Vitality fertilization balanced tree nutrition and mitigated severity of Sirococcus shoot blight on mature Norway spruce

The effects of vitality fertilization on needle nutrient status and symptom development of Sirococcus shoot blight (SSB) were examined in a long-term, single-tree fertilization experiment in a 90-yr old Norway spruce stand on acidic soil. Trees showing severe symptoms of SSB (“symptomatic trees”, N=72) and trees with less than 5% of the current-year shoots symptomatic of SBB (“asymptomatic trees”, N=72) were selected among the dominant and co-dominant trees within the stand and randomly assigned to three fertilization treatments: unfertilized control, dolomitic liming and gypsum & kieserite fertilization. Concentrations of macronutrients were determined in healthy current year needles of symptomatic and asymptomatic trees 3 and 6years after fertilization and compared with the status in autumn prior to fertilizer treatment. Severity of SSB was rated yearly from 2001 to 2006 by estimating the percentage of affected current-year shoots in the crown in order to assess evolution of disease severity due to changes of nutritional status of the host.Both fertilizer treatments significantly increased foliar Ca and Mg concentrations, and as a result the primary differences in nutrient status between symptomatic and asymptomatic trees disappeared. Thus, vitality fertilization mitigated insufficient Ca and Mg supply that characterized symptomatic trees prior to fertilization, and balanced tree nutrition. Application of the water soluble Ca- and Mg-fertilizer (gypsum & kieserite-variant) induced a quick but only short-term fertilization effect whereas dolomitic liming resulted in significantly higher Ca and Mg concentrations compared to control 3 as well as 6years after fertilization. In contrast, Ca- and Mg-deficiencies further intensified in the unfertilized trees.Balanced tree nutrition also mitigated disease severity of the fertilized trees and promoted tree recovery, indicating that the elements Ca and Mg are putatively involved in disease tolerance. Best results were achieved by fertilization with a water soluble Ca- and Mg-fertilizer which resulted in an 18.9% reduction of disease severity, assessed by the number of diseased shoots per tree, within five years. While dolomitic liming also promoted tree recovery (reduction in disease severity was 11.8%), in the unfertilized control variant a 3.5% increase was observed in the same period. Reduction in disease severity differed significantly from the control for both fertilizer treatments whereas differences between fertilizer treatments were not significant.Results of this study revealed that balanced nutritional status of mature Norway spruce due to vitality fertilization was associated to reduced symptom development of Sirococcus shoot blight.