Biological control of bacterial spot of tomato under field conditions at several locations in North America

Following the relatively successful biological control of bacterial speck of tomato under field conditions at several locations (Phytopathology 92 (2002) 1284), similar selection and testing strategies were employed in an effort to isolate an effective biological control agent for bacterial spot of tomato. Fifty potential biological control agents were isolated from tomato foliage in Alabama (AL) and Florida (FL) and tested under greenhouse conditions in AL for the ability to reduce the foliar severity of bacterial spot of tomato (Lycopersicon esculentum), which is caused by either Xanthomonas campestris pv. vesicatoria or Xanthomonas vesicatoria. Three pseudomonads that provided protection against bacterial speck also were included in the tests. The strains which were most efficacious (i.e., high mean percentage reduction) and consistent (i.e., low standard deviation) in reducing bacterial spot severity in repeated greenhouse experiments were selected for field experiments conducted over the period 1996–1998. Among these strains were Cellulomonas turbata BT1, which provided the highest mean reduction in disease severity [45.2% (SD = 21.0)], and Pseudomonas syringae Cit7 [36.4% (SD = 12.2)], which was the most consistent. Field experiments were conducted in Shorter, AL; Bradenton and Sanford, FL; Clinton, North Carolina; Wooster, Ohio; and London, Ontario, Canada. The highest mean reductions in severity of bacterial spot on foliage, averaged across all locations, were provided by P. syringae Cit7 [28.9% (SD = 11.6)] and Pseudomonas putida B56 [23.1% (SD = 7.4)]. The efficacy and consistency of P. syringae Cit7 against bacterial spot were very similar to those achieved against bacterial speck [28.3% (SD = 12.7)] (Phytopathology 92 (2002) 1284). Unfortunately, neither the bacterial strains nor the standard copper bactericides consistently reduced disease incidence on fruit.     

Response of tomato genotypes to bacterial speck (Pseudomonas syringae pv. tomato)

Two genotypes of tomato A 100 and Ontario 7710 which were inoculated separately with four strains of Pseudomonas syringae pv. tomato differed significantly in disease severity (susceptibility) to bacterial speck. At both concentrations of inoculum of each strain used (10⁷ and 10⁸ cfu/ml) A 100 appeared to be highly susceptible whereas Ontario 7710 showed very low or no susceptibility. The significant differences in virulence between strains and in response of tomato plants in three replicate experiments were found. Generally, concentration of inoculum 10⁷ cfu/ml was too low to induce consistent level of disease severity. The obtained results indicate the importance of consistent and favorable conditions for disease development in screening of tomato resistance to bacterial speck.     

Application of plant growth-promoting rhizobacteria to control Papaya ringspot virus and Tomato chlorotic spot virus

Papaya ringspot virus (PRSV-W) and Tomato chlorotic spot virus (TCSV) are responsible for severe losses in cucurbits and tomato production in south Florida and other regions in the USA. Traditional chemicals are not effective to control these viruses. Using plant growth-promoting rhizobacteria (PGPR) may present an alternative to control these viruses. Results from this study demonstrated that applying mixtures of PGPR strains is more efficient to control PRSV-W and TCSV compared to individual PGPR strain only. The application method significantly affected the efficiency of PGPR to control PRSV-W and TCSV. The highest reduction in disease severity of both PRSV-W and TCSV occurred in case of soil drenching with PGPR, followed by root dipping and seed coating treatments. Application of PGPR mixtures of (IN937a & SE34) or (IN937a &, SE34 & T4) were the most efficient methods to control these viral diseases.     

Integration of chitosan and plant growth-promoting rhizobacteria to control Papaya ringspot virus and Tomato chlorotic spot virus

Papaya ringspot virus-W (PRSV-W) and Tomato chlorotic spot virus (TCSV) are common viruses infecting vegetables in south Florida. Application of plant growth-promoting rhizobacteria (PGPR) has emerged as a potential alternative of chemical pesticides to control these viruses. But, it is not sufficient to completely replace chemical control. This study aimed to investigate the synergistic effect of chitosan and PGPR to control PRSV-W and TCSV. The efficiency of PGPR to suppress PRSV-W and TCSV was significantly improved when they were accompanied with chitosan treatment. The highest reduction in disease severity of both PRSV-W and TCSV was achieved when chitosan treatment was accompanied with mixture of two PGPR (IN937a + SE34) or three PGPR strains (IN937a + SE34 + SE56). The results of this study proved that implementation of chitosan and PGPR could significantly restrict losses due to PRSV-W and TCSV in squash and tomato, in Florida and the United States.     

Biocontrol of tomato late blight with the combination of epiphytic antagonists and rhizobacteria

Control of tomato late blight (LB) in Brazil is heavily based on chemicals. However, reduction in fungicide usage is required in both conventional and organic production systems. Assuming that biological control is an alternative for LB management, 208 epiphytic microorganisms and 23 rhizobacteria (RB) were isolated from conventional and organically grown tomato plants and tested for antagonistic activity against Phytophthora infestans. Based on in vitro inhibition of sporangia germination and detached leaflet bioassays, four EP microorganisms (Aspergillus sp., Cellulomonas flavigena, Candida sp., and Cryptococcus sp.) were selected. These microorganisms were applied either singly or combined on tomato plants treated or not with the RB Bacillus cereus. On control plants, LB progress rate (r), area under disease progress curve, and final disease severity were high. Lowest values of final severity were recorded on plants colonized by B. cereus and treated with C. flavigena, Candida sp. and Cryptococcus sp. There was no reduction on disease severity in plants treated only with RB. Biological control of LB resulted in low values of r and final severity. Integration of biological control with fungicides, cultural practices, and other measures can contribute to manage LB on tomato production systems.     

Towards the biological control of fungal and bacterial diseases of tomato using antagonistic Streptomyces spp.

This investigation was designed to explore the potential of microbial antagonism in the control of some tomato diseases including bacterial, Fusarium and Verticillium wilts; early blight; bacterial canker. Three Streptomyces spp. were used: S. pulcher, S. canescens and S. citreofluorescens.The in vitro studies showed that an 80% concentration of the culture filtrate of either S. pulcher or S. canescens significantly inhibited spore germination, mycelial growth and spotulation of Fusarium oxysporum f.sp. lycopersici, Verticillium albo-atrum and Alternaria solani. The same concentration of filtrate of either S. pulcher or S. citreofluorescens was detrimental to the bacterial populations of Clavibacter michiganensis subsp. michiganensis and Pseudomonas solanacearum.The in vivo studies involved different treatments: soaking tomato seeds in filtrate of the antagonist prior to sowing, inoculation of the soil with the antagonist 7 days before sowing, and coating of tomato seeds with spores of the antagonist before sowing. The seed-coating treatment was the most effective in controlling all the pathogens at 42 and 63 days after sowing. Soil inoculation with the antagonist 7 days prior to sowing was less effective in controlling the tomato pathogens as compared to seed-coating. The seed-soaking treatment was the least effective in controlling the diseases concerned.The results also revealed that seed-coating with antagonistic Streptomyces spp. significantly improved tomato growth.     

Metabolic changes in tomato fruits and seeds after viral, bacterial and fungal infections

The metabolic changes in tomato fruits and seeds separately infected with cucumber mosaic virus, Pseudomonas syringae pv. tomato or Botrytis cinerea were investigated cytochemically. The changes of peroxidase (E.C. 1.11.1.7) and β-glucosidase (E.C. 3.2.1.21) were investigated biochemically as well. Tomato fruits were involved in the study because of their high economic value. Tomato seeds were investigated since they have been used most extensively as a model system for studying the physiology and biochemistry of seed development. The diseases caused by the pathogens under study are of special importance for yield reduction in tomato. The three pathogens provoked local changes in the activities of enzymes under study that affect the infected pericarp tissues and neighboring seeds. It was established non-specific and specific responses. The non-specific responses of invaded tissues were expressed as a local enhancement of peroxidase activity in both pericarp tissues and seeds as well as a decrease in activities of: i. enzymes taking part in aerobic and anaerobic respiration, ii. hydrolases esterase and acid phosphatase involved in the basic metabolism as well as an enhancement of their activities in neighboring tissues. Furthermore, it was observed an enhancement of α-galactosidase activity in infected area was observed. The specific responses depending on the type of the pathogen consisted in an enhancement of glucose-6-phosphate dehydrogenase activity by virus infection and an increase of β-glucosidase activity by fungal invasion.     

Three native Pseudomonas fluorescens strains tested under growth chamber and field conditions as biocontrol agents against damping-off in alfalfa

Alfalfa (Medicago sativa) is one of the most important crops used in Uruguay for livestock feeding. Seedling diseases, particularly damping-off, are a critical factor which limits its establishment. Three native Pseudomonas fluorescens strains, UP61.2, UP143.8 and UP148.2, previously isolated from Lotus corniculatus, were evaluated to determine their efficacy as biological control agents for alfalfa seedling diseases in the field. Their compatibility with the alfalfa-Sinorhizobium meliloti symbiosis was also assessed. In growth chamber conditions seed inoculation with Pseudomonas strains did not affect different parameters of alfalfa-rhizobium symbiosis as shown by nodulation rate and shoot dry weight of plants. The presence of the commercial inoculant strains of S. meliloti did not impair colonization by the P. fluorescens and vice versa. In field trials the dynamics of rhizobial rhizospheric populations were not affected by the presence of P. fluorescens. Each P. fluorescens strain successfully colonized alfalfa roots at adequate densities for biocontrol activity. Results showed that P. fluorescens strains provided a 10–13% increase in the number of established plants relative to the control, an intermediate result compared to the fungicide treatment (24%). The alfalfa above-ground biomass was increased by 13% and 15–18% in the presence of the fungicide and P. fluorescens strains, respectively. Therefore, results from this study demonstrated that the three P. fluorescens strains provided effective control against soil-borne pathogens and suggest a potential use in the development of a commercial inoculant to be applied for the control of legume seedling diseases.     

Identification and fine-mapping of a bacterial brown spot disease resistance gene in maize

Bacterial brown spot (BBS) in maize (Zea mays L.) is caused by Pseudomonas syringae pv. syringae Van Holl (Pss). In China, this disease is not prevalent in maize at present. Here, we report the identification and fine mapping of the gene, referred to as Psy1, which confers resistance to BBS. An F₂ population, derived from the cross P25/F349, was used for linkage analysis and mapping of the resistance gene Psy1. Analysis of a BC₈F₂ population, derived from the same parents, confirmed that Psy1 was located on chromosome 10L and inherited as a single dominant gene. For fine mapping of Psy1, two introgression lines, X41 and X44, homozygous at the resistant gene locus, were introduced to hybridize with the susceptible parent P25 respectively, and developed a mixed BC₁ population. We found the closest markers to Psy1 are EST1 and FG29-3, which located on two adjacent BACs respectively, based on the B73 BAC sequence. Sequence analysis of these two BAC sequences (~300 kb) revealed the presence of a homologous sequence of receptor-like kinase. Also a co-segregation marker was developed based on this homologous sequence. These results will be useful for cloning of Psy1 and for transferring or pyramiding Psy1 through MAS in maize breeding programs.     

Analysis of pathogenicity mutants of a bacteriocin producing Xanthomonas perforans

Since the initial discovery of Xanthomonas perforans on tomato in 1991, it has completely displaced Xanthomonas euvesicatoria as the bacterial spot of tomato pathogen in Florida. Previous research has shown that X. perforans produces at least three different bacteriocin-like compounds (BcnA, BcnB, BcnC) antagonistic toward X. euvesicatoria strains. In this study pathogenicity-attenuated, bacteriocin-producing mutants of X. perforans were created to determine their potential as biological control agents for control of X. euvesicatoria. Several candidate genes were chosen based on previous studies in which mutant phenotypes exhibited reduced virulence in either X. perforans (OpgH_Xcv) or the closely related X. euvesicatoria strain 85-10 (hpaB, hpaC, xopA, xopD, avrBs2 and gumD). Each candidate gene in X. perforans was amplified and PCR-assisted deletion mutagenesis was performed in the wild-type (wt) X. perforans strain to create potential attenuation mutants. Each mutant was tested for growth rate, disease severity and antagonism toward X. euvesicatoria strains. Three mutants, XopA, opgH, and gumD were significantly less pathogenic than the wild-type strain with the opgH mutant reaching significantly lower internal populations than all other mutants except hpaC. The opgH-strain was the most affected in its ability to grow internally in plant tissue while inhibiting X. euvesicatoria populations equal to or more than the other mutant strains. This mutant strain could potentially be used as part of an effective biological control strategy.     

Effect of <Emphasis Type="Italic">Trichoderma</Emphasis> spp. isolates for biological control of tan spot of wheat caused by <Emphasis Type="Italic">Pyrenophora tritici-repentis</Emphasis> under field conditions in Argentina

Trichoderma spp. have been used as biocontrol agents to protect plants against foliar diseases in several crops, but information from field assays is scarce. In the present work, experiments were carried out to determine the effect of six isolates of Trichoderma harzianum and one isolate of T. koningii on the incidence and severity of tan spot, caused by Pyrenophora tritici-repentis (anamorph: Drechslera tritici-repentis) under field conditions. Significant differences between years, wheat cultivars and treatments were found. In 2003, two of the isolates assayed (T5, T7) showed the best performance against the disease applied as seed treatments or sprayed onto wheat leaves at different stages. The application of six of the treatments on wheat plants significantly reduced disease severity by 16 to 35% in comparison with the control. Disease control provided by isolate T7 was similar to that provided by the fungicide treatment (56% reduction). This is the first report on the efficacy of Trichoderma spp. against tan spot under field conditions in Argentina.