Studies on the chemical control of chrysanthemum white rust caused by Puccinia horiana

Experiments evaluating the curative and protective effect of sprays of benodanil, bitertanol, oxycarboxin, triadimefon, triforine and propiconazole, and intensive oxycarboxin fog and triforine spray regimes, against chrysanthemum white rust are described. All treatments reduced the disease when applied before symptoms developed but only propiconazole completely prevented disease development. A single spray of propiconazole applied 4–8 days after inoculation was sufficient to eradicate infection. This fungicide was also found to eliminate symptomless infection from cuttings when used as a dip prior to planting, or as two sprays after planting. When sporulation had occurred, three sprays of propiconazole or benodanil killed the telia, but viable telia were still present after three sprays of oxycarboxin, triadimefon or triforine. When applied as single protective sprays none of these fungicides consistently prevented infection, although levels were considerably reduced when application was made 2–3 days before exposure to the disease. Observational trials indicate that although propiconazole has some growth retardant effect it does not appear to present any significant phytotoxicity problems, either as a spray or as soil residue. As a result of this work propiconazole has been successfully used in a statutory eradication campaign against this rust. To avoid the possible development of resistance, it is suggested that its use should be confined to actual outbreaks, or when it is suspected that plants or cuttings may have become infected.