Resistance to Sphaerotheca pannosa in roses induced by 2,6‐dichloroisonicotinic acid

The susceptible rose cv. Madelon and the partially resistant cv. Sonia both responded with reduced development of rose powdery mildew when they were treated with the synthetic inducer 2,6‐dichloroisonicotinic acid (INA). The EC 50 for number of colonies cm −2 was approximately 0.4 mg L −1 in both cultivars when treated 4 days prior to inoculation. However, conspicuous differences were observed with respect to number of spores per cm 2 . For sporulation, the EC 50 was 0.37 mg L −1 in cv. Madelon and only 0.08 mg L −1 in cv. Sonia. A comparison with the pathosystems cucumber/ Sphaerotheca fuliginea and red cabbage/ Peronospora parasitica is made and the importance of the observed phenomenon for the selection of parents in a breeding programme for (partial) resistance is discussed.