Managing Botrytis Fruit Rot Fungicide Resistance in Berry Crops

2025 Update:

Fungicide applications are the primary method for controlling botrytis, but they are not the only tools producers can and should use. Pruning improves air flow through the plant canopy reducing hours of leaf and fruit wetness, potentially reducing disease infections (most diseases require surface moisture for a certain amount of time at a certain temperature for an infection to occur). Mulching or disking pruned branches and canes reduces the number of fungal spores in the field. Although cultural methods won’t control diseases, good cultural management practices do reduce and delay their growth.

Fungicide resistance development depends on the fungicide use history of your farm. The heavier the use of one or several fungicides, the higher risk of fungicide resistance. The ways to delay resistance are to rotate between different fungicide classes or groups (FRAC Groups), use premix products that contain more than one active ingredient with activity on Botrytis, and / or tank mixing products that have activity on the disease (in general group M, multi-site, fungicides are recommended in tank mixes due to their low chance of resistance development). When disease pressure is low, avoid using fungicides that are more likely to develop resistance.

When this paper was initially written in 2018, botrytis resistance to Boscalid (Cantus, Pristine), Cyprodinil (one of the active ingredients in Switch) and Fenhexamid (Elevate) had been found in Washington State berry fields, and was potentially present in BC berry fields too. While there was resistance to Boscalid in Washington state, Kenja and Luna Tranquility, which are also group 7 fungicides, continued to work. Unfortunately, recent research has shown that the fungal population in Washington State is shifting and fungi that are not only resistant to Boscalid, but also Kenja, Luna Tranquility and all other groups 7 fungicides are becoming more prevalent.

To ensure that the available fungicides continue to work, we need to not only rotate between products but also reduce our reliance on group 7 (and group 9) fungicides. Apply group 7 or 9 fungicides only 1 to at most 2 times per season and tank mix them with products from another group with activity on Botrytis (even when the product is a premix of 2 or more fungicide groups).

Botrytis is a difficult disease to control and there are not a lot of new chemicals coming. Most of the new products being registered either contain a group 7 or 9 fungicide or are biological products that tend to have lower efficacy when compared to the traditional options. We must manage the fungicides that are available so that they can continue to be used in the future. While using different, potentially more expensive products may cost more, loss of disease control will be more expensive in the long run.

Use the chart below to help choose chemistries to rotate between and help postpone resistance. Note that some chemical brands contain ingredients from more than one group. This is intended to increase the product’s activity and reduce development of resistance; however, if a product contains either a group 7 or 9 fungicide it is still recommended to tank mix the product with a group M or BM fungicide to further reduce the chance of resistance development.

While some fungicides have post-infection activity (when applied shortly after a rainfall), as a general rule, all fungicides should be applied prior to infection (ie prior to rainfall) to ensure the best results.

*FRAC Group: Fungicide Resistance Action Committee chemical groups. Products within a group have similar chemistry and therefore mode of action. Rotate between products in different groups in order to delay / decrease development of resistance. If you are interested in learning more about fungicide resistance please check here. 

Written by: Mike Hanna