Pacific Southwest

Title: Microbial- and isothiocyanate-mediated control of Phytophthora and Pythium species

Author: Cohen, M.F.; Yamamoto, E.; Condeso, E.; Anacker, B.L.; Rank, N.; Mazzola, M.

Date: 2008

Source: In: Frankel, Susan J.; Kliejunas, John T.; Palmieri, Katharine M., tech. coords. 2008. Proceedings of the sudden oak death third science symposium. Gen. Tech. Rep. PSW-GTR-214. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. pp. 337-340

Station ID: GTR-PSW-214

Description: Plant pathogens of the oomycete lineage share common susceptibilities to many biotic and abiotic stresses. We are investigating the potential of antagonistic bacteria, isothiocyanates, and mycophagous amoebae to control diseases caused by Phytophthora spp., including the etiologic agent of sudden oak death, Phytophthora ramorum (Rizzo and others 2005), and Pythium spp., which cause seedling root rotting diseases across the plant kingdom (Hendrix and Campbell 1973).

Antagonistic bacteria. Inoculation of plant growth media with surfactant-producing bacteria is an established method for preventing oomycete diseases in hydroponic cultures. For instance, infection of water hyacinth by Pythium ultimum can be controlled by Pseudomonas fluorescens strain SS101, which releases a cyclic lipopeptide surfactant that lyses oomycete zoospores (deSouza and others 2003). We sought to determine whether P. fluorescens SS101 treatment could be effective in non-hydroponic systems by investigating its capacity to control root infection by soil-borne Pythium spp. and leaf infection by P. ramorum.

Cells of strain SS101 and the surfactant-deficient mutant strain 17.18 were inoculated into orchard soil, with concomitant addition of soy flour to stimulate amplification of resident Pythium spp. populations. Twelve wheat seeds were sown per treatment. After 24 days eedlings were harvested and 10 1 cm root segments per seedling assayed for infection by Pythium spp. Observed infection frequencies of 0.83 percent and 1.7 percent in the SS101- and 17.18 treatments, respectively, were both significantly lower (P < 0.001) than the 26.7 percent infection frequency in the nontreated control, implying that the surfactant is not necessary for disease control. These results are consistent with our recent findings of protection against Pythium spp. root infection conferred by another surfactant-deficient mutant of strain SS101.⁵

Although we observed that pretreatment with strain SS101 can dramatically lower the incidence P. ramorum zoospore-mediated infection of detached bay and rhododendron leaves, the pretreatment was not effective at reducing disease incidence in field trials, most likely due to the diminution of bacterial inoculum on the leaf surface resulting from precipitation events.

Keywords: Oomycetes, Brassica seed meal, mycophagous amoeba, Pseudomonas fluorescens

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Citation

Cohen, M.F.; Yamamoto, E.; Condeso, E.; Anacker, B.L.; Rank, N.; Mazzola, M.  2008.  Microbial- and isothiocyanate-mediated control of Phytophthora and Pythium species.   In: Frankel, Susan J.; Kliejunas, John T.; Palmieri, Katharine M., tech. coords. 2008. Proceedings of the sudden oak death third science symposium. Gen. Tech. Rep. PSW-GTR-214. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. pp. 337-340.