Fungicide Efficacy Against Calonectria pseudonaviculata, Causal Agent of Boxwood Blight

Calonectria pseudonaviculata causes leaf spot and stem lesions resulting in defoliation and dieback of boxwood. Fungicides representing 20 different active ingredients from 13 different Fungicide Resistance Action Committee groups were evaluated for their effects on conidial germination and mycelial growth using in vitro assays, and the concentration that suppressed fungal growth to 15% of that on unamended media (EC₈₅) values were determined. A number of fungicides strongly inhibited mycelial growth of C. pseudonaviculata. Four demethylation inhibitor fungicides had EC₈₅ values of 1.2 μg a.i./ml or less. Thiophanate-methyl, fludioxonil, pyraclostrobin, trifloxystrobin, kresoxim-methyl, mancozeb, and chlorothalonil also had activity against mycelial growth. Fludioxonil + cyprodinil had a lower EC₈₅ than the same rate of fludioxonil alone, suggesting that cyprodinil had activity against mycelial growth. Fungicides that inhibited C. pseudonaviculata conidial germination include pyraclostrobin, trifloxystrobin, and kresoxim-methyl as well as fludioxonil, mancozeb, chlorothalonil, and boscalid. Quinoxyfen, etridiazole, fenhexamid, hymexazol, famoxadone, and cymoxanil did not inhibit either C. pseudonaviculata conidial germination or mycelial growth. In comparison with values found in the literature, EC₅₀ values for kresoxim-methyl were up to 10 times higher than reported previously, suggesting that fungicide insensitivity may have developed. Protectant fungicides with activity against conidial germination and systemic fungicides with activity against mycelial growth, such as those identified here, may be complementary to achieve the high levels of pathogen management required for control of this disease. In addition, multiple fungicide active ingredients from different mode-of-action groups used in mixtures or over time may also act to slow selection for fungicide insensitivity.