Effects of azoxystrobin, difenoconazole, polyoxin B (polar) and trifloxystrobin on germination and growth of Alternaria alternata and decay in red delicious apple fruit

Alternaria alternata is the predominant fungal pathogen responsible for moldy-core in red delicious strains of apple. In this study, we report on the effects of the polyoxin B compound Polar, the strobilurin fungicides, azoxystrobin and trifoxystrobin, and the sterol inhibitor difenoconazole on spore germination, mycelial growth and fruit decay on detached fruits caused by A. alternata. Germination was most sensitive to polyoxin B and trifloxystrobin among the tested compounds. The 50% and 95% effective concentration (EC50, and EC95), values for in vitro inhibition of conidial germination of the fungus were lowest for polyoxin B and trifloxystrobin, ranging from <0.01 to 0.15 mug/ml and 180 mug/ml, respectively. Germination was least sensitive to difenoconazole and azoxystrobin (EC50 and EC95 values ranging from 25 to 72 mug/ml and 720 mug/ml, respectively). Mycelial growth in vitro of A. alternata was most sensitive to difenoconazole (EC50 and EC95 values of 0.8 and 12 mug/ml, respectively) and least sensitive to both strobilurin fungicides (EC95 > 1000 mug/ml). Decay formation by A. alternata on mature detached fruits was most affected by trifloxystrobin and azoxystrobin (EC50 and EC95 values of 0.015-0.087 mug/ml and 8 mug/ml, respectively), intermediate in sensitivity to Polyoxin B (EC50 and EC95 from 1 to 33 mug/ml, respectively), and difenoconazole was the least effective (EC50 and EC95 from 20 to 490 mug/ml, respectively). Polyoxin B applied at 24 or 48 h post-inoculation inhibited fruit decay development. Difenoconazole and trifloxystrobin were less effective and azoxystrobin was ineffective when applied at 48 h post inoculation. The activity of azoxystrobin, difenoconazole, Polyoxin B and trifloxystrobin on one or more stages of the life cycle of A. alternata and on decay development in fruits suggests that these compounds potentially could provide control of moldy-core disease in apple. (C) 2002 Elsevier Science Ltd. All rights reserved.