Fungicide resistance in a diversity of fruit crops

As we continue to walk on really expensive eggshells in 2023, it’s important to recognize that fungicide availability along with the effectiveness of available fungicides (and their price) will impact orchard, vineyard and berry patch management 2023.

For all growers, the number 1 problem continues to be fungicide resistance in a diversity of fruit crops.

Table 1. Fungicide resistance reported or suspected in Indiana.

Fungicide-resistance remains a growing problem, increasing the cost of disease management, while resulting in lower levels of disease control.  The FRAC 3 fungicides (Nova, Procure) have been relied upon in all areas of fruit production, especially for apple scab and powdery mildew. Newer FRAC 3 fungicides (Inspire Super, Indar, Cevya, Topguard Specialty Crop) provide control even against some FRAC 3 resistant strains—at least for now. The post-infection activity of these fungicides provide some ‘wiggle-room’ for growers as far as timing, providing flexibility in both timing and coverage. However, too many growers have relied too heavily on this wiggle room at their peril. Any post-infection use of the FRAC 3 should be limited to 48 hr (not the previous 96) to reduce pressure and minimize resistance risks. Early season use of Syllit (dodine) continues to provide this flexibility to apple growers, prior to pink, and remains an excellent tool if resistance is not an issue.

Issue of fungicide resistance are also impacting the FRAC 11 (strobilurin, or strobies) fungicides (Flint and Sovran), resulting in a decreased level of activity in apple scab, bitter rot, powdery mildew (in apple, grape, strawberry), brown rot, and downy mildew control. Complete failures aren’t a question of if, but when. Resistance is starting to be an issue in the FRAC 7 group (Endura, along with the boscalid component of Pristine), particularly for botrytis control in grape and strawberry. Tank-mixes or tight rotations are the best ways to minimize the risk of this occurring, using mancozeb and captan to knock down any fungicide resistant isolates that may have evolved.

There are a lot of new fungicides registered that growers should consider, but buyer beware! Over a decade ago, the US Environmental Protection Agency (EPA) no longer required efficacy data. This makes it challenging not just for growers, but for those of us putting together the Midwest Fruit Pest Management Guide. Fruit pathologists from New York/New Jersery, south to Georgia and throughout the Midwest have trialed these products and published their findings. These results determined what was and was not included in the Midwest Fruit Pest Management Guide, and efficacy results were standardized for comparison to help growers make informed decisions about what product should work best for them. Among new fungicides to consider are Aprovia, Kenja, Miravis (FRAC 7); Aprovia Top (grapes) and Miravis Duo (FRAC 7+3); Evito (FRAC 11) for berries; Gatten for powdery mildew on many fruit crops (FRAC U13).

Keep in mind that there are still supply chain issues, although not as bad as last year. Limited availability coupled with higher prices are becoming the norm for many tank mix partners (captan, mancozeb, and some formulation of phosphorous acid). Rotation and tank-mixes are the best strategy to manage not only disease problems but also fungicide resistance.