Strategies for using plant protection products against apple scab(Venturia inaequalis) to avoid resistance development

Abstract: Using plant protection products (PPPs) with curative action to control apple scab canlead to resistance development. We therefore tested strategies for effective scab control thatavoided the use of curative PPPs. In 2010 and 2011, the efficacy of dithianon (Delan), sulphur(Thiovit) and potassium bicarbonate (Armicarb) on leaves and fruits using different strategieswas tested on the apple cultivar Jonagold at the Randwijk research station. In 2011, the curativeaction of difenoconazol (Score) was also tested. In the untreated controls, 52.5% (2010) and56.7% (2011) of the leaves were infected, and 52.5% (2010) and 90.9% (2011) of the fruits. Inboth years, stop-spray treatments (application of treatments immediately at the start of aninfection) with sulphur and dithianon resulted in equal infection incidence and severity on leavesand fruits compared to the preventive application of dithianon. The effect of the curativeapplication of potassium bicarbonate was comparable to the other treatments in 2010, but thenumber of lesions per leaf was higher than with the preventive treatment using dithianon in thatyear. The number of applications was lower with the stop-spray strategy than with the preventivestrategy. However, the stop-spray treatments sometimes took place outside normal workinghours, even at night. We concluded from the trials that effective protection against apple scab ispossible without using curative PPPs, which carry a risk of resistance development. Dithianonand sulphur can be used effectively as part of a stop-spray strategy.