Efficacy of Fungicides and Rotational Programs for Management of Powdery Mildew on Cantaloupe

Powdery mildew of cucurbits, caused by Podosphaera xanthii (syn. Sphaerotheca fuliginia auct. p.p. (Schltdl.) Pollacci), is a common and often severe disease in most areas of the world. Field trials were conducted with cantaloupe to compare disease management success provided by conventional fungicides and biofungicides having different inherent efficacies and modes of action, when applied alone throughout the treatment period or as components of fungicide application programs. Additionally, the portion of total disease control provided by each component fungicide within selected rotational programs was determined. When applied alone throughout the treatment period, disease severity in 2008 and 2009 compared with nontreated plants was reduced by values of 100, 99.3, and 98.1% by wettable sulfur (Microthiol Disperss), triflumizole (Procure), and quinoxyfen (Quintec), respectively; 83.9, 76.4, and 57.4% by trifloxystrobin (Flint), pyraclostrobin (Cabrio), or azoxystrobin (Quadris), respectively; and 39.8, 31.1, 30.0, and 28.6% by thiophanate-methyl (Topsin M), potassium bicarbonate (Kaligreen), kresoxim-methyl (Sovran), and Bacillus subtilis (Serenade), respectively. Rotational application programs composed of Microthiol Disperss, Procure, and Quintec reduced powdery mildew severity on cantaloupe by 97.5 to 100% in both trials. In comparison, disease reduction of 86.0 to 100% was achieved when the first and third fungicide applications were Quintec or Procure and the second and fourth applications were Cabrio, Flint, Kaligreen, Quadris, Serenade, Sovran, or Topsin M. In field trials designed to elucidate the portion of total disease control provided by each component fungicide within a rotational program, application sequences of Procure, Streptomyces lydicus (Actinovate), Procure, and Actinovate or Procure, Kaligreen, Procure, and Kaligreen resulted in reductions in powdery mildew severity of 69.1 and 78.7%, respectively. In comparison, inclusion of only the two Procure applications brought about a mean disease reduction of 85%, whereas inclusion of only the two Actinovate or Kaligreen applications reduced the level of powdery mildew control to 17.6 and 12.9%, respectively. The usefulness of fungicides with low inherent efficacy as resistance management partners when applied with highly efficacious at-risk fungicides may be questioned; however, the importance of any fungicide as a resistance management partner should be measured by its ability to dilute the selection pressure of the at-risk fungicide and to inhibit the growth of any resistant biotypes that may arise, factors which may not be reflected by its inherent efficacy in controlling disease. Because less efficacious fungicides did not increase the overall level of disease control, encouraging growers and pest control advisors to incorporate them into powdery mildew treatment programs may be a challenge, because resistance management is not their top priority. An ongoing educational effort emphasizing the benefits of resistance management programs with respect to prolonging the effectiveness of single-site mode of action fungicides is essential.