Are Your Strobilurin Fungicides Effective Against Anthracnose?

Strobilurin-resistant strains of Colletotrichum have been detected in avocado orchards, raising concerns about the continued effectiveness of strobilurin (Group 11) fungicides, such as azoxystrobin (for example, Amistar, Agristar, Stellar or others). As these fungicides are a key component of an integrated anthracnose management program, it’s crucial to ensure they are still providing effective control on your farm.

Have you tested the effectiveness of strobilurin fungicides on your farm?

Resistance testing is essential for managing resistance and ensuring that your investment in spraying is delivering results. Below is a simple method to assess whether strobilurin fungicides are effective against anthracnose when applied before harvest:

1. Select and tag trees: Choose several trees or 1-2 rows in each block that have fruit at commercial maturity.
2. Apply fungicide: Spray the selected trees or rows with a strobilurin fungicide.
3. Harvest fruit: 1-2 weeks later, harvest at least 1-2 trays of fruit from both the sprayed and unsprayed trees. Keep the fruit from each group in separate, clearly labelled trays. Ensure the fruit is taken from all sides of the canopy.
4. Incubate fruit: Store the trays in a warm, humid environment to ripen. You can place the trays inside loosely tied garbage bags to create this environment.
5. Evaluate disease development: Once the fruit is ripe — or slightly overripe — check for anthracnose symptoms. In the case of Hass avocados, you may need to cut and peel the fruit to observe any signs of disease.
6. Interpret the results:

• Similar disease levels in both groups: If the disease level is comparable in both the strobilurin-sprayed and unsprayed fruit, it’s likely that the Colletotrichum in your orchard is resistant to the fungicide, and continuing to use these sprays may not be worthwhile. In such cases, consider switching to systemic fungicides like Score® and/or Switch®.
• Minimal disease in both groups: This may indicate that you have an effective protectant spray program in place, are dealing with early-season fruit (which usually has less disease), or that disease pressure is currently low due to unfavourable conditions for fungal infection and disease development.

This protocol was developed by Liz Dann from the University of Queensland. For more detailed information on a revised orchard fungicide program for managing anthracnose in avocados, you can access the full paper here.

Information in this article is most relevant in North Queensland and Central Queensland growers given their known resistance issues. Although growers in Tristate and Western Australia have historically applied fewer strobilurin sprays, we know there are low levels of resistance in Colletotrichum in WA, and periodic rotation with the new chemistry available on permit will ensure the longevity of all fungicides in the future.

This article appeared as part of the 6 Sept 2024 issue of Guacamole, AAL’s enewsletter.

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