Consumer demand for organically produced food and the desire by many farmers to eliminate chemica... more Consumer demand for organically produced food and the desire by many farmers to eliminate chemical fertilizers and pesticides is increasing the need for research and educational programs to support organic farmers. To date, the land-grant universities and the cooperative extension service have been viewed by organic farmers as unresponsive to this need. The primary reason for the unresponsiveness has been inadequate training and resource materials available to extension agents. In 1998, we conducted an intensive training for agriculture agents in North Carolina. Funding was provided by the USDA SARE Professional Development Program. More than 50 agents participated in a series of workshops that were offered together as a graduate course worth four NCSU credits. Much of the training was conducted on the Organic Unit at The Center for Environmental Farming Systems (CEFS), a 100-acre facility dedicated to research and education in organic farming systems. The hands-on training consiste...
Strawberry (Fragaria xananassa) plant phenology was evaluated at two sites in North Carolina in o... more Strawberry (Fragaria xananassa) plant phenology was evaluated at two sites in North Carolina in order to assess the relationship of plant growth and the development of gray mold fruit rot (Botrytis cinerea). Site 1 consisted of three cultivars: Camarosa (CA), Chandler (CH), and Sweet Charlie (SC) from a single nursery; site 2 consisted of cultivar CH obtained from three distinct sources. Weekly records were kept of leaf senescence, leaf emergence, flowering, and fruit set, as well as a monthly whole-plant analysis. Leaf senescence and emergence was uniform for all treatments, with >90% of transplant foliage senescing between November and December. Analysis of leaf area and plant weights reveal an increase in dry mass beginning in February and continuing through April. Although first bloom appeared in December, primary floral development occurred in late February for cultivar SC and in March for cultivars CA and CH, with blooms continuing through late May. Marketable yield was low...
Marker assisted selection (MAS) has not been initiated in tomato (Solanum lycopersicum L.) for se... more Marker assisted selection (MAS) has not been initiated in tomato (Solanum lycopersicum L.) for septoria leaf spot (SLS) resistance caused by Septoria lycopersici Speg due to lack of molecular markers. We studied the inheritance of SLS resistance and identified molecular markers linked to SLS resistance using bulked segregant analysis (BSA) in a segregating F2 population. Tomato inbred lines, NC 85L-1W (2007), susceptible to SLS and NC 839-2(2007)-1, resistant to SLS were used to develop the segregating population. A total of 250 F2 plants, and 10 plants each of P1, P2 and F1 were grown at the Mountain Horticultural Crops Research and Extension Center (MHCREC), Mills River NC in the summer of 2009. Disease severity was scored using a scale of 0 to 5, where 0 = no disease and 5 = complete development of disease. DNA was extracted from 2-3 week old plants and parental lines were screened with a total of 197 random amplified polymorphic DNA (RAPD) primers, of which 34 were polymorphic. Two DNA bulks, called resistant bulk (RB) and susceptible bulk (SB) were prepared from the F2 individuals. The RB and SB consisted of 8 individuals each with disease scores of 0, and 4.0 or 4.5, respectively. The segregation ratio of resistant and susceptible plants in F2 generation fit the expected Mendelian ratio of 3:1 for a single dominant gene. Five RAPD markers were linked to the SLS disease reaction, of which two were linked to susceptibility and three to the resistance. Subject to verification in independent populations, these markers may be useful for MAS of SLS resistance in tomato.
Bacterial spot (BS), caused by four species of Xanthomonas in tomato (Solanum lycopersicum L.), i... more Bacterial spot (BS), caused by four species of Xanthomonas in tomato (Solanum lycopersicum L.), is challenging to manage by chemical methods under favorable environmental conditions. Development of resistant genotypes is a desirable solution to limit losses. A total of 64 genotypes were planted in the field at the Mountain Horticultural Crops Research and Extension Center (MHCREC), NC in a randomized complete block design with two replications consisting of six plants per plot. The genotypes were inoculated with a local and prevalent strain of Xanthomonas and phenotypic data on foliar disease severity was scored using the Horsfall-Barratt (HB) scale and on fruit incidence. Foliar severity ranged from 0 to over 75% defoliation and fruit incidence ranged from 0% to 100% and these traits were not correlated. The response of tomato genotypes for Xanthomonas-specific pathogen associated molecular pattern (PAMP) molecules in production of reactive oxygen species (ROS) was also scored for ...
Tomato (Solanum lycopersium L.) is the second most important vegetable crop in the world. In the ... more Tomato (Solanum lycopersium L.) is the second most important vegetable crop in the world. In the U.S., fresh market tomato production farm value was $0.86 billion in 2012, with Florida and North Carolina (NC) producing 38.6% of national production (34.7% and 3.9% respectively). In both states (2007), 99% of tomato growing acres were harvested for fresh market. Bacterial Wilt (BW) caused by Ralstonia solanacearum (Rs) is a devastating disease of tomato in tropical and subtropical environments around the world with losses of 70-100% in the Southeast U.S. in heavily infested fields. Vegetable grafting of commercial scions onto disease resistant rootstocks has been developed as a viable management strategy for BW in tomato, particularly for fresh market. Bacterial wilt resistance in tomato rootstock germplasm, however, is quantitative and partial. A greenhouse resistance study using 10 variably resistant tomato rootstocks lines (not grafted; mixture of open-pollinated and commercial hyb...
Recent Developments in Management of Plant Diseases, 2009
Major shifts in agricultural practices are complex with highly interdependent biological, environ... more Major shifts in agricultural practices are complex with highly interdependent biological, environmental, social, economic, business, and other (agri)culture factors. Seeking alternatives to methyl bromide (MeBr) to manage soilborne pathogens provided a model system that could be simulated where other large scale and dramatic changes need to be made. Parallel priorities were set in place. The first priority was to assemble an inter-disciplinary and interstate response and vision team of key private and public sector stakeholders. The second priority was to determine the risk-aversion and biological basis for fumigation. The third priority was to implement a plan that did not simply focus on chemical alternatives, but sought to advance the science of plant pathology and conduct discovery research about the biology, ecology and management of the primary plant pathogens and cropping systems. Therefore, three strategic levels of research and extension were identified: (1) Tactic substitution-addressing short term needs of growers who sought non-ozone depleting fumigant alternatives; (2) Tactic Diversification-focused on medium term alternatives that included non-fumigant and IPM based tactics; (3) Tactic Development-focused on long-term goals to explore microbial ecology and farming systems-based approaches to replace MeBr-dependent production systems. The fourth priority was to effectively extend research based information to primary clientele. Combined efforts resulted in technically and economical feasibility assessments of alternatives, exploration of viable diversification and development of plant disease management tactics, and a region-wide advanced understanding of the biology and ecology of key plant pathogens.
Four hundred thirty-three xanthomonad strains isolated from tomato or pepper plants from 32 diffe... more Four hundred thirty-three xanthomonad strains isolated from tomato or pepper plants from 32 different fields in four Caribbean and Central American countries were screened for the ability to hydrolyze starch and sodium polypectate and for resistance to copper and streptomycin. Of these, 95 representative strains were further characterized by various phnetic tests, and 63 of these strains were then analyzed by genomic fingerprinting. Most of the strains (>90%) were tolerant to copper. However, there was much more variability in sensitivity to streptomycin. All strains in Guadeloupe and 93% of the strains in Barbados were sensitive to streptomycin. The majority of strains were typical Xanthomonas campestris pv. vesicatoria group A strains. In Barbados, however, a unique group of strains was identified that was serologically similar to group A strains but was amylolytic. These strains were designated A1. The occurrence of X. campestris pv. vesicatoria group B strains in Central Amer...
▪   The advent of molecular biology in general and the polymerase chain reaction in particular ha... more ▪   The advent of molecular biology in general and the polymerase chain reaction in particular have greatly facilitated genomic analyses of microorganisms, provide enhanced capability to characterize and classify strains, and facilitate research to assess the genetic diversity of populations. The diversity of large populations can be assessed in a relatively efficient manner using rep-PCR-, AFLP-, and AP-PCR/RAPD-based genomic fingerprinting methods, especially when combined with computer-assisted pattern analysis. Genetic diversity maps provide a framework to understand the taxonomy, population structure, and dynamics of phytobacteria and provide a high-resolution framework to devise sensitive, specific, and rapid methods for pathogen detection, plant disease diagnosis, as well as management of disease risk. A variety of PCR-based fingerprinting protocols such as rDNA-based PCR, ITS-PCR, ARDRA, T-RFLPs, and tRNA-PCR have been devised, and numerous innovative approaches using specif...
Organic and heirloom tomatoes are high-value products with growing demand but there are many chal... more Organic and heirloom tomatoes are high-value products with growing demand but there are many challenges to successful cultivation. A systems comparison study was carried out to evaluate the production of the popular heirloom tomato ‘Cherokee Purple’ (Solanum lycopersicum L.) under high tunnel and open field systems in North Carolina from 2007 to 2008. Management of the high tunnel (i.e., temperature and irrigation), weather events as well as pest and disease pressure influenced crop quality and yield. The high tunnel and field systems achieved similar total yields (100 t·ha−1) the first season but yields were 33% greater in the high tunnel system than the field system in the second year (100 t·ha−1 and 67 t·ha−1, respectively). Both years, the tomatoes were planted in high tunnels 1 month earlier and harvested 3 weeks earlier than the field. The accumulation of ≈1100 growing degree-days (GDD) was required in both systems before 50% of the fruit was harvested. Fruit cracking, cat-fac...
Partial budget analysis was used to evaluate soil treatment alternatives to methyl bromide (MeBr)... more Partial budget analysis was used to evaluate soil treatment alternatives to methyl bromide (MeBr) based on their cost-effectiveness in the production of strawberries (Fragaria ×ananassa). The analysis was conducted for two geographical areas: the piedmont and coastal plain area (including North Carolina and Georgia) and the mountain area of western North Carolina, based on 7 years of field test data. The fumigation alternatives evaluated were Telone-C35 (1,3-dichloropropene 61.1% + chloropicrin 34.7%), Telone II (1,3-dichloropropene 94%), chloropicrin (Chlor-o-pic 99% and TriClor EC), InLine (1,3-dichloropropene 60.8% + chloropicrin 33.3%), and metam sodium (Vapam or Sectagon 42, 42% sodium methyldithiocarbamate). The MeBr formulation was 67% MeBr and 33% chloropicrin (Terr-O-Gas) with the exception of the earlier trials where a 98:2 ratio was used. In the piedmont and coastal plain area, the soil treated with chloropicrin showed the best results with an additional return of $1670/a...
Reactive oxygen species (ROS) made by plants in response to pathogen infection not only initiate ... more Reactive oxygen species (ROS) made by plants in response to pathogen infection not only initiate local and systemic defenses, they are also antimicrobial. A number of fungi are hypothesized to secrete the antioxidant mannitol to protect against this antimicrobial ROS during infection. This hypothesis is supported by reports that overexpression of the mannitol catabolic enzyme mannitol dehydrogenase (MTD) in plants increases resistance to mannitol-secreting pathogens like Botrytis cinerea and Alternaria alternata. To extend this hypothesis and test the general utility of this approach, we overexpressed celery MTD in a tomato breeding line (NC1 Grape) currently used in our program. Although we reported earlier that MTD overexpression provides resistance to Botrytis gray mold in a greenhouse tomato, this is the first report of overexpression in an elite breeding variety providing heritable, whole-plant resistance to A. solani (tomato early blight). In this study, progeny from a high-MT...
The oxidative burst is a critical early event in plant-pathogen interactions that leads to a loca... more The oxidative burst is a critical early event in plant-pathogen interactions that leads to a localized, programmed cell death (PCD) called the hypersensitive response (HR). The HR and associated PCD retard infection by biotrophic pathogens, but can, in fact, enhance infection by necrotrophic pathogens like Botrytis cinerea. In addition to signaling the induction of the HR, reactive oxygen species (ROS) produced during the oxidative burst are antimicrobial. We hypothesize that pathogens such as B. cinerea survive the antimicrobial effects of ROS, at least partially by secreting the antioxidant mannitol during infection. This is supported by the previous observation that overexpression of the catabolic enzyme mannitol dehydrogenase (MTD) can decrease a plants susceptibility to mannitol-secreting pathogens like B. cinerea. To extend the above hypothesis, and test the general utility of this approach in an important horticultural crop, we overexpressed celery MTD in tomato (Solanum lycopersicum cv. "Moneymaker"). In these studies, we observed a significant increase (up to 90%) in resistance to B. cinerea in transgenic tomatoes expressing high amounts of MTD.
In order to demonstrate that allelopathic interactions are occurring, one must, among other thing... more In order to demonstrate that allelopathic interactions are occurring, one must, among other things, demonstrate that putative phytotoxins move from plant residues on or in the soil, the source, through the bulk soil to the root surface, a sink, by way of the rhizosphere. We hypothesized that the incorporation of phytotoxic plant residues into the soil would result in a simultaneous inhibition of seedling growth and a stimulation of the rhizosphere bacterial community that could utilize the putative phytotoxins as a sole carbon source. If true and consistently expressed, such as relationship would provide a means of establishing the transfer of phytotoxins from residue in the soil to the rhizosphere of a sensitive species under field conditions. Presently, direct evidence for such transfer is lacking. To test this hypothesis, cucumber seedlings were grown in soil containing various concentrations of wheat or sunflower tissue. Both tissue types contain phenolic acids, which have been ...
DNA primers corresponding to conserved motifs in bacterial repetitive (REP, ERIC, and BOX) elemen... more DNA primers corresponding to conserved motifs in bacterial repetitive (REP, ERIC, and BOX) elements and PCR were used to show that REP-, ERIC-, and BOX-like DNA sequences are widely distributed in phytopathogenic Xanthomonas and Pseudomonas strains. REP-, ERIC, and BOX-PCR (collectively known as rep-PCR) were used to generate genomic fingerprints of a variety of Xanthomonas and Pseudomonas isolates and to identify pathovars and strains that were previously not distinguishable by other classification methods. Analogous rep-PCR-derived genomic fingerprints were generated from purified genomic DNA, colonies on agar plates, liquid cultures, and directly from lesions on infected plants. REP, ERIC, and BOX-PCR-generated fingerprints of specific Xanthomonas and Pseudomonas strains were found to yield similar conclusions wtih regard to the identity of and relationship between these strains. This suggests that the distribution of REP-, ERIC, and BOX-like sequences in these strains is a refle...
Consumer demand for organically produced food and the desire by many farmers to eliminate chemica... more Consumer demand for organically produced food and the desire by many farmers to eliminate chemical fertilizers and pesticides is increasing the need for research and educational programs to support organic farmers. To date, the land-grant universities and the cooperative extension service have been viewed by organic farmers as unresponsive to this need. The primary reason for the unresponsiveness has been inadequate training and resource materials available to extension agents. In 1998, we conducted an intensive training for agriculture agents in North Carolina. Funding was provided by the USDA SARE Professional Development Program. More than 50 agents participated in a series of workshops that were offered together as a graduate course worth four NCSU credits. Much of the training was conducted on the Organic Unit at The Center for Environmental Farming Systems (CEFS), a 100-acre facility dedicated to research and education in organic farming systems. The hands-on training consiste...
Strawberry (Fragaria xananassa) plant phenology was evaluated at two sites in North Carolina in o... more Strawberry (Fragaria xananassa) plant phenology was evaluated at two sites in North Carolina in order to assess the relationship of plant growth and the development of gray mold fruit rot (Botrytis cinerea). Site 1 consisted of three cultivars: Camarosa (CA), Chandler (CH), and Sweet Charlie (SC) from a single nursery; site 2 consisted of cultivar CH obtained from three distinct sources. Weekly records were kept of leaf senescence, leaf emergence, flowering, and fruit set, as well as a monthly whole-plant analysis. Leaf senescence and emergence was uniform for all treatments, with >90% of transplant foliage senescing between November and December. Analysis of leaf area and plant weights reveal an increase in dry mass beginning in February and continuing through April. Although first bloom appeared in December, primary floral development occurred in late February for cultivar SC and in March for cultivars CA and CH, with blooms continuing through late May. Marketable yield was low...
Marker assisted selection (MAS) has not been initiated in tomato (Solanum lycopersicum L.) for se... more Marker assisted selection (MAS) has not been initiated in tomato (Solanum lycopersicum L.) for septoria leaf spot (SLS) resistance caused by Septoria lycopersici Speg due to lack of molecular markers. We studied the inheritance of SLS resistance and identified molecular markers linked to SLS resistance using bulked segregant analysis (BSA) in a segregating F2 population. Tomato inbred lines, NC 85L-1W (2007), susceptible to SLS and NC 839-2(2007)-1, resistant to SLS were used to develop the segregating population. A total of 250 F2 plants, and 10 plants each of P1, P2 and F1 were grown at the Mountain Horticultural Crops Research and Extension Center (MHCREC), Mills River NC in the summer of 2009. Disease severity was scored using a scale of 0 to 5, where 0 = no disease and 5 = complete development of disease. DNA was extracted from 2-3 week old plants and parental lines were screened with a total of 197 random amplified polymorphic DNA (RAPD) primers, of which 34 were polymorphic. Two DNA bulks, called resistant bulk (RB) and susceptible bulk (SB) were prepared from the F2 individuals. The RB and SB consisted of 8 individuals each with disease scores of 0, and 4.0 or 4.5, respectively. The segregation ratio of resistant and susceptible plants in F2 generation fit the expected Mendelian ratio of 3:1 for a single dominant gene. Five RAPD markers were linked to the SLS disease reaction, of which two were linked to susceptibility and three to the resistance. Subject to verification in independent populations, these markers may be useful for MAS of SLS resistance in tomato.
Bacterial spot (BS), caused by four species of Xanthomonas in tomato (Solanum lycopersicum L.), i... more Bacterial spot (BS), caused by four species of Xanthomonas in tomato (Solanum lycopersicum L.), is challenging to manage by chemical methods under favorable environmental conditions. Development of resistant genotypes is a desirable solution to limit losses. A total of 64 genotypes were planted in the field at the Mountain Horticultural Crops Research and Extension Center (MHCREC), NC in a randomized complete block design with two replications consisting of six plants per plot. The genotypes were inoculated with a local and prevalent strain of Xanthomonas and phenotypic data on foliar disease severity was scored using the Horsfall-Barratt (HB) scale and on fruit incidence. Foliar severity ranged from 0 to over 75% defoliation and fruit incidence ranged from 0% to 100% and these traits were not correlated. The response of tomato genotypes for Xanthomonas-specific pathogen associated molecular pattern (PAMP) molecules in production of reactive oxygen species (ROS) was also scored for ...
Tomato (Solanum lycopersium L.) is the second most important vegetable crop in the world. In the ... more Tomato (Solanum lycopersium L.) is the second most important vegetable crop in the world. In the U.S., fresh market tomato production farm value was $0.86 billion in 2012, with Florida and North Carolina (NC) producing 38.6% of national production (34.7% and 3.9% respectively). In both states (2007), 99% of tomato growing acres were harvested for fresh market. Bacterial Wilt (BW) caused by Ralstonia solanacearum (Rs) is a devastating disease of tomato in tropical and subtropical environments around the world with losses of 70-100% in the Southeast U.S. in heavily infested fields. Vegetable grafting of commercial scions onto disease resistant rootstocks has been developed as a viable management strategy for BW in tomato, particularly for fresh market. Bacterial wilt resistance in tomato rootstock germplasm, however, is quantitative and partial. A greenhouse resistance study using 10 variably resistant tomato rootstocks lines (not grafted; mixture of open-pollinated and commercial hyb...
Recent Developments in Management of Plant Diseases, 2009
Major shifts in agricultural practices are complex with highly interdependent biological, environ... more Major shifts in agricultural practices are complex with highly interdependent biological, environmental, social, economic, business, and other (agri)culture factors. Seeking alternatives to methyl bromide (MeBr) to manage soilborne pathogens provided a model system that could be simulated where other large scale and dramatic changes need to be made. Parallel priorities were set in place. The first priority was to assemble an inter-disciplinary and interstate response and vision team of key private and public sector stakeholders. The second priority was to determine the risk-aversion and biological basis for fumigation. The third priority was to implement a plan that did not simply focus on chemical alternatives, but sought to advance the science of plant pathology and conduct discovery research about the biology, ecology and management of the primary plant pathogens and cropping systems. Therefore, three strategic levels of research and extension were identified: (1) Tactic substitution-addressing short term needs of growers who sought non-ozone depleting fumigant alternatives; (2) Tactic Diversification-focused on medium term alternatives that included non-fumigant and IPM based tactics; (3) Tactic Development-focused on long-term goals to explore microbial ecology and farming systems-based approaches to replace MeBr-dependent production systems. The fourth priority was to effectively extend research based information to primary clientele. Combined efforts resulted in technically and economical feasibility assessments of alternatives, exploration of viable diversification and development of plant disease management tactics, and a region-wide advanced understanding of the biology and ecology of key plant pathogens.
Four hundred thirty-three xanthomonad strains isolated from tomato or pepper plants from 32 diffe... more Four hundred thirty-three xanthomonad strains isolated from tomato or pepper plants from 32 different fields in four Caribbean and Central American countries were screened for the ability to hydrolyze starch and sodium polypectate and for resistance to copper and streptomycin. Of these, 95 representative strains were further characterized by various phnetic tests, and 63 of these strains were then analyzed by genomic fingerprinting. Most of the strains (>90%) were tolerant to copper. However, there was much more variability in sensitivity to streptomycin. All strains in Guadeloupe and 93% of the strains in Barbados were sensitive to streptomycin. The majority of strains were typical Xanthomonas campestris pv. vesicatoria group A strains. In Barbados, however, a unique group of strains was identified that was serologically similar to group A strains but was amylolytic. These strains were designated A1. The occurrence of X. campestris pv. vesicatoria group B strains in Central Amer...
▪   The advent of molecular biology in general and the polymerase chain reaction in particular ha... more ▪   The advent of molecular biology in general and the polymerase chain reaction in particular have greatly facilitated genomic analyses of microorganisms, provide enhanced capability to characterize and classify strains, and facilitate research to assess the genetic diversity of populations. The diversity of large populations can be assessed in a relatively efficient manner using rep-PCR-, AFLP-, and AP-PCR/RAPD-based genomic fingerprinting methods, especially when combined with computer-assisted pattern analysis. Genetic diversity maps provide a framework to understand the taxonomy, population structure, and dynamics of phytobacteria and provide a high-resolution framework to devise sensitive, specific, and rapid methods for pathogen detection, plant disease diagnosis, as well as management of disease risk. A variety of PCR-based fingerprinting protocols such as rDNA-based PCR, ITS-PCR, ARDRA, T-RFLPs, and tRNA-PCR have been devised, and numerous innovative approaches using specif...
Organic and heirloom tomatoes are high-value products with growing demand but there are many chal... more Organic and heirloom tomatoes are high-value products with growing demand but there are many challenges to successful cultivation. A systems comparison study was carried out to evaluate the production of the popular heirloom tomato ‘Cherokee Purple’ (Solanum lycopersicum L.) under high tunnel and open field systems in North Carolina from 2007 to 2008. Management of the high tunnel (i.e., temperature and irrigation), weather events as well as pest and disease pressure influenced crop quality and yield. The high tunnel and field systems achieved similar total yields (100 t·ha−1) the first season but yields were 33% greater in the high tunnel system than the field system in the second year (100 t·ha−1 and 67 t·ha−1, respectively). Both years, the tomatoes were planted in high tunnels 1 month earlier and harvested 3 weeks earlier than the field. The accumulation of ≈1100 growing degree-days (GDD) was required in both systems before 50% of the fruit was harvested. Fruit cracking, cat-fac...
Partial budget analysis was used to evaluate soil treatment alternatives to methyl bromide (MeBr)... more Partial budget analysis was used to evaluate soil treatment alternatives to methyl bromide (MeBr) based on their cost-effectiveness in the production of strawberries (Fragaria ×ananassa). The analysis was conducted for two geographical areas: the piedmont and coastal plain area (including North Carolina and Georgia) and the mountain area of western North Carolina, based on 7 years of field test data. The fumigation alternatives evaluated were Telone-C35 (1,3-dichloropropene 61.1% + chloropicrin 34.7%), Telone II (1,3-dichloropropene 94%), chloropicrin (Chlor-o-pic 99% and TriClor EC), InLine (1,3-dichloropropene 60.8% + chloropicrin 33.3%), and metam sodium (Vapam or Sectagon 42, 42% sodium methyldithiocarbamate). The MeBr formulation was 67% MeBr and 33% chloropicrin (Terr-O-Gas) with the exception of the earlier trials where a 98:2 ratio was used. In the piedmont and coastal plain area, the soil treated with chloropicrin showed the best results with an additional return of $1670/a...
Reactive oxygen species (ROS) made by plants in response to pathogen infection not only initiate ... more Reactive oxygen species (ROS) made by plants in response to pathogen infection not only initiate local and systemic defenses, they are also antimicrobial. A number of fungi are hypothesized to secrete the antioxidant mannitol to protect against this antimicrobial ROS during infection. This hypothesis is supported by reports that overexpression of the mannitol catabolic enzyme mannitol dehydrogenase (MTD) in plants increases resistance to mannitol-secreting pathogens like Botrytis cinerea and Alternaria alternata. To extend this hypothesis and test the general utility of this approach, we overexpressed celery MTD in a tomato breeding line (NC1 Grape) currently used in our program. Although we reported earlier that MTD overexpression provides resistance to Botrytis gray mold in a greenhouse tomato, this is the first report of overexpression in an elite breeding variety providing heritable, whole-plant resistance to A. solani (tomato early blight). In this study, progeny from a high-MT...
The oxidative burst is a critical early event in plant-pathogen interactions that leads to a loca... more The oxidative burst is a critical early event in plant-pathogen interactions that leads to a localized, programmed cell death (PCD) called the hypersensitive response (HR). The HR and associated PCD retard infection by biotrophic pathogens, but can, in fact, enhance infection by necrotrophic pathogens like Botrytis cinerea. In addition to signaling the induction of the HR, reactive oxygen species (ROS) produced during the oxidative burst are antimicrobial. We hypothesize that pathogens such as B. cinerea survive the antimicrobial effects of ROS, at least partially by secreting the antioxidant mannitol during infection. This is supported by the previous observation that overexpression of the catabolic enzyme mannitol dehydrogenase (MTD) can decrease a plants susceptibility to mannitol-secreting pathogens like B. cinerea. To extend the above hypothesis, and test the general utility of this approach in an important horticultural crop, we overexpressed celery MTD in tomato (Solanum lycopersicum cv. "Moneymaker"). In these studies, we observed a significant increase (up to 90%) in resistance to B. cinerea in transgenic tomatoes expressing high amounts of MTD.
In order to demonstrate that allelopathic interactions are occurring, one must, among other thing... more In order to demonstrate that allelopathic interactions are occurring, one must, among other things, demonstrate that putative phytotoxins move from plant residues on or in the soil, the source, through the bulk soil to the root surface, a sink, by way of the rhizosphere. We hypothesized that the incorporation of phytotoxic plant residues into the soil would result in a simultaneous inhibition of seedling growth and a stimulation of the rhizosphere bacterial community that could utilize the putative phytotoxins as a sole carbon source. If true and consistently expressed, such as relationship would provide a means of establishing the transfer of phytotoxins from residue in the soil to the rhizosphere of a sensitive species under field conditions. Presently, direct evidence for such transfer is lacking. To test this hypothesis, cucumber seedlings were grown in soil containing various concentrations of wheat or sunflower tissue. Both tissue types contain phenolic acids, which have been ...
DNA primers corresponding to conserved motifs in bacterial repetitive (REP, ERIC, and BOX) elemen... more DNA primers corresponding to conserved motifs in bacterial repetitive (REP, ERIC, and BOX) elements and PCR were used to show that REP-, ERIC-, and BOX-like DNA sequences are widely distributed in phytopathogenic Xanthomonas and Pseudomonas strains. REP-, ERIC, and BOX-PCR (collectively known as rep-PCR) were used to generate genomic fingerprints of a variety of Xanthomonas and Pseudomonas isolates and to identify pathovars and strains that were previously not distinguishable by other classification methods. Analogous rep-PCR-derived genomic fingerprints were generated from purified genomic DNA, colonies on agar plates, liquid cultures, and directly from lesions on infected plants. REP, ERIC, and BOX-PCR-generated fingerprints of specific Xanthomonas and Pseudomonas strains were found to yield similar conclusions wtih regard to the identity of and relationship between these strains. This suggests that the distribution of REP-, ERIC, and BOX-like sequences in these strains is a refle...
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