We conducted dose-response studies of the toxicity of salicylate (SA; a putative signal molecule for enhancement of plant disease resistance [EPDR]) toward plant-pathogenic fungi. SA concentrations ≥10.0 mM were required for complete inhibition of fungal growth from mycelial plugs. SA doses of 2.0–5.0 mM typically reduced fungal growth by 50%, whereas doses of 0.5 mM or lower had little or no effect on fungal growth. However, growth of several test fungi was completely inhibited by 2.0 mM SA combined with concentrations of cupric chloride, antifungal bacterial culture fluids, or neem extract that were otherwise only slightly to moderately inhibitory. We conclude that (1) endogenous SA concentrations (up to 10.0–100.0 μM) are unlikely to directly inhibit fungi in plants, (2) concentrations of exogenous SA applied for EPDR (2.0–10.0 mM) are likely to be only moderately inhibitory to fungi, and (3) additions of other antifungal materials with which SA synergizes may enhance the antifungal activity of SA applied to plant surfaces for EPDR. The latter conclusion provides a rationale for further study of the synergistic interactions of moderately active antifungal materials for practical plant disease control.
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Vol. 66 • No. 2