Induction of systemic resistance to Sclerotinia sclerotiorum by oxalic acid in oilseed rape

Treatment of the lowermost leaf of flowering plants of oilseed rape cv. Linetta with 20 m M oxalic acid induced systemic disease resistance in the three next youngest leaves. Resistance was not caused by the translocation of fungitoxic concentrations of oxalic acid. Resistance was also expressed in the stem; in oxalic acid-treated plants the vertical spread of stem lesions was halted and the signal travelled both acropetally and basipetally. Expression of resistance in the leaves was detected 6 h after treatment and continued for at least 5 weeks. Twenty-four h after treatment, resistance was detectable in the leaves following the use of as little as 0·5 mM oxalic acid. Local resistance in the oxalic acid-sprayed leaf was detectable only at certain times (12 and 48 but not 24 h after treatment) and at very low (0·25 mM ) or high ($40 mM ) oxalic acid concentrations. When oxalic acid was applied to a discrete area of the leaf, significant local resistance was expressed in the surrounding leaf tissue, maximum resistance being exhibited by the tissue closest to the site of petiole attachment.