Chemical control of Cylindrocladium and Rhizoctonia root rots in blueberry propagation

Abstract

Cylindrocladium and Rhizoctonia root rots, caused by Cylindrocladium parasiticum and binucleate Rhizoctonia spp., respectively, are common soilborne diseases observed during blueberry propagation in Georgia, USA. Six trials with artificially inoculated propagation medium were completed in a shade-house to determine the efficacy of select fungicides to control these diseases. For Cylindrocladium root rot, two trials with single-drench fungicide applications were made at the time the cuttings (southern highbush blueberry cultivar Rebel) were stuck. In two additional trials, a second drench application was added two weeks later. In all trials, fludioxonil (Medallion) significantly reduced lesion incidence and lesion length. Neither triflumizole (Terraguard) nor thiophanate-methyl (Cleary's 3336) consistently reduced lesion incidence with one application, indicating that a single application of these materials was not sufficient for disease suppression. The second fungicide application improved disease control for all fungicides. However, fludioxonil always performed statistically better than thiophanate-methyl and numerically better than triflumizole as indicated by lesion incidence, lesion length and girdling incidence. For Rhizoctonia root rot, two trials were completed, each with two fungicide applications (when cuttings were stuck and 2 weeks later). Fludioxonil and azoxystrobin (Heritage) significantly reduced lesion incidence and lesion length in both trials. Flutolanil (Prostar) reduced lesion incidence in both trials and also reduced lesion length in the second trial. Pending labeling for this use pattern, fludioxonil, which has provided good control of both diseases in our trials, may form the basis of a fungicide rotation program to supplement sanitation practices for managing Cylindrocladium and Rhizoctonia root rots in blueberry propagation.