Title: Within-field spread of Phytophthora ramorum on rhododendron in nursery settings
Author: Heungens, Kurt; De Dobbelaere, Isabelle; Gehesquière, Bjorn; Vercauteren, Annelies; Maes, Martine;
Source: In: Frankel, Susan J.; Kliejunas, John T.; Palmieri, Katharine M. 2010. In: Proceedings of the Sudden Oak Death Fourth Science Symposium. Gen. Tech. Rep. PSW-GTR-229. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. pp. 72-75
Publication Series: General Technical Report (GTR)
Description: In Europe, Phytophthora ramorum has mostly been detected on rhododendron plants in nurseries. European Union (EU) phytosanitary measures state that potential host plants within a radius of 2 m of an infected plant must be destroyed, and remaining host plants within a radius of 10 m must be quarantined. Despite the lack of research on the spread characteristics of P. ramorum in nurseries, these distances have been generally accepted.
To test the distances over which P. ramorum can spread between potted rhododendron plants in a nursery setting, dispersal was monitored from individual infected plants in a mock nursery. The distance over which the disease could spread was monitored in separate experiments during spring, summer, and fall seasons. Dispersal of the pathogen was mainly to neighboring plants, and plant-to-plant contact was an important factor for successful spread. Aerial detection of P. ramorum with a Burkard spore sampler was consistently negative. In contrast, pathogen spread via drain water films on the non-draining growing surface took place even at a distance of several meters. Splash dispersal from water films and direct inoculation during tipping over of plants seem to play a more important role in pathogen spread than aerial dispersal.
Disease spread was also monitored during the course of 2 seasons at a commercial nursery where a large number of findings of P. ramorum had been made during the EU-mandated survey. Plant parts with Phytophthora-like symptoms were collected at regular intervals, field coordinates of samples were taken, and the pathogen was isolated and identified. Out of more than 1300 samples, P. citricola was the most abundant Phytophthora species present, but a total of 281 P. ramorum-positive samples were identified. The location of P. ramorum findings was correlated with cultivar block within the field and did not show a clear pattern of focal spread. Microsatellite-based genotyping of the P. ramorum isolates revealed two sets of isolates with a unique genotype. Distance in the field between isolates of the same genotype was not limited to 10 m. Presence of P. ramorum in drain water was confirmed.
These data again suggest that aerial dispersal of P. ramorum is limited, but spread via drainage water can occur over several meters on the non-draining surfaces, and is the most likely means of spread among potted plants grown under such conditions. These data have implications for both quarantine measures and practical pest management.
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Heungens, Kurt; De Dobbelaere, Isabelle; Gehesquière, Bjorn; Vercauteren, Annelies; Maes, Martine. 2010. Within-Field Spread of Phytophthora ramorum on Rhododendron in Nursery Settings. In: Frankel, Susan J.; Kliejunas, John T.; Palmieri, Katharine M. 2010. In: Proceedings of the Sudden Oak Death Fourth Science Symposium. Gen. Tech. Rep. PSW-GTR-229. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. pp. 72-75
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