Plant innate immunity is activated upon pathogen attack by recognizing their avirulent (avr) genes by Resistant (R) genes leading to R-gene resistance or host resistance. Another form of innate immunity is non-host resistance that is exhibited by a given plant species to most strains of a microbial species. R-gene resistance activates salicylic acid (SA) that is synthesized from methyl salicylic acid (MeSA) by Salicylic Acid Binding Protein 2 (SABP2). It was hypothesized that SABP2 plays the similar role in non-host resistance also. Growth experiments and non-host related gene analysis experiments were conducted on tobacco plants using P.s tabaci and P.s. phaseolicola that are host and non-host pathogens on tobacco respectively. Tobacco control plant C3 that expresses SABP2 and 1-2 that is RNAi silenced in SABP2 expression were used in this study. Results suggest that SABP2 may not have any significant role in tobacco non-host resistance.

Strawberry (Fragaria x ananassa, Duch.) is an important crop in California, with more than 35,000 acres planted in 2018 resulting in a farm gate value of $3.1 billion. In 2020, California strawberry production accounted for more than 85% of national strawberry production and faces serious threats to production due to various soil-borne diseases. One such disease, Verticillium wilt, is caused by the fungal pathogen Verticillium dahliae and is commonly found in temperate zones around the world where strawberries are grown. Due to the phase-out of efficacious fumigants like methyl bromide, alternative disease management methods have become necessary to alleviate threats to production. Alternative fumigation practices such as crop termination have recently been investigated, and the integration of crop termination with bed fumigation and host resistance can play an integral role in control of Verticillium wilt. A field trial was established at California Polytechnic State University, San Luis Obispo to examine the efficacy of integrative management solutions for control of Verticillium wilt of strawberry in a naturally infested field. The efficacy of sequential fumigation applications of crop termination and bed fumigation was examined. Further, the integration of a resistant cultivar was also implemented in hopes of further decreasing plant mortality and increasing yield. Different fumigant products such as metam potassium, metam sodium, and chloropicrin were used to assess their performance in different fumigation applications. Metam potassium and metam sodium were used for crop termination. When used for crop termination, both products delivered significant reduction in soil inoculum density and adequate crop injury. Metam potassium, metam sodium, and chloropicrin were used for bed fumigation. All products reduced soil inoculum density. Lower plant mortality and higher yield resulted from sequential applications of crop termination and bed fumigation, with average plant mortality for non-treated control plots and sequentially fumigated plots being 67.2% and 24.1%, respectively. There were no significant increases in yield for plots bed fumigated and sequentially crop terminated and bed fumigated, but significant increases in yield for all plots treated versus the non-treated plots were found. The integration of a moderately resistant cultivar Valiant after the fumigation series showed lower mortality and higher yield versus a susceptible cultivar Seascape. A two-year study was also conducted in order to evaluate host resistance to Verticillium wilt in 74 cultivars and elite breeding lines from five strawberry breeding programs. Genotypes were established in a field naturally infested with V. dahliae on the campus farm at California Polytechnic State University, San Luis Obispo. All five breeding programs had a wide range of susceptibility to Verticillium wilt, ranging from 1.5% to 100% mortality for both years of the trial. Twenty-three cultivars and elite breeding lines were common to both years of the trial; of these, five cultivars showed vastly different results between the two years. For example, ‘Monterey’ showed 78.8% mortality in 2021 and 11.5% mortality in 2022. This demonstrates the importance of evaluating host resistance over multiple years under different environmental conditions and field locations.

Eastern hemlock hosts the hemlock woolly adelgid, an introduced sap-feeding insect that causes rapid deterioration of the host. Like most conifers, eastern hemlock produces a variety of constitutive and induced defenses, primarily terpenoids. To explore the relationship of terpenoid defenses with adelgid infestations, we artificially infested hemlocks at a forest site and a plantation site, and compared their terpenoid concentrations to those in control trees. Infested trees showed lower terpenoid concentrations than control trees, suggesting that eastern hemlock not only fails to induce production of terpenoids in response to adelgid infestation, but becomes less able to produce carbon-based defenses due to loss of carbon resources to the adelgid. Greater light intensity may account for consistently higher terpenoid concentrations at the plantation site, supporting the explanation that carbon limitation restricts terpenoid production. Recent studies have identified a small number of individual eastern hemlock trees that demonstrate relative resistance to the hemlock woolly adelgid. We compared concentrations of terpenoids in susceptible and relatively resistant trees, both in the forest and in propagated cuttings in a common-garden setting. Terpenoid concentrations were higher in twig tissue of resistant versus susceptible trees, across six sampling dates and at both sites. Because the common-garden cuttings were free of herbivores, the higher terpenoid concentrations are interpreted as a constitutive defense. Increased levels of monoterpenes and sesquiterpenes imply an overall increase in the input of carbon precursors to both terpenoid synthesis pathways. This result suggests either an altered growth-defense balance favoring allocation of carbon resources towards production of defenses, or overall greater carbon availability in growing twig tissue of adelgid-resistant eastern hemlock individuals. We contribute detailed terpenoid data to the study of the eastern hemlock – hemlock woolly adelgid system. Our solvent extraction method permits us to examine needle and twig tissues separately, capture minor components at low concentrations, and focus on stored rather than volatilized terpenoids. By relating terpenoid concentrations to insect densities, we explore the relationships of tentatively defensive chemistry to insect population dynamics. The question remains which terpenoids, if any, directly affect hemlock woolly adelgid and what role phenols may play in the system.

Developing a flue-cured tobacco cultivar with high resistance to tobacco cyst nematode (TCN) is an important initiative in the Southern Piedmont of Virginia, where this pathogen causes severe yield losses. One hundred twenty eight lines representing a diverse geographic array of tobacco accessions, including cultivars from several types of tobacco, flue-cured-type tobacco introductions, and wild Nicotiana species were evaluated for TCN resistance under greenhouse conditions. Inheritance of TCN resistance has been reported to be closely linked or pleiotropic to inheritance of wildfire resistance. Consequently, accessions were also screened for wildfire resistance under greenhouse conditions to evaluate this relationship among a diverse group of tobacco accessions. Twenty-one accessions were identified with resistance to TCN. Response to the two pathogens was highly correlated. However, there was no relationship between resistance to the two pathogens for several accessions. 'KY 190', a fire-cured cultivar, possessed the N. longiflora source of wildfire resistance, but was found susceptible to TCN. This result seems to rule out pleiotropy and is consistent with the assertion that the two resistance genes are closely linked. six TCN resistant lines and two susceptible lines were selected and a diallel study was conducted to determine the inheritance of resistance to TCN. Additive gene action contributed significantly to inheritance of TCN resistance. Three accessions, 'Burley 64', 'Kutsaga 110', and 'Tl 1597', were determined to be the most promising parents for use in a breeding program designed to develop a flue-cured cultivar with a high level of TCN resistance. / Master of Science

To improve understanding of the effectiveness of host resistance against Leptosphaeria spp., three aspects of effectiveness of resistance were investigated. With focus on the major Rlm-mediated resistance against L. maculans, changes in effectiveness of Rlm7-mediated resistance to prevent initiation of disease epidemics at the leaf spot stage were investigated in winter oilseed rape field experiments at five sites in the UK over the period with the cropping seasons 2009/2010 - 2013/2014. L. maculans isolates virulent against Rlm7 were identified in the UK. This may be associated with observed changes in lesion phenotypes on the Rlm7 cultivars in field conditions. However, despite increased severity of phoma leaf spotting on Rlm7 cultivars, there was no associated increase in phoma stem canker severity at the end of the cropping seasons. The effectiveness of winter oilseed rape cultivars for control of phoma stem canker (caused by L. maculans or L. biglobosa) was affected by the coexistence of the two Leptosphaeria species in oilseed rape crops. Weather conditions influenced ascospore release of both species and favoured L. biglobosa ascospore release in 2011, resulting in subsequent increased L. biglobosa phoma leaf spotting and stem canker severity. However, coexistence of Leptosphaeria spp. on oilseed rape crops was affected by the cultivar resistance against L. maculans. CE experiments showed that there were interactions between the two Leptosphaeria spp. in planta. Their coexistence on B. napus was influenced by the different host responses that they trigger during host colonisation. Effects of increased temperature on effectiveness of resistance against L. maculans and on severity of symptoms by Leptosphaeria spp. on B. napus were investigated. Increased temperature affected both Rlm4- and Rlm7-mediated resistance, when assessed by phenotypic and molecular techniques. Increased temperature was associated with increased symptom severity, for both L. maculans and L. biglobosa lesions on plants. Cultivar quantitative resistance background increased effectiveness of resistance against phoma stem canker pathogens at increased temperature and should be deployed in in strategies for adaptation to climate change to avoid increased phoma stem canker epidemics in the future.

No description available.

The production of strawberries can be severely limited by soilborne plant pathogens, insects and weeds. Macrophomina phaseolina is a problematic soilborne fungal pathogen in California strawberry production inciting the disease Macrophomina crown rot. When established, the pathogen can cause extensive plant decline and mortality. Host resistance will be a critical tool for managing this disease and guiding breeding programs in the post methyl bromide era. Evaluation of host resistance in strawberry germplasm to M. phaseolina was evaluated through phenotypic assessments of disease incidence. A total of 90 strawberry cultivars and elite selections were included in a replicated field trial conducted in artificially inoculated soils to assess host resistance. Significant differences in levels of resistance and susceptibility were observed among genotypes tested in this trial. The five most resistant strawberry genotypes from highest to lowest in percent plant mortality were: UC-R, UC-G, UC-V, Manresa, and Osceola. The five most susceptible strawberry genotypes with the highest percent mortality in ranking order from highest to lowest were: UC-J, Ruby June, Festival, UC-Y, and UC-A. Of the genotypes tested in this trial UC-V, Manresa and Osceola could be characterized as highly resistant, but no complete resistance was observed. An additional study was conducted to correlate host symptom expression with the extent of pathogen colonization in different strawberry tissues, and to determine if resistant germplasm can contribute to secondary inoculum production in the field. An established qPCR method was utilized to quantify M. phaseolina colonization of strawberry tissues. There were significant effects for cultivar (P < 0.0001) as well as a significant two-way interaction of cultivar x sample time (P= 0.0083) on the concentration of M. phaseolina DNA detected in strawberry tissues. Expression of the resistant phenotype in strawberry cultivars was associated with limited plant colonization by M. phaseolina. The extent of colonization of a specific cultivar by M. phaseolina was dependent on the sample time after inoculation with the pathogen. In addition, the roots and crowns of a specific strawberry cultivar were equally colonized on a per plant tissue weight basis, but this provides only speculation into the mechanisms conferring host resistance. A third study was conducted to integrate host resistance of strawberry genotypes with the use of organic amendments in effort to mutually enhance the efficacy of each factor for the control of Macrophomina crown rot. Artificially inoculated potting substrate was amended with Brassica juncea mustard seed meal at a rate of 4.94 tons ha-1(MSM), and anaerobic soil disinfestation utilizing rice bran at a rate of 22.24 tons ha-1 (ASD) were compared to a non-amended (UTC) and steam controls. The soil assay indicated that the ASD and steam treatments were able to reduce the CFU g-1 potting substrate of M. phaseolina by 99.7-100%. In addition, there were significant effects of soil treatment on the fresh biomass of weed seedlings recovered from the potting substrate. However, disease severity and host colonization of multiple strawberry cultivars by M. phaseolina was not reduced when grown in the treated potting substrate. The effect of strawberry cultivar on the extent of pathogen colonization was highly significant (P < 0.0001), in which cultivars characterized as resistant from phenotypic screenings possessed lower concentrations of M. phaseolina DNA. The suppression of M. phaseolina in response to organic amendments was limited but this study supports findings from previous experiments that genotype specific host resistance minimizes host colonization and reduces the production of secondary inoculum.

: Les plantes ont développé au cours de leur évolution un système d’immunité innée constitué de barrières préformées et de réponses de défense induites contre les agents pathogènes. Ce travail s’inscrit dans l’étude des résistances induites chez Arabidopsis thaliana soit naturellement contre les agents pathogènes racinaires Fusarium oxysporum spp. (Fo), ou après application de phosphite (Phi), contre le parasite foliaire Hyaloperonospora arabidopsidis (Hpa). Dans la première partie du travail, les rôles des métabolites secondaires (MS) et des formes réactives de l’oxygène (ROS) dans les résistances racinaires basale et non-hôte, respectivement aux formes spéciales conglutinans (Foco) et melonis (Fom) de Fo, ont été analysés. Nous avons démontré la participation de la camalexine, une phytoalexine indolique, dans la résistance basale d’Arabidopsis à Foco. En revanche, la scopolétine, une phytoalexine phénolique, et les ROS jouent des rôles essentiels dans la résistance non-hôte à Fom. Ces données soulignent le rôle clé des MS et des ROS dans les résistances hôte et non-hôte d’Arabidopsis. Dans une deuxième partie de ce travail, le mode d’action du Phi, un oxyanion de l’acide phosphoreux (H3PO3) protégeant Arabidopsis contre l’isolat Noco2 de Hpa, a été étudié. L’effet de faibles doses de Phi est aboli chez des mutants d’Arabidopsis affectés dans la voie de transduction du signal acide salicylique (SA) indiquant que l’induction de résistance à Hpa est médiée par des mécanismes dépendants du SA. Le Phi potentialise les réponses de défense contre Hpa Noco2 via EDS1-PAD4, deux composants essentiels de la résistance basale, NPR1 et la protéine de défense PR1. L’expression de la MAP kinase MPK4, un régulateur négatif de la résistance à Hpa, est diminuée par le Phi, lors de l’inoculation par Hpa Noco2. Nos résultats démontrent que la potentialisation des réponses de défense par le Phi est associée à la régulation négative de MPK4. / Plants have developed during their evolution an innate immunity system consisting of preformed barriers and induced defence responses against pathogens. This work studies resistances in Arabidopsis thaliana induced either naturally against the root pathogen Fusarium oxysporum spp. (Fo), or after application of phosphite (Phi) against the leaf pathogen Hyaloperonospora arabidopsidis (Hpa). In a first part, roles of secondary metabolites (SM) and reactive oxygen species (ROS) in basal and non-host resistances of roots to the special forms conglutinans (Foco) and melonis (Fom) of Fo, respectively, were analyzed. We demonstrated the involvement of the indolic phytoalexin camalexin, in basal resistance of Arabidopsis to Foco. In contrast, the phenolic phytoalexin, scopoletin, and ROS play essential roles in non-host resistance to Fom. These data underscore the key role of MS and ROS in basal and non-host resistances of Arabidopsis. In a second part, the mode of action of Phi, an oxyanion of phosphorous acid (H3PO3) protecting Arabidopsis against the Hpa isolate Noco2 was studied. Effect of low doses of Phi is abolished in Arabidopsis mutants affected in salicylic acid (SA) signalling, indicating that induced resistance to Hpa is mediated by SA-dependent mechanisms. Phi primes defence responses against Hpa Noco2 via EDS1-PAD4, two essential components of basal resistance, as well as NPR1 and PR1. Expression of the MAP kinase MPK4, a negative regulator of resistance to Hpa, is decreased by Phi after inoculation with Hpa Noco2. Our results demonstrate that priming of defence responses by Phi is associated with down-regulation of MPK4.

No description available.

No description available.