Compatibility of systemic acquired resistance and microbial biocontrol for suppression of plant disease in a laboratory assay

Systemic acquired resistance (SARI and microbial biocontrol each hold promise as alternatives to pesticides for control of plant diseases. SAR and Bacillus cereus UW85, a microbial biocontrol agent, separately suppress seedling damping-off diseases caused by oomycete pathogens. The purposes of this study were to investigate how expression of SAR affected the efficacy of biocontrol by UW85 and if U W 8 5 treatment of plants induced SAR. We devised a laboratory assay in which seedling damping-off disease, induction of SAR, and growth of UW85 could be quantified. Seedlings of Nicotiana tabacum Xanthi nc were germinated on moist filter paper and transferred after 7 days to water agar plates (40 seedlings per plate). Zoospores of oomycete pathogens (Pythium tomlosum, Pythium aphanidermafum, or Phytophthora parasitical were applied at concentrations that caused = 80% seedling mortality within 10 days. Seedling mortality was dependent on zoospore inoculum concentration. The level of disease suppression caused by treatment with UW85 depended on the UWS5 dose applied. SAR was induced with 0.5-m~ salicylic acid or O.l-mM 2,6-dichloroisonicotc acid. Expression of an SAR-related gene was confirmed by northern analysis with a probe prepared from a tobacco PR-la cDNA. Induction of SAR suppressed disease caused by each of the oomycete pathogens, but did not alter the growth of W 8 5 on roots. Treatment of seedlings with UW85 did not induce the expression of PR-la. The combination of induction of S A R and treatment with UW85 resulted in additive suppression of disease as measured by seedling survivaI.