Sensitivity of two sugarcane rust fungi to fungicides in urediniospore germination and detached leaf bioassays

Management of brown (Puccinia melanocephala) and orange (Puccinia kuehnii) rust of sugarcane in Florida primarily relies on fungicide applications. Using two different in vitro bioassays, we evaluated the efficacy of five fungicides registered for control of these diseases. Inhibition of P. kuehnii and P. melanocephala urediniospore germination varied from 96 to 100% at 0.01–10 μg per ml of pyraclostrobin and fluxapyroxad + pyraclostrobin. However, only 48% of the P. kuehnii spores were inhibited at 10 μg per ml of fluxapyroxad. Capacity to inhibit sugarcane infection and rust uredinia formation was tested with a detached leaf bioassay where fungicides were applied at different times with respect to inoculation with P. kuehnii or P. melanocephala. When fungicides were sprayed immediately after inoculation of leaf fragments, uredinia formation of P. melanocephala and P. kuehnii was suppressed at 99–100% regardless of the fungicide, except for metconazole (36–50% reduction). Uredinia formation of P. kuehnii was also nil when all fungicides (except metconazole) were sprayed 1 or 2 days prior to inoculation. However, maximum reduction in uredinia formation was 82% and 48% when leaves were sprayed with fungicides one or two days after inoculation, respectively. Therefore, these fungicides are more efficient in preventive than curative management of sugarcane orange rust