Reducing Primary Inoculum of Apple Scab Using Foliar Application of Urea in Autumn

Abstract

The effect of applications of urea 5% after harvest but before leaf-fall, as foliarapplication, in order to restrict perithecial production by Venturia inaequalis in a commercial superintensive apple orchard situated near Cluj-Napoca, Romania was studied. The urea works in two ways:directly by inhibiting the development of ascospores, and by stimulating the development ofantagonistic microorganisms from against V. inaequalis. There are several possibilities to reduce theamount of overwintering apple scab, such as removing fallen leaves by raking, vacuuming, or usingleaf blowers, chopping or shredding the leaf litter in the spring. Primary infection of scab conidia isproduced by infected leaf litter on the orchard floor. Any modalities that reduce the amount of leaflitter will help to reduce the amount of inoculum for primary infection with scab. Urea-treated leavesdecomposed rapidly infected leaf litter, thus destroying the overwintering substrate for the V.inaequalis fungus. The results have shown large reductions in spore production, often as high as 70 to80%, following application of 5% urea. Spraying the surface of the leaves on the ground with urea 5%reduced primary infection by about 60%. In conclusion, urea applications proved to be effective toreduce apple scab inoculum. Combining the urea with other modalities of leaf shredding provides thegreatest reductions in ascospores.