Protecting fungicide chemistry used in Australian strawberry production for more sustainable control of powdery mildew and leaf blotch

Abstract

Powdery mildew (caused by Podosphaera aphanis) and leaf blotch (caused by Gnomoniopsis fructicola) are important diseases of strawberry in nursery and fruit production regions of Australia. Myclobutanil, trifloxystrobin and penthiopyrad are registered for control of powdery mildew in Australia, but are not restricted to either nursery or fruit production. Due to the specific mode of action of these fungicides against P. aphanis, their repeated use over the successive stages of strawberry runner and then fruit production may increase the risk of fungicide resistance in the pathogen. Currently, there is no fungicide registered for control of G. fructicola in strawberry in Australia. A series of field trials in Queensland and Victorian strawberry nurseries over two seasons (2013/14 to 2014/15) evaluated the efficacy of 19 fungicide and biorational products, applied alone or as part of a program (in combination with other products), against powdery mildew and leaf blotch. Our aim was to manage the risk of fungicide resistance by developing disease control programs specifically for use in the strawberry runner nurseries with different activity groups to the fungicides used in strawberry fruit production. Treatments were applied weekly using a knapsack sprayer starting December/January for approximately 16 weeks until prior to harvest of runners in March/April. Runners were randomly selected and rated for the incidence of powdery mildew and leaf blotch prior to harvest. Where single fungicide treatments were applied (2013/14), the field trial results identified bupirimate, cyflufenamid, quinoxyfen, proquinazid, azoxystrobin + difenoconazole, as equally or more effective in controlling powdery mildew as the industry standard program of sulphur, myclobutanil and trifloxystrobin. Field trials in 2014/15 assessed spray programs that were based on sulphur in different combinations with quinoxyfen, bupirimate, cyflufenamid and/or with reduced applications of myclobutanil and trifloxystrobin. All programs were as effective against powdery mildew as the industry standard program. Azoxystrobin + difenoconazole, prochloraz, and chlorothalonil were the most effective against leaf blotch in trials in 2013/14 and 2014/15. It is anticipated that the coordinated use of different chemistries across the strawberry nursery and fruit industries will reduce the risk of fungicide resistance developing in pathogen populations, and increase the sustainability of disease protection across strawberry crops.