COMPATIBILITY OF SYSTEMIC ACQUIRED-RESISTANCE AND MICROBIAL BIOCONTROL FOR SUPPRESSION OF PLANT-DISEASE IN A LABORATORY ASSAY

Systemic acquired resistance (SAR) and microbial biocontrol each hold promise as alternatives to pesticides for control of plant diseases. S AR and Bacillus cereus UW85, a microbial biocontrol agent, separately suppress seedling damping-off diseases caused by oomycete pathogens. T he purposes of this study were to investigate how expression of SAR af fected the efficacy of biocontrol by UW85 and if UW85 treatment of pla nts induced SAR. We devised a laboratory assay in which seedling dampi ng-off disease, induction of SAR, and growth of UW85 could be quantifi ed. Seedlings of Nicotiana tabacum Xanthi nc were germinated on moist filter paper and transferred after 7 days to water agar plates (40 see dlings per plate). Zoospores of oomycete pathogens (Pythium torulosum, Pythium aphanidermatum, or Phytophthora parasitica) were applied at c oncentrations that caused approximate to 80% seedling mortality within 10 days. Seedling mortality was dependent on zoospore inoculum concen tration. The level of disease suppression caused by treatment with UW8 5 depended on the UW85 dose applied. SAR was induced with 0.5-mM salic ylic acid or 0.1-mM 2,6-dichloroisonicotinic acid. Expression of an SA R-related gene was confirmed by northern analysis with a probe prepare d from a tobacco PR-1a cDNA. Induction of SAR suppressed disease cause d by each of the oomycete pathogens, but did not alter the growth of U W85 on roots. Treatment of seedlings with UW85 did not induce the expr ession of PR-1a. The combination of induction of SAR and treatment wit h UW85 resulted in additive suppression of disease as measured by seed ling survival.