Although fungicide treatment did not completely prevent rust infection, it afforded sufficient reduction in severity to discriminate the rust effect from variety and nitrogen effects. Consistent with previous studies [1] and [2], increased rates of N increased the severity of stripe rust during grain filling. N application also increased yield and grain protein content in all varieties in both years, and generally there was no interaction between N rate and disease. This finding suggests that stripe rust has the same effect on yield at all rates of N, even though rust severity increased as N rate increased. This correspondence may arise because higher

levels of N lead to higher leaf area index (LAI [10]). Robert et al. [11] showed for leaf rust of wheat

that photosynthesis in green parts of the leaf was unaffected STA-9090 cell line by the presence of rust elsewhere in the leaf. It is possible that despite higher stripe rust severity at high N, with the higher LAI the total amount of green leaf was not reduced. Stripe rust reduced yield of the susceptible wheat variety in both years, but it reduced grain protein content only in HM in 2006. This difference could be due either to environment, with yields in 2006 being almost twice as high PD98059 in vitro as in 2007, or to genotype. The effect of stripe rust on the proportion of added N recovered in the grain differed between the two years. In 2006, when both yield and GPC were reduced by disease, the rate of return on added N was approximately halved.

This was a much larger effect than would be expected from a 10% reduction in yield and a reduction in mean grain protein from 11.7% to 11.2% by the presence of stripe rust. However, in 2007, when yield was reduced by disease, protein content was unaffected. These conditions resulted in almost no difference in the marginal N yield in grain with the addition of varying N rates. The mechanisms by which rusts reduce N yield remain uncertain. Yield reductions are due to loss of photosynthetic area [11]. Normally, reduced carbohydrate ADP ribosylation factor available for grain filling would be expected to increase relative protein content, as is typically seen when necrotrophic foliar diseases reduce yield [6]. However, our experiments with stripe rust showed a reduction in yield accompanied by either no change or a reduction in protein content, indicating that the total amount of N entering the grain was reduced. There are three possible mechanisms for this effect. One is removal of N from the plant tissue by the pathogen, principally as spores. Robert et al. [12] found that N content of leaf rust spores was lower, and C content higher, than those of wheat leaves, suggesting that rusts do not remove N from the plant at a higher rate than C. The other mechanisms are reduced uptake of N and reduced remobilisation from vegetative tissue into the grain after anthesis. Both uptake and remobilisation are reduced by late infections with foliar diseases [13].