Inhibitory activities of five fungicides on Alternaria suffruticosae and their field control efficacy against tree peony black spot.

Abstract

Tree peony black spot (TPBS), mainly caused by Alternaria suffruticosae, is a common leaf disease on the ornamental peony, which posed a great threat on the flower buds in the current year and the flowering quality in the next year. However, there was only one fungicide registered for the control of the disease, difenoconazole. In order to avoid the severe problem of pathogen resistance caused by long-term use of difenoconazole, it is necessary to screen more chemical fungicides for the prevention and control of TPBS. In the paper, the biological activities of flutolanil, phenamacril, pyraclostrobin, and boscalid on mycelial growth, conidial germination, germ tube elongation and sporulation quantity of A. suffruticosae were determined, and field control efficacy were conducted to evaluate the preventive and therapeutic activities. Difenoconazole, was used as a control simultaneously. The results showed that pyraclostrobin had the strongest inhibitory effects on the conidial germination, mycelium growth, germ tube elongation and sporulation quantity, with the average EC50 of 0.0517, 0.5343, 0.0008 and 0.8068 μg/mL respectively. The inhibitory activity of flutolanil on the four developmental stages of A. suffruticosae was weaker than the other three fungicides. Compared with flutolanil, boscalid, the other succinate dehydrogenase inhibitors, had more srtong inhibitory effects on the mycelial growth and sporulation quantity, with the average EC50 of 3.8603 and 1.4760 μg/mL respectively. Phenamacril had a moderate inhibitory level, which had more inhibitory activity on conidial germination and germ tube elongation, with the average EC50 of 31.5349 and 5.2597 μg/mL. All of the four fungicides had no significant effects on the shape of spores and germ tubes. The control fungicide difenoconazole had the strongest inhibitory activity on mycelial growth, and the average EC50 was only 0.3297 μg/ml. However, its inhibitory activity on the other three growth stages was not high. In the field trials, pyraclostrobin had high control efficacy on TPBS even at low concentrations, reaching a minimum of 62.6293%, which was higher than that of difenoconazole. The other three fungicides had higher control efficacy at high concentrations, but decreased significantly at low concentrations. Considering the dosage and control efficacy, pyraclostrobin was the first choice for the control of TPBS. Pyraclostrobin is the preferred alternative fungicide of difenoconazole for the prevention and control of TPBS in production.