Oral Technical Session: Disease Control and Pest Management

54-O

Fungicide impact on in vitro germination of Puccinia horiana, the causal agent of Chrysanthemum White Rust.
C. L. PALMER (1), S. E. Nester (2), J. M. Revell (2), M. R. Bonde (2)
(1) IR-4, Princeton, NJ, U.S.A.; (2) USDA ARS, Frederick, MD, U.S.A.

Puccinia horiana is an actionable pathogen which upon diagnosis triggers an eradication protocol combining destruction of symptomatic chrysanthemums with a strict fungicide regime for symptomless plants. Appearance of symptoms typically occurs during the fall just as growers are preparing to ship their chrysanthemum crops. To expand the list of fungicide tools in the eradication protocol and develop fungicide sensitivity baselines, we screened in vitro germination of P. horiana basidiospores in 0.05% water agar amended with varying concentrations of 12 fungicides: azoxystrobin, fluoxistrobin, mancozeb, mandestrobin, metconazole, myclobutanil, propiconazole, pyraclostrobin + boscalid, tebuconazole, trifloxystrobin, trifloxystrobin + triademefon, and triticonazole. Leaves with pustules ready to sporulate were affixed to petri plate lids over bases containing fungicide-amended agar. After at least 16 hours in the dark at 17oC, percent germination was assessed. Concentrations required for 50% germination (LC50) grouped according to fungicide mode of action class. Benzimidazole fungicides (FRAC 3) exhibited LC50s ranging from 20 to 72 ppm, while strobilurin fungicides (FRAC 11) ranged from 2 to 16 ppb. Mancozeb (FRAC M3) exhibited an LC50 of 7 ppm. Combinations of FRAC 11 fungicides with other modes of action exhibited LC50s in the same range as the strobilurins. These data provide a baseline for monitoring development of fungicide resistance in P. horiana over time.