Le développement de maladies d'origine tellurique relève de mécanismes épidémiologiques particuliers, propres au cycle biologique et aux traits d'histoire de vie des agents pathogènes incriminés. La caractérisation des processus qui déterminent le développement des épidémies est une étape clé dans la conception et l'optimisation de méthodes de protection contre ces maladies. Les cultures légumières de plein champ constituent des agrosystèmes particulièrement sensibles aux attaques parasitaires. Les Pythium sont responsables du cavity spot de la carotte, une maladie racinaire qui affecte la qualité des récoltes dans plusieurs bassins de production en France et dans le monde. L'objectif des recherches présentées ici est d'identifier, comprendre et hiérarchiser les processus déterminant la dynamique spatio-temporelle de cette maladie, essentiellement due à Pythium violae, en association avec d'autres espèces du complexe parasitaire. L'analyse de la composition d'un de ces complexes, complétée par la caractérisation biologique des principales espèces pathogènes, suggère que par souci de simplification, il est raisonnable de négliger les interactions entre espèces au cours de la phase infectieuse. L'existence d'infections secondaires (auto- et allo-infections) chez P. violae, et donc la nature polycyclique d'une épidémie de cavity spot, sont démontrées expérimentalement. Cette hypothèse est initialement étayée par deux étapes de l'analyse, l'une portant sur l'examen de relations pathométriques, et l'autre sur les ajustements de modèles aux données illustrant des cinétiques de la maladie. L'ensemble des résultats obtenus permet de concevoir un modèle épidémiologique exploratoire, basé sur l'occurrence des infections primaires et secondaires. Les effets de différents facteurs sur ces processus, comme l'application d'un fongicide, l'humidité du sol et la densité de semis, sont testés expérimentalement et discutés. Ils permettent d'envisager l'association de moyens de lutte complémentaires dans le cadre de stratégies de protection intégrée.

This study was undertaken to determine the sensitivity of grape downy mildew (DM, Plasmopara viticola) and powdery mildew (PM, Erysiphe necator) to commonly used single-site fungicides in Virginia and nearby states. DM and PM isolates were collected from 2005 to 2007. In grape leaf disc bioassays, 92% of the DM isolates were QoI (azoxystrobin)-resistant, but none were resistant to mefenoxam. Eighty-two percent of the PM isolates were QoI-resistant, but none were resistant to boscalid and quinoxyfen. The frequency of the G143A point mutation, which confers high levels of QoI resistance, was quantified in DM and PM isolates by real-time PCR. Most of the QoI-resistant DM and PM isolates contained >95% of the 143A allele. QoI-sensitive DM isolates contained less than 1% of 143A. One out of 145 and 14 out of 154 QoI-resistant DM and PM isolates (able to grow on azoxystrobin concentration ï ³ 1 µg/ml), respectively, contained less than 1% 143A. Most PM isolates exhibited reduced sensitivity to five DMI fungicides when compared to a sensitive subgroup (n=9) and compared to published reports for unexposed populations; the resistance factor (median EC50 of the entire isolate collection divided by median EC50 of sensitive subgroup) was highest for tebuconazole (360) and myclobutanil (350), followed by triflumizole (79), triadimefon (61), and fenarimol (53). Sensitivities to all five DMI fungicides, but also azoxystrobin, were moderately to strongly correlated (pairwise r-values ranging from 0.60 to 0.88). / Master of Science in Life Sciences

New Jersey, Georgia, and the Eastern Shore of Virginia (ESV) are important snap bean (Phaseolus vulgaris L.) growing regions, but profitability is threatened by Pythium blight. Causal agents of Pythium blight on snap bean were identified using morphological characterization and sequence analysis of the rDNA-internal transcribed spacer (ITS) regions of 100 isolates. Most isolates were Pythium aphanidermatum (Edson) Fitzp. (53%), and also included Pythium deliense Meurs (31%; all from Georgia), Pythium ultimum Trow (12%), Pythium myriotylum Drechsler (2%), Pythium catenulatum Matthews (1%), and unknown Pythium sp. (1%). To our knowledge, this is the first report of P. deliense in Georgia and on common bean and squash (Cucurbita pepo L.); as well as the first report of P. catenulatum on lima bean (Phaseolus lunatus L.) and in New Jersey. Fungicide labeling and cultivar selection for Pythium blight management is hindered by difficulties associated with conducting successful trials, because the disease occurs sporadically and clustered in the field. Three P. aphanidermatum-infested inoculum substrates were evaluated at three concentrations. The vermiculite/V8 juice (5:3 weight to volume) inoculum (10,000 ppg/0.3 m) consistently caused at least 50% disease in 3 field trials. Sensitivity of the Pythium blight pathogens was determined in vitro against five fungicides. Twenty-two Pythium isolates representing P. aphanidermatum, P. deliense, P. ultimum, and P. myriotylum were inoculated to media amended with each active ingredient at 0, 100μg/ml, the concentration equivalent to the field labeled rate if applied on succulent beans at 187 L/ha, and the equivalent if applied at 374 L/ha. All isolates were completely sensitive (100% growth reduction, or GR) to all active ingredients at the labeled rates, except azoxystrobin. At 100μg/ml azoxystrobin, one P. deliense isolate had 8.9% GR. All isolates had 100% GR to copper hydroxide at 100μg/ml, and the lowest GR on mefenoxam-amended medium was 91.9%. At 100μg/ml cyazofamid, all P. deliense isolates were completely sensitive and variation was observed in P. aphanidermatum isolates. At 100μg/ml potassium phosphite, significant GR similarities were recorded within isolates of the same species, and less than 50% GR was observed in all P. deliense isolates. / Ph. D.

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