Abstract
Factors that would account for a noted decline in the N status of lucerne with increasing stand age in Canterbury, New Zealand were investigated. Changes in the soil pH, numbers of rhizobia capable of nodulating lucerne in the rhizosphere and changes in the N 2-fixing effectiveness of Rhizobium with stand age were eliminated as possible factors. In a series of field experiments, soil-borne fungi were found to cause a significant reduction in percentage foliar N. nodule weight and foliar production in a 5-yr-old Sitona disroideus Gyllenhal affected stand. The absence of a S. discoideus × fungal interaction in a 2-yr-old stand was explained by the lower occurrence of fungi both on the rhizoplane and in surface sterilized root sections compared to the 5-yr-old stand. It is speculated that the replacement of nodules consumed by S. discoideus larvae is inhibited in older (⩾5yr) stands by soil-borne fungi causing necrosis of the sites of Rhizobium infection; this in turn lowering the N status of lucerne.
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