Nitrogen Effects on the Pathogenicity ofDrechslera sorokinianaandCurvularia geniculataon Germinating Seed ofFestuca rubra

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Abstract
The effects of (NH4)2SO4 and Ca(NO3)2 on the pathogenicity of D. sorokiniana and C. geniculata, singly and in combination on germinating seed of F. rubra were evaluated. Both N sources at 10-2 M reduced the rate of seedling emergence and total emergence from uninoculated seed in autoclaved and nonautoclaved soil. Total seedling emergence from uninoculated seed in nonautoclaved soil was less than that in autoclaved soil in response to both N sources at 10-2 M. These responses were suggestive of direct N toxicity to the germinating seed in autoclaved soil and of stimulation of unknown biotic factors in nonautoclaved soil that, when combined with direct N toxicity, additively reduced total emergence. A concentration of 10-3 M of both N sources generally stimulated rate of seedling emergence and had no effect on total seedling emergence from uninoculated seed. The lowest concentration (10-4 M) of the N sources tested showed mixed effects on rate of seedling emergence and total emergence from uninoculated seed in autoclaved and nonautoclaved soil. The highest concentration of both N sources enhanced the pathogenicity of D. sorokiniana and C. geniculata, inoculated on seed both alone and in combination. Rate of seedling emergence was slowed by D. sorokiniana and the combination of D. sorokiniana + C. geniculata, and each pathogen and their combination also reduced total seedling emergence. The combination of the pathogens, together with the highest concentration of either N source, produced the most severe reduction in total seedling emergence. These results suggest a combination of direct toxicity to germinating seed and an enhancement of the pathogenicity of D. sorokiniana and C. geniculata induced by N-containing compounds.

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