Influence of sulfur fertilization on sulfur metabolites, disease incidence and severity of fungal pathogens in oilseed rape in Scotland

A harmonic mineral nutrition of plants that fully satisfies their nutrient demand is an important component of natural resistance mechanisms against biotic and abiotic stress. In Scotland, where atmospheric sulfur (S) depositions are extremely low, the infection pressure of various fungal pathogens proved to be higher than in regions with a higher S input. Soil applied S fertilization as sulfate significantly increased the natural resistance against fungal pathogens, but the trigger mechanisms are so far unknown and consequently, S induced resistance (SIR) can not be prompted regularly by S amendments. Field experiments were conducted in Aberdeen and Inverness from 2000 to 2003 in order to evaluate the relationship between S applications and oilseed rape resistance to the three main fungal pathogens in this area (i.e. Pyrenopeziza brassicae, Leptosphaeria maculans, Peronospora parasitica). Generally, significant differences existed between sites and seasons with view to the infection of oilseed rape plants with fungal pathogens. In all three years and on both experimental sites, disease progression, expressed as disease incidence and disease severity, was not consistently reduced by S applications. However, a positive effect of S fertilization in spring on infections with P. parasitica was found on both sites in 2002. S fertilization increased the cysteine (0.70 μmol g^-1 vs. 1.07 μmol g^-1), glutathione (15.2 μmol g^-1 vs. 18.5 μmol g^-1) and glucosinolate content (3.81 μmol g^-1 vs. 4.76 μmol g^-1) in young fully developed leaves of oilseed rape. Several lines of evidence show that these S compounds play an important role in the plant defence reactions. Factors possibly related to the initiation of SIR under natural conditions are discussed.