Gary Vallad - Academia.edu (original) (raw)

Papers by Gary Vallad

Research paper thumbnail of Characterization and elimination of linkage-drag associated with Fusarium wilt race 3 resistance genes

Theoretical and Applied Genetics

Key message Reducing the size of the I-3 introgression resulted in eliminating linkage-drag contr... more Key message Reducing the size of the I-3 introgression resulted in eliminating linkage-drag contributing to increased sensitivity to bacterial spot and reduced fruit size. The I-7 gene was determined to have no effect on bacterial spot or fruit size, and germplasm is now available with both the reduced I-3 introgression and I-7. Abstract Tomato (Solanum lycopersicum) production is increasingly threatened by Fusarium wilt race 3 (Fol3) caused by the soilborne fungus, Fusarium oxysporum f. sp. lycopersici. Although host resistance based on the I-3 gene is the most effective management strategy, I-3 is associated with detrimental traits including reduced fruit size and increased bacterial spot sensitivity. Previous research demonstrated the association with bacterial spot is not due to the I-3 gene, itself, and we hypothesize that reducing the size of the I-3 introgression will remedy this association. Cultivars with I-7, an additional Fol3 resistance gene, are available but are not wi...

Research paper thumbnail of Integrated Management of Southern Blight in Vegetable Production

EDIS

PP272, a 6-page illustrated fact sheet by Chenzhao Xie and Gary Vallad, guides vegetable growers ... more PP272, a 6-page illustrated fact sheet by Chenzhao Xie and Gary Vallad, guides vegetable growers to recognize and manage this serious fungal disease affecting diverse crops grown around the world, especially in tropical and subtropical regions — symptoms and signs, pathogen biology, disease cycle, epidemiology, and integrated management. Includes references. Published by the UF Department of Plant Pathology, March 2010.

Research paper thumbnail of Whole genome sequences reveal the Xanthomonas perforans population is shaped by the tomato production system

The ISME Journal, 2021

Modern agricultural practices increase the potential for plant pathogen spread, while the advent ... more Modern agricultural practices increase the potential for plant pathogen spread, while the advent of affordable whole genome sequencing enables in-depth studies of pathogen movement. Population genomic studies may decipher pathogen movement and population structure as a result of complex agricultural production systems. We used whole genome sequences of 281 Xanthomonas perforans strains collected within one tomato production season across Florida and southern Georgia fields to test for population genetic structure associated with tomato production system variables. We identified six clusters of X. perforans from core gene SNPs that corresponded with phylogenetic lineages. Using whole genome SNPs, we found genetic structure among farms, transplant facilities, cultivars, seed producers, grower operations, regions, and counties. Overall, grower operations that produced their own transplants were associated with genetically distinct and less diverse populations of strains compared to gro...

Research paper thumbnail of Breeding for Resistance to Fusarium Wilt of Tomato: A Review

Genes, 2021

For over a century, breeders have worked to develop tomato (Solanum lycopersicum) cultivars with ... more For over a century, breeders have worked to develop tomato (Solanum lycopersicum) cultivars with resistance to Fusarium wilt (Fol) caused by the soilborne fungus Fusarium oxysporum f. sp. lycopersici. Host resistance is the most effective strategy for the management of this disease. For each of the three Fol races, resistance has been introgressed from wild tomato species, predominately in the form of R genes. The I, I-2, I-3, and I-7 R genes have each been identified, as well as the corresponding Avr effectors in the fungus with the exception of Avr7. The mechanisms by which the R gene protein products recognize these effectors, however, has not been elucidated. Extensive genetic mapping, gene cloning, and genome sequencing efforts support the development of tightly-linked molecular markers, which greatly expedite tomato breeding and the development of elite, Fol resistant cultivars. These resources also provide important tools for pyramiding resistance genes and should support the...

Research paper thumbnail of Assessing Changes and Associations in theXanthomonas perforansPopulation Across Florida Commercial Tomato Fields Via a Statewide Survey

Phytopathology®, 2021

Before 1991, Xanthomonas euvesicatoria was the causal agent of bacterial spot of tomato in Florid... more Before 1991, Xanthomonas euvesicatoria was the causal agent of bacterial spot of tomato in Florida but was quickly replaced by X. perforans. The X. perforans population has changed in genotype and phenotype despite lack of a clear selection pressure. To determine the current Xanthomonas population in Florida, we collected 585 Xanthomonas strains from 70 tomato fields, representing 22 farms across eight counties, in the Florida tomato production region. Strains were isolated from 23 cultivars across eight seed producers and were associated with eight transplant facilities during the fall 2017 season. Our collection was phenotypically and genotypically characterized. Only X. perforans was identified, and all strains except one (99.8%) were tolerant to copper sulfate and 25% of strains were resistant to streptomycin sulfate. Most of the strains (99.3%) that were resistant to streptomycin sulfate were sequence type 1. The X. perforans population consisted of tomato races 3 (8%) and 4 (9...

Research paper thumbnail of Fusarium Wilt (Fusarium oxysporum f. sp. niveum) of Watermelon

EDIS, 2019

Fusarium wilt of watermelon is one of the most serious and difficult diseases to manage and occur... more Fusarium wilt of watermelon is one of the most serious and difficult diseases to manage and occurs in most production regions worldwide. The fungus can be seedborne and has great longevity in the soil, allowing infested soil to also serve as a source of infection. This new 4-page publication of the UF/IFAS Plant Pathology Department signs, symptoms, and the disease cycle of Fusarium wilt and provides recommendations for cultural and chemical management. Written by Pamela Roberts, Nicholas Dufault, Robert Hochmuth, Gary Vallad, and Mathews Paret. https://edis.ifas.ufl.edu/pp352

Research paper thumbnail of Iceberg Lettuce Breeding Lines with Resistance to Verticillium Wilt Caused by Race 1 Isolates of Verticillium dahliae

HortScience, 2011

, announce the release of two breeding lines of lettuce (Lactuca sativa L.). Lines RH08-0472 and ... more , announce the release of two breeding lines of lettuce (Lactuca sativa L.). Lines RH08-0472 and RH08-0475 are F 9 iceberg-type lettuce breeding lines with resistance to verticillium wilt caused by V. dahliae. They were selected from the cross 'Tiber' • ('La Brillante' • 'Pacific'). Resistance is derived from 'La Brillante', a Batavia-type lettuce cultivar with resistance to Race 1 isolates of V. dahliae. These breeding lines are suitable for commercial production in both V. dahliaeinfested and non-infested fields. They are being released for commercialization of verticillium wilt-resistant iceberg cultivars and for use as parents in further breeding of resistant cultivars.

Research paper thumbnail of Association of the Fusarium Wilt Race 3 Resistance Gene, I-3, on Chromosome 7 with Increased Susceptibility to Bacterial Spot Race T4 in Tomato

Journal of the American Society for Horticultural Science, 2014

Bacterial spot of tomato (Solanum lycopersicum), caused by several Xanthomonas species, is one of... more Bacterial spot of tomato (Solanum lycopersicum), caused by several Xanthomonas species, is one of the most important diseases of the crop in humid production regions of the world. Conventional breeding approaches for resistance to bacterial spot previously identified race-specific resistances, but current efforts also seek to use quantitative trait loci (QTLs) effecting broad-spectrum resistance. Resistance QTLs and candidate QTLs have been reported on several chromosomes, including a major QTL on chromosome 11. Fusarium wilt (Fusarium oxysporum f. sp. lycopersici) race 3 resistance gene, I-3, is associated with smaller fruit size and has been implicated in other associations with negative characteristics. We evaluated four F2 populations involving the bacterial spot-tolerant breeding lines Fla. 8517, Fla. 8233, and Fla. 8326 across two field seasons to validate and quantify previously identified loci and to test for an effect of I-3 on bacterial spot sensitivity. The chromosome 11 ...

Research paper thumbnail of A Novel Phylogroup of Pseudomonas cichorii Identified Following an Unusual Disease Outbreak on Tomato

Phytopathology, Jan 15, 2017

Recently, in Central Florida tomato production fields, tomato foliage and fruit were observed wit... more Recently, in Central Florida tomato production fields, tomato foliage and fruit were observed with symptoms similar to bacterial speck. Fluorescent pseudomonads were consistently isolated and the strains were characterized by standard LOPAT tests, pathogenicity tests, and genetic characterization using 16S ribosomal RNA (rRNA) sequences and multilocus sequence analysis (MLSA) of conserved housekeeping genes. LOPAT test results indicated that the strains were likely Pseudomonas cichorii. These strains were pathogenic on tomato and were also pathogenic on lettuce, the host for the type strain of P. cichorii. Likewise, strains of P. cichorii isolated in Florida since the early 1980s from hosts other than tomato, along with the type strain, were also pathogenic on tomato. Genetic characterization using 16S rRNA and MLSA confirmed that the strains were most closely related to P. cichorii but varied significantly from the type strain. The Florida P. cichorii strains formed a separate phyl...

Research paper thumbnail of Plant foliar disease suppression mediated by composted forms of paper mill residuals exhibits molecular features of induced resistance

Physiological and Molecular Plant Pathology, 2003

Arabidopsis thaliana grown in soil from field plots amended with composted forms of paper mill re... more Arabidopsis thaliana grown in soil from field plots amended with composted forms of paper mill residuals (PMR) exhibited reduced symptoms of bacterial speck caused by Pseudomonas syringae pv. tomato (Pst) compared with plants grown in soil from field plots amended with a non-composted PMR or non-amended soils. Similar results were obtained with tomato (Lycopersicon esculentum Mill.). No relationship between foliar disease suppression and plant nutrition or stature was observed. In Arabidopsis, the reduction of foliar disease symptoms ranged between 34 and 65%, depending on the type of composted PMR amendment, and was associated with reduced Pst titers in planta. An Arabidopsis npr1 defense mutant and a NahG transgenic line, both of which exhibit disrupted systemic acquired resistance, were also disrupted in their suppression of Pst disease symptoms in composted PMR treatments. Arabidopsis grown in soil amended with composted PMR also displayed an increased expression of pathogenesis-related defense genes prior to pathogen inoculation. We conclude that plants grown in soils with composted PMR-amendments were more resistant to disease caused by Pst due to the induction of plant defenses, similar to systemic acquired resistance. The identity of the PMR elicitor(s) is as yet unknown, but was shown to be heat labile.

Research paper thumbnail of Organically Managed Soils Reduce Internal Colonization of Tomato Plants by Salmonella enterica Serovar Typhimurium

Phytopathology®, 2013

A two-phase experiment was conducted twice to investigate the effects of soil management on movem... more A two-phase experiment was conducted twice to investigate the effects of soil management on movement of Salmonella enterica Typhimurium in tomato plants. In the first phase, individual leaflets of 84 tomato plants grown in conventional or organic soils were dip inoculated two to four times before fruiting with either of two Salmonella Typhimurium strains (109 CFU/ml; 0.025% [vol/vol] Silwet L-77). Inoculated and adjacent leaflets were tested for Salmonella spp. densities for 30 days after each inoculation. Endophytic bacterial communities were characterized by polymerase chain reaction denaturing gradient gel electrophoresis before and after inoculation. Fruit and seed were examined for Salmonella spp. incidence. In phase 2, extracted seed were planted in conventional soil, and contamination of leaves and fruit of the second generation was checked. More Salmonella spp. survived in inoculated leaves on plants grown in conventional than in organic soil. The soil management effect on S...

Research paper thumbnail of Soil-based systemic delivery and phyllosphere in vivo propagation of bacteriophages

Research paper thumbnail of Bacterial spot of tomato and pepper: diverseXanthomonasspecies with a wide variety of virulence factors posing a worldwide challenge

Molecular Plant Pathology, 2015

Host range: Causes bacterial spot disease on plants belonging to the Solanaceae family, primarily... more Host range: Causes bacterial spot disease on plants belonging to the Solanaceae family, primarily tomato (Solanum lycopersicum), pepper (Capsicum annuum) and chilli peppers (Capsicum frutescens). Disease symptoms: Necrotic lesions on all above-ground plant parts. Distribution: Worldwide distribution of X. euvesicatoria and X. vesicatoria on tomato and pepper; X. perforans and X. gardneri increasingly being isolated from the USA, Canada, South America, Africa and Europe. A wide diversity within the bacterial spot disease complex, with an ability to cause disease at different temperatures, makes this pathogen group a worldwide threat to tomato and pepper production. Recent advances in genome analyses have revealed the evolution of the pathogen with a plethora of novel virulence factors. Current management strategies rely on the use of various chemical control strategies and sanitary measures to minimize pathogen spread through contaminated seed. Chemical control strategies have been a challenge because of resistance by the pathogen. Breeding programmes have been successful in developing commercial lines with hypersensitive and quantitative resistance. However, durability of resistance has been elusive. Recently, a transgenic approach has resulted in the development of tomato genotypes with significant levels of resistance and improved yield that hold promise. In this article, we discuss the current taxonomic status, distribution of the four species, knowledge of virulence factors, detection methods and strategies for disease control with possible directions for future research.

Research paper thumbnail of Mycelial Compatibility and Pathogenic Diversity Among Sclerotium rolfsii Isolates in the Southern United States

Plant Disease, 2014

Sclerotium rolfsii is a soilborne fungus that causes southern blight on a wide range of plants in... more Sclerotium rolfsii is a soilborne fungus that causes southern blight on a wide range of plants in tropical and subtropical regions of the world. Eighty-four isolates collected from Florida, Georgia, Louisiana, South Carolina, Texas, and Virginia were paired and assigned to 23 mycelial compatibility groups (MCGs), of which 11 MCGs consisted of a single isolate. Isolates within an MCG typically originated from different hosts and different geographical areas, with the exception of MCG 11. In all, 13 of the 15 isolates in MCG 11 originated from peanut in Georgia and Florida, while the other 2 isolates originated from potato in Virginia and from the ornamental Barlaeria cristata in Florida. Significant differences in the size and number of sclerotia produced in vitro existed between isolates from peanut and other hosts. Nineteen isolates representative of the most common MCGs (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 13, 14, 15, 16, 17, and 18) were tested for pathogenicity on tomato, pepper, and...

Research paper thumbnail of Dynamics of Verticillium Species Microsclerotia in Field Soils in Response to Fumigation, Cropping Patterns, and Flooding

Phytopathology®, 2015

Verticillium dahliae is a soilborne, economically significant fungal plant pathogen that persists... more Verticillium dahliae is a soilborne, economically significant fungal plant pathogen that persists in the soil for up to 14 years as melanized microsclerotia (ms). Similarly, V. longisporum is a very significant production constraint on members of the family Brassicaceae. Management of Verticillium wilt has relied on methods that reduce ms below crop-specific thresholds at which little or no disease develops. Methyl bromide, a broad-spectrum biocide, has been used as a preplant soil fumigant for over 50 years to reduce V. dahliae ms. However, reductions in the number of ms in the vertical and horizontal soil profiles and the rate at which soil recolonization occurs has not been studied. The dynamics of ms in soil before and after methyl bromide + chloropicrin fumigation were followed over 3 years in six 8-by-8-m sites in two fields. In separate fields, the dynamics of ms in the 60-cm-deep vertical soil profile pre- and postfumigation with methyl bromide + chloropicrin followed by var...

Research paper thumbnail of Characterization of race-specific interactions among isolates of Verticillium dahliae pathogenic on lettuce

Verticillium wilt, caused by Verticillium dahliae, poses a major threat to lettuce (Lactuca sativ... more Verticillium wilt, caused by Verticillium dahliae, poses a major threat to lettuce (Lactuca sativa) production in California. Incorporation of resistance into commercial lettuce cultivars offers the least expensive technique of sustaining production in infested areas. To test the breadth of the resistance identified in field experiments, a pair of susceptible ('Salinas' and 'Sniper') and resistant ('La Brillante' and 'Little Gem') lettuce cultivars were used as differentials and individually inoculated with 29 isolates of V. dahliae and two isolates of V. albo-atrum from several hosts, including lettuce, in replicated greenhouse experiments. The reactions of the four cultivars were determined based on the disease severity at maturity. None of the V. albo-atrum isolates or V. dahliae isolates from cruciferous hosts caused significant disease on lettuce. Both Salinas and Sniper were susceptible to many isolates of V. dahliae (21 of 23) from noncruciferous hosts, and the isolates varied in their overall virulence. However, of these, only three isolates caused significant disease on the resistant cvs. La Brillante and Little Gem. These three isolates also were distinct from the other V. dahliae isolates based on sequence data from the intergenic spacer (IGS) region of the nuclear ribosomal RNA gene, suggesting that they form a phylogenetically distinct subgroup that differs in virulence toward specific lettuce genotypes. Accordingly, isolates of V. dahliae virulent on all tested cultivars, including the resistant La Brillante and Little Gem, were designated as race 2, whereas those virulent only on the susceptible Salinas and Sniper were designated as race 1. Although a range of virulence among isolates has been described in other hosts, this is the first description of distinct virulence phenotypes in V. dahliae since a similar race structure was described in tomato in the 1960s.

Research paper thumbnail of Molecular Variation Among Isolates of Verticillium dahliae and Polymerase Chain Reaction-Based Differentiation of Races

Phytopathology®, 2010

Verticillium dahliae is a soilborne fungal pathogen that causes vascular wilt in a variety of eco... more Verticillium dahliae is a soilborne fungal pathogen that causes vascular wilt in a variety of economically important crops worldwide. There are two races of V. dahliae that infect tomato and lettuce. Although race-1-specific resistance has been identified in both tomato and lettuce, no resistant sources are available for race 2. Molecular analyses were employed to characterize the genetic variability and race structure of 101 isolates of V. dahliae from a variety of hosts, mainly from central and coastal California, and 10 isolates exotic to this area. Analyses of the 16 simple sequence repeat (SSR) markers illustrated that tomato subpopulations from central California were distinct relative to the marigold subpopulations. In contrast, cotton and olive isolates showed admixture with tomato isolates. Analyses of both the ribosomal DNA intergenic spacer regions and SSR markers revealed high genetic variability among isolates but were unable to delineate races of V. dahliae. However, a...

Research paper thumbnail of Genetic variance, coefficient of parentage, and genetic distance of six soybean populations

Theoretical and Applied Genetics, 1997

Plant breeders would like to predict which biparental populations will have the largest genetic v... more Plant breeders would like to predict which biparental populations will have the largest genetic variance. If the population genetic variance could be predicted using coefficient of parentage or genetic distance estimates based on molecular marker data, breeders could choose parents that produced segregating populations with a large genetic variance. Three biparental soybean {Glycine max (L.) Merr.} populations were developed by crossing parents that were closely related, based on pedigree relationships. Three additional biparental populations were developed by crossing parents that were assumed to be unrelated. The genetic variance of each population was estimated for yield, lodging, physiological maturity, and plant height. Coefficient of parentage was calculated for each pair of parents used to develop the segregating populations. Genetic distance was determined, based on the number of random amplified polymorphic markers (RAPD) that were polymorphic for each pair of parents. Genetic distance was not associated with the coefficient of parentage or the magnitude of the genetic variance. The genetic variance pooled across the three closely related populations was smaller than the genetic variance pooled across the three populations derived from crossing unrelated parents for all four traits that were evaluated.

Research paper thumbnail of Transgenic Resistance Confers Effective Field Level Control of Bacterial Spot Disease in Tomato

PLoS ONE, 2012

We investigated whether lines of transgenic tomato (Solanum lycopersicum) expressing the Bs2 resi... more We investigated whether lines of transgenic tomato (Solanum lycopersicum) expressing the Bs2 resistance gene from pepper, a close relative of tomato, demonstrate improved resistance to bacterial spot disease caused by Xanthomonas species in replicated multi-year field trials under commercial type growing conditions. We report that the presence of the Bs2 gene in the highly susceptible VF 36 background reduced disease to extremely low levels, and VF 36-Bs2 plants displayed the lowest disease severity amongst all tomato varieties tested, including commercial and breeding lines with host resistance. Yields of marketable fruit from transgenic lines were typically 2.5 times that of the non-transformed parent line, but varied between 1.5 and 11.5 fold depending on weather conditions and disease pressure. Trials were conducted without application of any copper-based bactericides, presently in wide use despite negative impacts on the environment. This is the first demonstration of effective field resistance in a transgenic genotype based on a plant R gene and provides an opportunity for control of a devastating pathogen while eliminating ineffective copper pesticides.

Research paper thumbnail of Variation for Resistance to Verticillium Wilt in Lettuce (Lactuca sativaL.)

Plant Disease, 2007

Host resistance offers the most cost-effective method of Verticillium wilt control in lettuce (La... more Host resistance offers the most cost-effective method of Verticillium wilt control in lettuce (Lactuca sativa). In 2004 and 2005, 107 and 22 lettuce cultivars, respectively, were screened for resistance in a field infested with Verticillium dahliae, and disease progress on resistant and susceptible cultivars was determined. Greenhouse experiments were conducted to evaluate 16 cultivars for resistance to a race 1 and a race 2 isolate. Significant differences for resistance were observed within cultivated lettuce. In susceptible cultivars, disease levels increased through the season, whereas disease in resistant cultivars remained constant. Resistance in greenhouse tests was dependant upon the race used. Seven cultivars were resistant to race 1, whereas all were susceptible to race 2. Cultivar reactions to race 1 in greenhouse and field experiments were correlated, indicating the utility of greenhouse evaluations. The identification of resistance in diverse lettuce types is beneficial...

Research paper thumbnail of Characterization and elimination of linkage-drag associated with Fusarium wilt race 3 resistance genes

Theoretical and Applied Genetics

Key message Reducing the size of the I-3 introgression resulted in eliminating linkage-drag contr... more Key message Reducing the size of the I-3 introgression resulted in eliminating linkage-drag contributing to increased sensitivity to bacterial spot and reduced fruit size. The I-7 gene was determined to have no effect on bacterial spot or fruit size, and germplasm is now available with both the reduced I-3 introgression and I-7. Abstract Tomato (Solanum lycopersicum) production is increasingly threatened by Fusarium wilt race 3 (Fol3) caused by the soilborne fungus, Fusarium oxysporum f. sp. lycopersici. Although host resistance based on the I-3 gene is the most effective management strategy, I-3 is associated with detrimental traits including reduced fruit size and increased bacterial spot sensitivity. Previous research demonstrated the association with bacterial spot is not due to the I-3 gene, itself, and we hypothesize that reducing the size of the I-3 introgression will remedy this association. Cultivars with I-7, an additional Fol3 resistance gene, are available but are not wi...

Research paper thumbnail of Integrated Management of Southern Blight in Vegetable Production

EDIS

PP272, a 6-page illustrated fact sheet by Chenzhao Xie and Gary Vallad, guides vegetable growers ... more PP272, a 6-page illustrated fact sheet by Chenzhao Xie and Gary Vallad, guides vegetable growers to recognize and manage this serious fungal disease affecting diverse crops grown around the world, especially in tropical and subtropical regions — symptoms and signs, pathogen biology, disease cycle, epidemiology, and integrated management. Includes references. Published by the UF Department of Plant Pathology, March 2010.

Research paper thumbnail of Whole genome sequences reveal the Xanthomonas perforans population is shaped by the tomato production system

The ISME Journal, 2021

Modern agricultural practices increase the potential for plant pathogen spread, while the advent ... more Modern agricultural practices increase the potential for plant pathogen spread, while the advent of affordable whole genome sequencing enables in-depth studies of pathogen movement. Population genomic studies may decipher pathogen movement and population structure as a result of complex agricultural production systems. We used whole genome sequences of 281 Xanthomonas perforans strains collected within one tomato production season across Florida and southern Georgia fields to test for population genetic structure associated with tomato production system variables. We identified six clusters of X. perforans from core gene SNPs that corresponded with phylogenetic lineages. Using whole genome SNPs, we found genetic structure among farms, transplant facilities, cultivars, seed producers, grower operations, regions, and counties. Overall, grower operations that produced their own transplants were associated with genetically distinct and less diverse populations of strains compared to gro...

Research paper thumbnail of Breeding for Resistance to Fusarium Wilt of Tomato: A Review

Genes, 2021

For over a century, breeders have worked to develop tomato (Solanum lycopersicum) cultivars with ... more For over a century, breeders have worked to develop tomato (Solanum lycopersicum) cultivars with resistance to Fusarium wilt (Fol) caused by the soilborne fungus Fusarium oxysporum f. sp. lycopersici. Host resistance is the most effective strategy for the management of this disease. For each of the three Fol races, resistance has been introgressed from wild tomato species, predominately in the form of R genes. The I, I-2, I-3, and I-7 R genes have each been identified, as well as the corresponding Avr effectors in the fungus with the exception of Avr7. The mechanisms by which the R gene protein products recognize these effectors, however, has not been elucidated. Extensive genetic mapping, gene cloning, and genome sequencing efforts support the development of tightly-linked molecular markers, which greatly expedite tomato breeding and the development of elite, Fol resistant cultivars. These resources also provide important tools for pyramiding resistance genes and should support the...

Research paper thumbnail of Assessing Changes and Associations in theXanthomonas perforansPopulation Across Florida Commercial Tomato Fields Via a Statewide Survey

Phytopathology®, 2021

Before 1991, Xanthomonas euvesicatoria was the causal agent of bacterial spot of tomato in Florid... more Before 1991, Xanthomonas euvesicatoria was the causal agent of bacterial spot of tomato in Florida but was quickly replaced by X. perforans. The X. perforans population has changed in genotype and phenotype despite lack of a clear selection pressure. To determine the current Xanthomonas population in Florida, we collected 585 Xanthomonas strains from 70 tomato fields, representing 22 farms across eight counties, in the Florida tomato production region. Strains were isolated from 23 cultivars across eight seed producers and were associated with eight transplant facilities during the fall 2017 season. Our collection was phenotypically and genotypically characterized. Only X. perforans was identified, and all strains except one (99.8%) were tolerant to copper sulfate and 25% of strains were resistant to streptomycin sulfate. Most of the strains (99.3%) that were resistant to streptomycin sulfate were sequence type 1. The X. perforans population consisted of tomato races 3 (8%) and 4 (9...

Research paper thumbnail of Fusarium Wilt (Fusarium oxysporum f. sp. niveum) of Watermelon

EDIS, 2019

Fusarium wilt of watermelon is one of the most serious and difficult diseases to manage and occur... more Fusarium wilt of watermelon is one of the most serious and difficult diseases to manage and occurs in most production regions worldwide. The fungus can be seedborne and has great longevity in the soil, allowing infested soil to also serve as a source of infection. This new 4-page publication of the UF/IFAS Plant Pathology Department signs, symptoms, and the disease cycle of Fusarium wilt and provides recommendations for cultural and chemical management. Written by Pamela Roberts, Nicholas Dufault, Robert Hochmuth, Gary Vallad, and Mathews Paret. https://edis.ifas.ufl.edu/pp352

Research paper thumbnail of Iceberg Lettuce Breeding Lines with Resistance to Verticillium Wilt Caused by Race 1 Isolates of Verticillium dahliae

HortScience, 2011

, announce the release of two breeding lines of lettuce (Lactuca sativa L.). Lines RH08-0472 and ... more , announce the release of two breeding lines of lettuce (Lactuca sativa L.). Lines RH08-0472 and RH08-0475 are F 9 iceberg-type lettuce breeding lines with resistance to verticillium wilt caused by V. dahliae. They were selected from the cross 'Tiber' • ('La Brillante' • 'Pacific'). Resistance is derived from 'La Brillante', a Batavia-type lettuce cultivar with resistance to Race 1 isolates of V. dahliae. These breeding lines are suitable for commercial production in both V. dahliaeinfested and non-infested fields. They are being released for commercialization of verticillium wilt-resistant iceberg cultivars and for use as parents in further breeding of resistant cultivars.

Research paper thumbnail of Association of the Fusarium Wilt Race 3 Resistance Gene, I-3, on Chromosome 7 with Increased Susceptibility to Bacterial Spot Race T4 in Tomato

Journal of the American Society for Horticultural Science, 2014

Bacterial spot of tomato (Solanum lycopersicum), caused by several Xanthomonas species, is one of... more Bacterial spot of tomato (Solanum lycopersicum), caused by several Xanthomonas species, is one of the most important diseases of the crop in humid production regions of the world. Conventional breeding approaches for resistance to bacterial spot previously identified race-specific resistances, but current efforts also seek to use quantitative trait loci (QTLs) effecting broad-spectrum resistance. Resistance QTLs and candidate QTLs have been reported on several chromosomes, including a major QTL on chromosome 11. Fusarium wilt (Fusarium oxysporum f. sp. lycopersici) race 3 resistance gene, I-3, is associated with smaller fruit size and has been implicated in other associations with negative characteristics. We evaluated four F2 populations involving the bacterial spot-tolerant breeding lines Fla. 8517, Fla. 8233, and Fla. 8326 across two field seasons to validate and quantify previously identified loci and to test for an effect of I-3 on bacterial spot sensitivity. The chromosome 11 ...

Research paper thumbnail of A Novel Phylogroup of Pseudomonas cichorii Identified Following an Unusual Disease Outbreak on Tomato

Phytopathology, Jan 15, 2017

Recently, in Central Florida tomato production fields, tomato foliage and fruit were observed wit... more Recently, in Central Florida tomato production fields, tomato foliage and fruit were observed with symptoms similar to bacterial speck. Fluorescent pseudomonads were consistently isolated and the strains were characterized by standard LOPAT tests, pathogenicity tests, and genetic characterization using 16S ribosomal RNA (rRNA) sequences and multilocus sequence analysis (MLSA) of conserved housekeeping genes. LOPAT test results indicated that the strains were likely Pseudomonas cichorii. These strains were pathogenic on tomato and were also pathogenic on lettuce, the host for the type strain of P. cichorii. Likewise, strains of P. cichorii isolated in Florida since the early 1980s from hosts other than tomato, along with the type strain, were also pathogenic on tomato. Genetic characterization using 16S rRNA and MLSA confirmed that the strains were most closely related to P. cichorii but varied significantly from the type strain. The Florida P. cichorii strains formed a separate phyl...

Research paper thumbnail of Plant foliar disease suppression mediated by composted forms of paper mill residuals exhibits molecular features of induced resistance

Physiological and Molecular Plant Pathology, 2003

Arabidopsis thaliana grown in soil from field plots amended with composted forms of paper mill re... more Arabidopsis thaliana grown in soil from field plots amended with composted forms of paper mill residuals (PMR) exhibited reduced symptoms of bacterial speck caused by Pseudomonas syringae pv. tomato (Pst) compared with plants grown in soil from field plots amended with a non-composted PMR or non-amended soils. Similar results were obtained with tomato (Lycopersicon esculentum Mill.). No relationship between foliar disease suppression and plant nutrition or stature was observed. In Arabidopsis, the reduction of foliar disease symptoms ranged between 34 and 65%, depending on the type of composted PMR amendment, and was associated with reduced Pst titers in planta. An Arabidopsis npr1 defense mutant and a NahG transgenic line, both of which exhibit disrupted systemic acquired resistance, were also disrupted in their suppression of Pst disease symptoms in composted PMR treatments. Arabidopsis grown in soil amended with composted PMR also displayed an increased expression of pathogenesis-related defense genes prior to pathogen inoculation. We conclude that plants grown in soils with composted PMR-amendments were more resistant to disease caused by Pst due to the induction of plant defenses, similar to systemic acquired resistance. The identity of the PMR elicitor(s) is as yet unknown, but was shown to be heat labile.

Research paper thumbnail of Organically Managed Soils Reduce Internal Colonization of Tomato Plants by Salmonella enterica Serovar Typhimurium

Phytopathology®, 2013

A two-phase experiment was conducted twice to investigate the effects of soil management on movem... more A two-phase experiment was conducted twice to investigate the effects of soil management on movement of Salmonella enterica Typhimurium in tomato plants. In the first phase, individual leaflets of 84 tomato plants grown in conventional or organic soils were dip inoculated two to four times before fruiting with either of two Salmonella Typhimurium strains (109 CFU/ml; 0.025% [vol/vol] Silwet L-77). Inoculated and adjacent leaflets were tested for Salmonella spp. densities for 30 days after each inoculation. Endophytic bacterial communities were characterized by polymerase chain reaction denaturing gradient gel electrophoresis before and after inoculation. Fruit and seed were examined for Salmonella spp. incidence. In phase 2, extracted seed were planted in conventional soil, and contamination of leaves and fruit of the second generation was checked. More Salmonella spp. survived in inoculated leaves on plants grown in conventional than in organic soil. The soil management effect on S...

Research paper thumbnail of Soil-based systemic delivery and phyllosphere in vivo propagation of bacteriophages

Research paper thumbnail of Bacterial spot of tomato and pepper: diverseXanthomonasspecies with a wide variety of virulence factors posing a worldwide challenge

Molecular Plant Pathology, 2015

Host range: Causes bacterial spot disease on plants belonging to the Solanaceae family, primarily... more Host range: Causes bacterial spot disease on plants belonging to the Solanaceae family, primarily tomato (Solanum lycopersicum), pepper (Capsicum annuum) and chilli peppers (Capsicum frutescens). Disease symptoms: Necrotic lesions on all above-ground plant parts. Distribution: Worldwide distribution of X. euvesicatoria and X. vesicatoria on tomato and pepper; X. perforans and X. gardneri increasingly being isolated from the USA, Canada, South America, Africa and Europe. A wide diversity within the bacterial spot disease complex, with an ability to cause disease at different temperatures, makes this pathogen group a worldwide threat to tomato and pepper production. Recent advances in genome analyses have revealed the evolution of the pathogen with a plethora of novel virulence factors. Current management strategies rely on the use of various chemical control strategies and sanitary measures to minimize pathogen spread through contaminated seed. Chemical control strategies have been a challenge because of resistance by the pathogen. Breeding programmes have been successful in developing commercial lines with hypersensitive and quantitative resistance. However, durability of resistance has been elusive. Recently, a transgenic approach has resulted in the development of tomato genotypes with significant levels of resistance and improved yield that hold promise. In this article, we discuss the current taxonomic status, distribution of the four species, knowledge of virulence factors, detection methods and strategies for disease control with possible directions for future research.

Research paper thumbnail of Mycelial Compatibility and Pathogenic Diversity Among Sclerotium rolfsii Isolates in the Southern United States

Plant Disease, 2014

Sclerotium rolfsii is a soilborne fungus that causes southern blight on a wide range of plants in... more Sclerotium rolfsii is a soilborne fungus that causes southern blight on a wide range of plants in tropical and subtropical regions of the world. Eighty-four isolates collected from Florida, Georgia, Louisiana, South Carolina, Texas, and Virginia were paired and assigned to 23 mycelial compatibility groups (MCGs), of which 11 MCGs consisted of a single isolate. Isolates within an MCG typically originated from different hosts and different geographical areas, with the exception of MCG 11. In all, 13 of the 15 isolates in MCG 11 originated from peanut in Georgia and Florida, while the other 2 isolates originated from potato in Virginia and from the ornamental Barlaeria cristata in Florida. Significant differences in the size and number of sclerotia produced in vitro existed between isolates from peanut and other hosts. Nineteen isolates representative of the most common MCGs (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 13, 14, 15, 16, 17, and 18) were tested for pathogenicity on tomato, pepper, and...

Research paper thumbnail of Dynamics of Verticillium Species Microsclerotia in Field Soils in Response to Fumigation, Cropping Patterns, and Flooding

Phytopathology®, 2015

Verticillium dahliae is a soilborne, economically significant fungal plant pathogen that persists... more Verticillium dahliae is a soilborne, economically significant fungal plant pathogen that persists in the soil for up to 14 years as melanized microsclerotia (ms). Similarly, V. longisporum is a very significant production constraint on members of the family Brassicaceae. Management of Verticillium wilt has relied on methods that reduce ms below crop-specific thresholds at which little or no disease develops. Methyl bromide, a broad-spectrum biocide, has been used as a preplant soil fumigant for over 50 years to reduce V. dahliae ms. However, reductions in the number of ms in the vertical and horizontal soil profiles and the rate at which soil recolonization occurs has not been studied. The dynamics of ms in soil before and after methyl bromide + chloropicrin fumigation were followed over 3 years in six 8-by-8-m sites in two fields. In separate fields, the dynamics of ms in the 60-cm-deep vertical soil profile pre- and postfumigation with methyl bromide + chloropicrin followed by var...

Research paper thumbnail of Characterization of race-specific interactions among isolates of Verticillium dahliae pathogenic on lettuce

Verticillium wilt, caused by Verticillium dahliae, poses a major threat to lettuce (Lactuca sativ... more Verticillium wilt, caused by Verticillium dahliae, poses a major threat to lettuce (Lactuca sativa) production in California. Incorporation of resistance into commercial lettuce cultivars offers the least expensive technique of sustaining production in infested areas. To test the breadth of the resistance identified in field experiments, a pair of susceptible ('Salinas' and 'Sniper') and resistant ('La Brillante' and 'Little Gem') lettuce cultivars were used as differentials and individually inoculated with 29 isolates of V. dahliae and two isolates of V. albo-atrum from several hosts, including lettuce, in replicated greenhouse experiments. The reactions of the four cultivars were determined based on the disease severity at maturity. None of the V. albo-atrum isolates or V. dahliae isolates from cruciferous hosts caused significant disease on lettuce. Both Salinas and Sniper were susceptible to many isolates of V. dahliae (21 of 23) from noncruciferous hosts, and the isolates varied in their overall virulence. However, of these, only three isolates caused significant disease on the resistant cvs. La Brillante and Little Gem. These three isolates also were distinct from the other V. dahliae isolates based on sequence data from the intergenic spacer (IGS) region of the nuclear ribosomal RNA gene, suggesting that they form a phylogenetically distinct subgroup that differs in virulence toward specific lettuce genotypes. Accordingly, isolates of V. dahliae virulent on all tested cultivars, including the resistant La Brillante and Little Gem, were designated as race 2, whereas those virulent only on the susceptible Salinas and Sniper were designated as race 1. Although a range of virulence among isolates has been described in other hosts, this is the first description of distinct virulence phenotypes in V. dahliae since a similar race structure was described in tomato in the 1960s.

Research paper thumbnail of Molecular Variation Among Isolates of Verticillium dahliae and Polymerase Chain Reaction-Based Differentiation of Races

Phytopathology®, 2010

Verticillium dahliae is a soilborne fungal pathogen that causes vascular wilt in a variety of eco... more Verticillium dahliae is a soilborne fungal pathogen that causes vascular wilt in a variety of economically important crops worldwide. There are two races of V. dahliae that infect tomato and lettuce. Although race-1-specific resistance has been identified in both tomato and lettuce, no resistant sources are available for race 2. Molecular analyses were employed to characterize the genetic variability and race structure of 101 isolates of V. dahliae from a variety of hosts, mainly from central and coastal California, and 10 isolates exotic to this area. Analyses of the 16 simple sequence repeat (SSR) markers illustrated that tomato subpopulations from central California were distinct relative to the marigold subpopulations. In contrast, cotton and olive isolates showed admixture with tomato isolates. Analyses of both the ribosomal DNA intergenic spacer regions and SSR markers revealed high genetic variability among isolates but were unable to delineate races of V. dahliae. However, a...

Research paper thumbnail of Genetic variance, coefficient of parentage, and genetic distance of six soybean populations

Theoretical and Applied Genetics, 1997

Plant breeders would like to predict which biparental populations will have the largest genetic v... more Plant breeders would like to predict which biparental populations will have the largest genetic variance. If the population genetic variance could be predicted using coefficient of parentage or genetic distance estimates based on molecular marker data, breeders could choose parents that produced segregating populations with a large genetic variance. Three biparental soybean {Glycine max (L.) Merr.} populations were developed by crossing parents that were closely related, based on pedigree relationships. Three additional biparental populations were developed by crossing parents that were assumed to be unrelated. The genetic variance of each population was estimated for yield, lodging, physiological maturity, and plant height. Coefficient of parentage was calculated for each pair of parents used to develop the segregating populations. Genetic distance was determined, based on the number of random amplified polymorphic markers (RAPD) that were polymorphic for each pair of parents. Genetic distance was not associated with the coefficient of parentage or the magnitude of the genetic variance. The genetic variance pooled across the three closely related populations was smaller than the genetic variance pooled across the three populations derived from crossing unrelated parents for all four traits that were evaluated.

Research paper thumbnail of Transgenic Resistance Confers Effective Field Level Control of Bacterial Spot Disease in Tomato

PLoS ONE, 2012

We investigated whether lines of transgenic tomato (Solanum lycopersicum) expressing the Bs2 resi... more We investigated whether lines of transgenic tomato (Solanum lycopersicum) expressing the Bs2 resistance gene from pepper, a close relative of tomato, demonstrate improved resistance to bacterial spot disease caused by Xanthomonas species in replicated multi-year field trials under commercial type growing conditions. We report that the presence of the Bs2 gene in the highly susceptible VF 36 background reduced disease to extremely low levels, and VF 36-Bs2 plants displayed the lowest disease severity amongst all tomato varieties tested, including commercial and breeding lines with host resistance. Yields of marketable fruit from transgenic lines were typically 2.5 times that of the non-transformed parent line, but varied between 1.5 and 11.5 fold depending on weather conditions and disease pressure. Trials were conducted without application of any copper-based bactericides, presently in wide use despite negative impacts on the environment. This is the first demonstration of effective field resistance in a transgenic genotype based on a plant R gene and provides an opportunity for control of a devastating pathogen while eliminating ineffective copper pesticides.

Research paper thumbnail of Variation for Resistance to Verticillium Wilt in Lettuce (Lactuca sativaL.)

Plant Disease, 2007

Host resistance offers the most cost-effective method of Verticillium wilt control in lettuce (La... more Host resistance offers the most cost-effective method of Verticillium wilt control in lettuce (Lactuca sativa). In 2004 and 2005, 107 and 22 lettuce cultivars, respectively, were screened for resistance in a field infested with Verticillium dahliae, and disease progress on resistant and susceptible cultivars was determined. Greenhouse experiments were conducted to evaluate 16 cultivars for resistance to a race 1 and a race 2 isolate. Significant differences for resistance were observed within cultivated lettuce. In susceptible cultivars, disease levels increased through the season, whereas disease in resistant cultivars remained constant. Resistance in greenhouse tests was dependant upon the race used. Seven cultivars were resistant to race 1, whereas all were susceptible to race 2. Cultivar reactions to race 1 in greenhouse and field experiments were correlated, indicating the utility of greenhouse evaluations. The identification of resistance in diverse lettuce types is beneficial...