Effects of cultivar mixtures on scab control in apple orchards

The effects of two mixtures of resistant and susceptible apple cultivars on the development of scab caused by Venturia inequalis were observed in an experimental orchard over four years, initially for two years without fungicides against scab, and subsequently for two years with a moderate fungicide schedule. The row-by-row and within-row mixtures included a susceptible cultivar and a resistant cultivar in equal proportions. Without fungicides, the results showed a significant reduction of disease incidence over both years (7.3 to 21.3%), and severity in the second year (35.4%) in the within-row mixtures, compared to the monoculture of the susceptible cultivar. The best results were obtained when the within-row mixture was associated with moderate fungicide treatments; in this case the reduction in disease incidence reached 75.1% on leaves and 69.7% on fruits during the growth phase. The characteristics of the Venturia inaequalis/Malus x domestica pathosystem and the results obtained in this experiment suggest a moderate but not negligible ability of cultivar mixtures for reducing epidemics of the disease.