Fire blight, caused by Erwinia amylovora, is a serious disease, which attacks plants within the family of Rosaceae. This bacterial disease causes major problems in the cultivation of pears (Pyrus communis) around the world. The occurrence of the disease is still geographically limited to certain areas in Sweden and neighbouring countries. However, increasing temperatures due to climate change and the relative unawareness of the public, augment the risk of its spread. Within the EU, there are no approved antibiotics for use in pear orchards. Within the species of P. communis, cultivars vary in their susceptibility to fire blight. Many scientists believe that new, resistant cultivars and rootstocks are important tools to enable pome fruit cultivation in the future.In this study,the susceptibility of some Swedish pear heirloom cultivars to E. amylovora was investigated by using immature pear fruit. The bacteria were introduced into immature pear fruit by using defined concentrations of inoculum. The fruit were incubated at 25°C, and disease severity was recorded over time. Disease was measured and recorded as the extent of lateral lesion size on the surfaces of the pear fruit. No definite differences in disease development were recorded across the concentrations tested. The cultivars were compared to one another, and ranked according to susceptibility. The most susceptible to the least susceptible were: `Sollerö´ > `Unknown Gävle´ = `Esperens herre´ = `Aspa´ > `Höstbergamott´ > `Göteborgs Diamant´ > `Conference´ = `Lilla dalpilen´ = `Carola´ > `Alexander Lukas´ = `Blodpäron´ = `Bonne Louise´. The results could have become clearer if immature fruit of younger physiological age had been used. Nevertheless, the ranking of some cultivars agrees with the published results of others.

Cotoneaster is an ornamental shrub valued for showy flowers, berries and architecture as well as the ability tolerate adverse conditions under which other taxa fail. Cotoneaster is a highly diverse genus of over 400 species, of which few are available in the US nursery trade. Some species commercially available have been identified as potentially invasive in the state of Oregon and also are susceptible to the bacterial disease fire blight. Cotoneaster selections with reduced fertility and disease resistance would be desirable characteristics for low input landscape plants. A goal of my research was to characterize Cotoneaster spp. to provide information for the rationale planning and development of novel clones to meet these horticultural goals. In the first study, genome sizes were estimated using flow cytometery and ploidy levels were inferred using holoploid genome size. Observed differences in monoploid genome sizes translate to a difference in chromosome size. Differences in chromosome size may present a reproductive barrier when they are large. This may pose a challenge in wide crosses, but may be utilized to achieve sterility in the F1 interspecific hybrid population. Differences in genome size are not related to taxonomic ordering, so wide inter sectional and inter subgeneric crosses may be possible. In the second study, susceptibility of Cotoneaster to fire blight was measured on plants inoculated with Erwinia amylovora strain Ea153. In greenhouse assays conducted over two years, plants were inoculated by cutting leaves with scissors infested with the pathogen. Some species were 'highly susceptible' to fire blight where plants were killed to the ground, and others were rated 'highly resistant' and no lesions were observed. Seventeen accessions were rated as resistant to fire blight This research provides the first report of ploidy, genome sizes, and susceptibility of species of Cotoneaster to fire blight. Collectively this research provides a toolbox for a breeder to tackle the challenge of creating disease resistant cultivars with reduced fertility. / Graduation date: 2013

El foc bacterià és una malaltia que afecta a plantes de la família de la rosàcies, causada pel bacteri Erwinia amylovora. El seu rang d'hostes inclou arbres fruiters, com la perera, la pomera o el codonyer, i plantes ornamentals de gran interès comercial i econòmic. Actualment, la malaltia s'ha dispersat i es troba àmpliament distribuïda en totes les zones de clima temperat del món. A Espanya, on la malaltia no és endèmica, el foc bacterià es va detectar per primer cop al 1995 al nord del país (Euskadi) i posteriorment, han aparegut varis focus en altres localitzacions, que han estat convenientment eradicats. El control del foc bacterià, és molt poc efectiu en plantes afectades per la malaltia, de manera que es basa en mesures encaminades a evitar la dispersió del patogen, i la introducció de la malaltia en regions no endèmiques. En aquest treball, la termoteràpia ha estat avaluada com a mètode d'eradicació d'E. amylovora de material vegetal de propagació asimptomàtic. S'ha demostrat que la termoteràpia és un mètode viable d'eradicar E. amylovora de material de propagació. Gairebé totes les espècies i varietats de rosàcies mantingudes en condicions d'humitat sobrevivien 7 hores a 45 ºC i més de 3 hores a 50 ºC, mentre que més d'1 hora d'exposició a 50 ºC amb calor seca produïa danys en el material vegetal i reduïa la brotació. Tractaments de 60 min a 45 ºC o 30 min a 50 ºC van ser suficients per reduir la població epífita d'E. amylovora a nivells no detectables (5 x 102 ufc g-1 p.f.) en branques de perera. Els derivats dels fosfonats i el benzotiadiazol són efectius en el control del foc bacterià en perera i pomera, tant en condicions de laboratori, com d'hivernacle i camp. Els inductors de defensa de les plantes redueixen els nivells de malaltia fins al 40-60%. Els intervals de temps mínims per aconseguir el millor control de la malaltia van ser 5 dies pel fosetil-Al, i 7 dies per l'etefon i el benzotiadiazol, i les dosis òptimes pel fosetil-Al i el benzotiadiazol van ser 3.72 g HPO32- L-1 i 150 mg i.a. L-1, respectivament. Es millora l'eficàcia del fosetil-Al i del benzotiadiazol en el control del foc bacterià, quan es combinen amb els antibiòtics a la meitat de la dosi d'aquests últims. Tot i que l'estratègia de barrejar productes és més pràctica i fàcil de dur a terme a camp, que l'estratègia de combinar productes, el millor nivell de control de la malaltia s'aconsegueix amb l'estratègia de combinar productes. Es va analitzar a nivell histològic i ultrastructural l'efecte del benzotiadiazol i dels fosfonats en la interacció Erwinia amylovora-perera. Ni el benzotiadiazol, ni el fosetil-Al, ni l'etefon van induir canvis estructurals en els teixits de perera 7 dies després de la seva aplicació. No obstant, després de la inoculació d'E. amylovora es va observar en plantes tractades amb fosetil-Al i etefon una desorganització estructural cel·lular, mentre que en les plantes tractades amb benzotiadiazol aquestes alteracions tissulars van ser retardades. S'han avaluat dos models (Maryblyt, Cougarblight) en un camp a Espanya afectat per la malaltia, per determinar la precisió de les prediccions. Es van utilitzar dos models per elaborar el mapa de risc, el BRS-Powell combinat i el BIS95 modificat. Els resultats van mostrar dos zones amb elevat i baix risc de la malaltia. Maryblyt i Cougarblight són dos models de fàcil ús, tot i que la seva implementació en programes de maneig de la malaltia requereix que siguin avaluats i validats per un període de temps més llarg i en àrees on la malaltia hi estigui present. / Fire blight, caused by the bacterium Erwinia amylovora, is a serious disease of rosaceous plants that affects fruit trees such as pear, apple or quince, and ornamental plants with great commercial and economic interest. The disease is spread and well distributed in all temperate regions of the world. In Spain, where the disease is non endemic, fire blight was first detected in 1995 in the North of the country (Euskadi) and later, several new outbreaks have appeared in other locations that have been properly eradicated. Control of fire blight is very slightly effective in affected plants and is based on measures to avoid the spread of pathogen, and the introduction of disease in non-endemic regions. In this work, thermotherapy has been evaluated as a method for eradication of E. amylovora from symptomless propagating plant material. It has been demonstrated that heat is a viable method for eradicating E. amylovora from the propagation material of the pear. Almost all rosaceous species and cultivars maintained under moist conditions survived 7 hours at 45 ºC and up to 3 hours at 50 ºC, while more than 1 hour of exposure at 50 ºC under dry heat injured plants and reduced shooting. However, 60 min at 45 ºC or 30 min at 50 ºC were enough to reduce epiphytic E. amylovora population on pear budwoods to non-detectable level (5 x 102 cfu g-1 f.w.). Phosphonate derivatives and benzothiadiazole were effective in fire blight control in pear and apple, under laboratory, greenhouse and field conditions. Plant defense inducers reduced disease levels to 40-60%. The minimal time intervals to achieve the best control of disease were 5 days for fosetyl-Al, and 7 days for ethephon and benzothiadiazole, and the optimal doses of fosetyl-Al and benzothiadiazole were 3.72 g HPO32- L-1 and 150 mg a.i. L-1, respectively. The efficacy of fosetyl-Al and benzothiadiazole in fire blight control was improved when consecutively sprayed (combined strategy) with a half-reduced dose of antibiotics. Although the mixed strategy is more practical and easier to apply in the orchard than the combined one, the best level of fire blight control was achieved with the combined strategy. The effect of benzothiadiazole and phosphonates in Erwinia amylovora-pear interaction was analyzed at histological and ultrastructural level. Neither benzothiadiazole, nor fosetyl-Al, nor ethephon induced structural changes in pear leaf tissues 7 days after their application. However, after E. amylovora inoculation structural cell disorganization was observed in fosetyl-Al and ethephon-sprayed plants, while in benzothiadiazole-sprayed plants these tissue alterations were delayed. Two predictive models (Maryblyt and Cougarblight) were evaluated in an orchard naturally affected by fire blight in Spain, to determine the accuracy of the predictions. The combined BRS-Powell model and the modified BIS95 model were also evaluated. Results showed two clearly differentiated geographical areas with high and low fire blight risk. Maryblyt and Cougarblight are easy models to use, but their implementation in disease management programs must be evaluated and validated for more seasons and in areas where the disease is present.