Foliar applications of boron to almond trees in dryland areas

Spain is the country with most almond (Prunus dulcis) cultivated area. As much as 93.5% of this area consists of dryland orchards, which are prone to the development of boron (B) deficiencies. Although correction of B deficiency can be achieved through soil or foliar B applications, foliar treatments are more effective under dry conditions due to the low root absorption rates from dry soils. The objective of this trial was to observe and quantify the response of almond trees to foliar B application under drought conditions. The experiment, consisting of 'Desmayo Largueta' and 'Marcona' almond cultivars, was located in Almatret (Lleida) in the northeastern Spain. The initial foliar B concentration was 28 ppm. During 1999, 2000 and 2001 rainfall was 380, 477 and 312 mm respectively, with 50-60% occurring during the growing season. Treatments applied were: (T-1) a single B spray in early spring, two months after full bloom; (T-2) a single B spray two weeks after fruit harvest; (T-3) two B sprays, one in early spring and the other after fruit harvest; and (T-0) control without B application. Boron was applied as Solubor, a commercial product containing 20.8% B, at the concentration equivalent to 832 ppm B in the volume of 7 L tree(-1). Foliar B concentrations increased in treated trees but declined in subsequent year, perhaps due to competition for B from fruit, low soil water availability limiting root B uptake and/or inadequate number of foliar B applications. Two foliar B sprays delayed bloom for 4-6 d, which may reduce the possibility of blossom damage from late winter or early spring frosts. Fruit set and final fruit number were improved in trees receiving foliar treatments, particularly those receiving two foliar sprays (T3 treatment). Cumulative yields were increased in response to B fertilization. Kernel production was approximately 21-24% higher in the T3 than in the T1 and T0 treatments. The post-harvest B treatment had intermediate effectiveness. Cumulative fruit number per tree was also significantly enhanced by B treatments. The results suggest that further increases in the number of B foliar sprays beyond the two used in this study should be explored in future studies to better alleviate B stress in almond trees, especially under dry conditions. Optimal B treatments, however, may differ among cultivars.