused by Phytophthora cinnamomi. The Somis-1 soil, which had consistent massive degradation of P. cinnamomi hyphal mats, low P. cinnamomi populations and good tree health, was shown to be a soil with a high level of microbial suppression to P. cinnamomi in greenhouse studies. Other soils chosen for study, which seemed to demonstrate some type of suppression in the field as evidenced by high P. cinnamomi soil populations yet with good tree health, did not demonstrate microbial suppression in greenhouse studies. These soils may represent a type of suppression which is ephemeral or highly dependent on specific environmental factors. The suppression of the Somis-1 soil was transferable to a conducive soil with as little as 1% natural soil mixed with 99% fumigated soil. The suppression was gradually eliminated in soil pre-treated at various temperatures from 25° to 90°C. The suppression in the Somis-1 soil did not correspond with cellulase or laminarinase activity or soil microbial activity. The suppression appeared to correspond with moderately well-drained soils as found in the Somis-1 soil, which drained at a rate that might be conducive to the growth and activity of microorganisms antagonistic to P. cinnamomi.
Abstract: The objectives of this investigation were to determine the potential of four isolates (B1, B3, B9, and B13) of Bacillus and their mixture for biocontrol of root rot pathogens under field conditions, as well as their effect on development and yield of pepper. The application of Bacillus increased plant height 20% and yield 270%. Also, it decreased incidence 80% and severity of root rot 39% in comparison to the check. Bacterial identification was carried out by biochemical tests, Biolog badges, and polymerase chain reaction (PCR) with specific primers for the ribosomal gene 16S. Isolates of Bacillus B1 were identified as Bacillus amyloliquefaciens, B3 as B. licheniformis, and B9 and B13 as B. subtilis. In a phylogenetic tree, these Bacilli were grouped separately with other isolates of Bacillus.
Abstract: Control of late blight by alternative products is important for reduction of fungicide inputs and potato production costs. The efficacy of essential oils and biopesticides for inhibition of growth of Phytophthora infestans on media and suppression of late blight on potato plants in growth chambers was examined. Growth of pathogen isolates of diverse genotypes was evaluated on Rye B media amended with essential oils (lavender, thyme, thyme borneal, and oregano) and the biopesticide Serenade (Bacillus subtilis strain QST 713). Over 90% inhibition of pathogen growth was achieved with oregano and Serenade amendments. The protective foliar application of Serenade, an aerated compost tea (ACT), Effective Microorganism mix (EM), and oregano, resulted in disease suppression of 5–40% relative to the untreated control. ACT had no significant suppressive effects (0–15% reduction), EM resulted in mild suppression (15–30% reduction), and oregano and Serenade consistently resulted in moderate disease reduction (20–40%). No oil or biological treatment produced disease control comparable to the chemical control chlorothalonil, which resulted in disease reductions of 80–98%. Both oregano and Serenade resulted in some phytotoxicity at high doses. These results suggest that the natural products and biological amendments tested are not sufficient for effective late blight control by themselves; however, when used in combination with other established disease control practices, these approaches may contribute to improved, integrated, and more sustainable management options for late blight.
Abstract: A combination of two compatible micro-organisms,Trichoderma harzianum and Streptomyces rochei, both antagonistic to the pathogen Phytophthora capsici, was used to control root rot in pepper. The population of the pathogen in soil was reduced by 75% as a result. Vegetative growth of the mycelium of P. capsici was
inhibited in vitro on the second day after P. capsici and T. harzianum were placed on the opposite sides of the same Petri plate. Trichoderma harzianum was capable of not only arresting the spread of the pathogen from a distance, but also after invading the whole surface of the pathogen colony, sporulating over it. Scanning electron microscopy showed the hyphae of P. capsici surrounded by those of T. harzianum,/i>, their subsequent disintegration, and the eventual suppression of the pathogen’s growth. Streptomyces rochei produced a zone of inhibition, from which was obtained a compound with antioomycete property secreted by the bacteria. When purified by high-pressure liquid chromatography, this compound was identified as 1-propanone, 1-(4-chlorophenyl), which seems to be one of the principal compounds involved in the antagonism. A formulation was prepared that maintained the compound’s capacity to inhibit growth of the
pathogen for up to 2 years when stored at room temperature in the laboratory on a mixture of plantation soil and vermiculite. The two antagonists, added as a
compound formulation, were effective at pH from 3.5 to 5.6 at 23–30C. The optimal dose of the antagonists in the compound formulation was 3.5 · 108 spores/ml of T. harzianum and 1.0 · 109 FCU/ml of S. rochei. This is the first report of a compound biocontrol formulation of these two antagonists with a potential to control root rot caused by P. capsici.
Abstract: Bacterial strains isolated from the rhizosphere of angelica trees were evaluated for their antagonistic activity against Phytophthora cactorum, a causal agent of Phytophthora root rot. Of these, three bacterial strains, designated as T-1-8, T-1-14 and T-1-23, strongly inhibited mycelial growth of P. cactorum ARE-862 in a dual-culture plate assay. Biocontrol activity of these strains was then examined by dipping root of young seedlings of angelica trees into a bacterial suspension. The incidence of Phytophthora root rot was markedly suppressed for at least 79 days in pot tests when treated seedlings were planted in naturally infested soil. The suppression was maintained through June of the next year. In addition, these strains significantly reduced the development of Phytophthora root rot up to 47 days in naturally infested field and up to 63 days (the last day of testing) in an artificially (moderately) infested field. Based on their main bacteriological properties, strain T-1-14 was identified as Enterobacter cloacae and T-1-8 and T-1-23 were identified as Serratia ficaria.
Abstract: The effects of various fungi, bacteria and different compost extracts on foliar infection of potato (Solarium tuberosum)with Phytophthora infestans were tested in detached leaf bioassays in 2001-2003. Application of microorganism inocula and compost extracts as well as copper oxychloride to excised leaves resulted in different degrees of blight control. In general, suppression of blight lesion growth was not improved by applying the antagonists before the leaves were inoculated with Phytophthora spores. There was some evidence that extracts made from different compost feed stocks of different ages suppressed leaflet infection with blight. However, results were very limited and inconsistent. Moreover, the effects were much smaller than where copper oxychloride was used. Improved efficacy of acceptable alternatives to copper fungicides especially in organic farming is required. No promising effective
alternative to the use of copper fungicides to reduce late blight infection in organic potato production systems was identified in the experiments.
Abstract: The three most important species of Phytophthora worldwide causing black pod disease of cacao are P. palmivora, P. megakarya, and P. capsici. Chemicals are effective in controlling this disease but more
natural methods would be preferred. One alternative is to use natural plant extracts. Rosemary and lavender leaf extracts were found to be effective in reducing germination of P. capsici, P. megakarya, and
P. palmivora zoospores when supplemented to agar plates at different dilutions. The extracts displayed the biggest impact on P. megakarya zoospores where it completely inhibited germination at a 25% dilution of
the prepared extract. When applied to cacao leaf disks, rosemary extract reduced necrosis caused by P. megakarya zoospores. In a bioassay, pears first treated with lavender extract showed no symptoms of P. megakarya infection compared with the non-treated controls. Based upon HPLC analyses, the active compound in these extracts was determined to be cafieic acid, rosmarinic acid or some simple derivative thereof. When added to agar plates, synthetic cafieic acid and rosmarinic acid completely inhibited germination of P. capsici, P. megakarya,and P. palmivora zoospores at concentrations of 3 and 6 g l-1, respectively. In addition, sage and rice bran extracts, which both contain cafieic acid, were also effective in reducing zoospore germination. In contrast, inhibition of Botrytis cinerea or Trichoderma asperellum conidia germination did not occur, displaying some level of specificity. These extracts could provide an economically safe method for reducing damage caused by black pod disease on cacao until resistant varieties are developed and released.
Abstract: More than 500 isolates of bacteria were obtained from the aerial part and rhizosphere of sweet pepper (Capsicum annuum L.) plants harvested from different places in the Region of Murcia (Spain). The isolates were purified and assayed in vitro against Phytophthora capsici and Alternaria alternata. Sixty isolates (12 %) produced an inhibition zone against at least one of the pathogens, while ten had a strongly inhibitory effect on both pathogens assayed. Microscopic observation of interactions zone showed cell vacuolisation, hyphae lysis and spilling of cytoplasm content of the pathogens in the culture media. These ten isolates were then chosen for biocontrol of Phytophthora root rot and Alternaria leaf spots of pepper plants in vivo. Four of them denominated HS93, LS234, LS523 and LS674 reduced P. capsici root rot by 80, 51, 49 and 54 %, respectively, and A. alternata leaf spots by 54, 74, 62 and 53 %. HS93 belongs to the genus Bacillus and probably the species subtilis, while LS234, LS523 and LS674 belong to the genus Bacillus and probably the species licheniformis. Dry mass of plants treated with these bacteria was significantly higher than that of non-treated and inoculated plants.
Abstract