Use pattern and limits of potassium bicarbonate for apple scab control in Québec orchards

Abstract: Scab (Venturia inaequalis) is an economically important disease of apples in many wet climate production areas including Québec, where orchards are almost half planted with a single susceptible cultivar (McIntosh), thereby increasing scab severity through selective pressure. Numerous fungicide sprays directed against the primary infections of this fungus are required every year for commercial production. However, V. inaequalis populations are becoming increasingly resistant to a number of products registered for scab control. Consequently, since 2009 alternative chemistries such as potassium bicarbonate (KBC) were tested under different use strategies. We observed that KBC can be effective in controlling scab only if precisely applied within 300 degree-hours (base 0 °C) from the onset of infection. KBC mixed with sulfur had a much higher efficacy that KBC applied as a standalone. Under an optimal use pattern, the KBC+sulphur mix showed similar efficacy to liquid lime sulfur (LLS) but less than penthiopyrad or solatenol (SDHI) or difenoconazole (DMI) applied with the same strategy. Nonetheless, because KBC + sulfur is presumably less prone to resistance than current systemic fungicides, less phytotoxic than LLS, and considerably less expensive than many other product, it is likely that growers will integrate this as an additional option when post-infection efficacy is needed.