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Title: Comparing necrosis of Rhododendron leaf tissue inoculated with Phytophthora ramorum sporangia or zoospores

item Widmer, Timothy

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/26/2008
Publication Date: 12/20/2008
Citation: Widmer, T.L. 2008. Comparing necrosis of Rhododendron leaf tissue inoculated with Phytophthora ramorum sporangia or zoospores. Plant Disease. 93:30-35.

Interpretive Summary: The pathogen that causes Sudden Oak Death is also a problem on many ornamental plants such as Rhododendrons, Viburnums, and Camellias. There is a lot of research to screen different cultivars of trees and ornamentals for susceptibility to this disease. This pathogen produces different structures that enable the organism to spread and survive. One structure, called a sporangium, can either germinate directly and infect the host or release many motile structures that can swim to new infection sites, depending upon the environmental conditions. This study examines if the motile structures or direct germination of the sporangium is more important in development of disease symptoms. Results showed that the motile structures produced a higher number of infected leaves per Rhododendron plant and more disease symptoms on the infected leaves than the sporangium alone. Scientists will benefit from this research because it demonstrates the most efficient infective structure of the pathogen that should be used in future host screening tests that give the most reliable results. These results will then help the plant industry to design an effective breeding program using reliable results.

Technical Abstract: Phytophthora species produce sporangia that either germinate directly or release zoospores depending upon environmental conditions. Phytophthora ramorum is no exception producing abundant sporangia that are disseminated by wind and rain. Past research involving inoculation trials or screening host material have used both sporangia and zoospores as the inoculum type. However, it is unknown what impact a specific propagule type may have on disease. Rhododendron leaf disks were inoculated with zoospores (75, 500, or 2400 per disk), sporangia (75 per disk) or sporangia plus trifluoperazine hydrochloride (TFP) (75 per disk), a chemical that inhibits zoospore formation. The highest concentration of zoospores induced the highest percentage of necrosis (97 percent), followed by sporangia and 500 zoospores per disk (88 and 89 percent, respectively). The sporangia plus TFP treatment had the lowest necrosis at 48 percent. In another test, rooted rhododendron cuttings were inoculated with sporangia, with or without TFP (3000 per ml), cysts (50,000 per ml) and zoospores (3000 or 50,000 per ml). After 5 days in a dew chamber at 20 degrees C, the percentage of leaves per plant that developed necrosis was highest with the plants inoculated with the zoospores and cysts. The percentage of leaf necrosis was highest when the cysts and zoospores were inoculated at the 50,000 per ml rate followed by the sporangia alone and zoospores at 3000 per ml treatment. Leaves inoculated with sporangia plus TFP had the lowest necrosis. The difference in necrosis between inoculation with sporangia and inoculation with zoospores at 50,000 per ml suggest that not all sporangia release zoospores in planta. This demonstrates that the full inoculum potential may not be achieved when sporangia are used as the inoculum propagule.