We had questions this winter concerning whether chlorothalonil products, when combined with copper products at the petal fall and shuck split application timeframes, would exacerbate or increase copper damage. Shane Breeden, a graduate student in the Plant Pathology Department, established a trial to review this premise in an O’Henry planting at the UGA Horticulture Farm. Treatments were as follows:
- Untreated control
- Bravo Weather Stik @ 4 pints/A at petal fall and shuck split
- Kocide 3000 @ 0.5 lb/A at petal fall and shuck split
- Bravo Weather Stik @ 4 pints/A + Kocide 3000 @ 0.5 lb/A at petal fall and shuck split
We did not observe any increase in damage with the combination, and any copper damage was very minimal. Though I have observed copper damage following these early applications in some commercial settings, in our recent field trials we have not seen much damage from copper at this stage without regard. However, as we have increased the number of copper applications over time, we do see a buildup of damage that can be significant.
Based on the results we observed, we do not believe there is a negative interaction of chlorothalonil and copper. Chlorothalonil is not likely to develop resistance, and it is critical to early scab management; though other fungicides are available for scab management, these are all prone to resistance development in fungi (scab and brown rot), so we prefer to utilize them sparingly and in the pre-harvest sprays for brown rot alone. Copper products are critical to suppression of bacterial spot during this early timeframe, so we do need both fungicides and bactericides to go out at the same time. Bottom line, I continue to recommend the use of chlorothalonil and copper for the petal fall and shuck split applications.