Integrated approach for control of foliar diseases in strawberry runner nurseries and management of chemical resistance

Abstract

Powdery mildew and leaf blotch are important diseases that affect strawberries in Australia. Both diseases can reduce transplant quality in the strawberry nursery sector, and decrease yield in the fruit sector. Disease control in the strawberry nursery and fruit sectors relies on the use of the fungicides myclobutanil and trifloxystrobin. Due to the way these fungicides act against the pathogen, their repeated use in the nurseries and fruit farms increases the risk of the fungus becoming resistant to the chemicals. Resistance is when a pathogen is no longer controlled by a fungicide when used as directed.
To address the risk of resistance, a series of field experiments were conducted in strawberry nurseries in Queensland and Victoria over three seasons to assess the efficacy of several fungicides, including biorationals, with different modes of action against the diseases. Biorational is a term given to products that are relatively non-toxic to humans and have a low impact on the environment, and include oils, soaps, microbials, minerals and botanicals.
Results showed that a number of fungicides, including biorationals, were effective against powdery mildew and leaf blotch. Three fungicidal actives, bupirimate, cyflufenamid and quinoxyfen, were granted minor-use permits for specific use against powdery mildew in strawberry runner production. Data from the trials supported two of the minor-use permits (bupirimate and cyflufenamid). Quinoxyfen was granted a minor-use permit not long after the commencement of the project after discussions between the industry and chemical company. The study supported quinoxyfen being effective against powdery mildew in strawberry runners.
Best practice chemical control of powdery mildew in nursery runner crops, is to avoid use of myclobutanil and trifloxystrobin (only apply early in the season if required), and rotate the recently permitted fungicides, quinoxyfen, bupirimate and cyflufenamid through the season. Nursery growers in Queensland and Victoria have now implemented this strategy. This will minimise the risk of the pathogen developing resistance to myclobutanil and trifloxystrobin in the fruit farms.
At the time of writing this final report, there are currently no permitted or registered fungicides for control of leaf blotch/stem-end rot in strawberry. However, results from trials showed that fluazinam, prochloraz and azoxystrobin+difenoconazole reduced the disease in strawberry nursery crops. Based on results from field trials, an application has been made for a minor-use permit for fluazinam against leaf blotch in the strawberry nursery sector only.
Access to a wide range of fungicides with different modes of action is important for managing fungicide resistance across the whole industry. Co-ordinated use of different fungicides between nursery and fruit production farms will assist in reducing the risk of fungicide resistance developing in pathogen populations. In the long term, it is anticipated that this practice will extend the effective life of current and new fungicides, and reduce the economic impact of diseases in the strawberry nursery and fruit sectors.
The coordination of fungicide use across both sectors can help guide future chemical use and registration of new products. This concept may also be applied to other horticultural industries with more than one generation of production/propagation (e.g. potato) to better manage the risk of chemical resistance.