Benzothiadiazole (BTH) induces resistance in cauliflower (Brassica oleracea var botrytis) to downy mildew of crucifers caused by Peronospora parasitica

Seedlings of Billabong, a downy mildew susceptible cauliflower Fl hybrid, were screened for their ability to develop induced resistance following treatment with CGA 245704, also known as benzothiadiazole (BTH). The seedlings were sprayed with a water solution containing the following doses of the elicitor: 0.0015, 0.005, 0.01, 0.015, 0.045, 0.075, 0.15 and 0.25 mg active ingredient (a.i.) per mi. In the dose-response experiments, the treated seedlings were challenged four days later with inocula prepared from either frozen or fresh produced spores of the virulent downy mildew isolate FP06. Results obtained showed that up to a concentration of 0.45 mg a.i./ml, the efficacies of the induced resistance was lower with fresh spores than with frozen ones. At 0.45 mg a.i./ml or higher, no differences were seen. Similar results were obtained on 30 day-old plants (young plants). In the time-course experiments, the treated seedlings were inoculated with isolate FP06 - 1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 15 and 30 days later. Results obtained showed that no resistance was induced when the inoculation was done just after BTH treatment or one day before. The seedlings as well as the young plants exhibited induced resistance when challenged 1-30 days after treatment and this resistance was shown to be sytemic. Furthermore, it was shown that BTH solutions kept at 4-5 degrees C were still able to induce resistance 30 days after their preparation. In all experiments, growth reduction affecting seedling's height was noticed and its importance was dose-dependent. The reduction was of 5.9% for the lowest concentration (0.0015 mg a.i./ml) to 38.3 for the highest one (0.25 mg a.i./ml). At concentrations of 0.045 and 0.05 mg a.i./ml, induction of resistance was maximum and the height reduction was of approximatively 22%. On young plants, the growth reduction concerned only leaf length and width. At a concentration of 0.05 mg a.i./ml, the corresponding leaf growth reduction were of 14.1 for the width and 12.3% for the length. No direct effect of BTH on spore morphology or germination was shown. (C) 1999 Elsevier Science Ltd. All rights reserved.