Response of Susceptible and Slow Leaf-Rusting Wheats to Infection by Puccinia recondita

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Abstract
Flag leaves of slow leaf-rusting winter wheat [Triticum aestivum] cultivars Suwon 85 and P6028 and susceptible cultivars Monon and Suwon 92 were inoculated uniformly with urediospores of P. recondita in the greenhouse to measure components of slow-rusting resistance. Uredia first appeared on ''Monon'' and ''Suwon 92'' on day 6 after inoculation and all uredia had appeared by day 9 or 10. Uredia first appeared on ''Suwon 85'' and ''P6028'' on day 7 but all uredia did not appear until day 12 or 13. Throughout most of the period of opening of uredia, development lagged 2-4 days on ''Suwon 85'' and ''P6028''. The final numbers of uredia per square centimeter on ''Suwon 85'' and ''P6028'' were less than on ''Monon'' and ''Suwon 92'', but the difference was not statistically significant. For all cultivars, approximately 12% of the spores applied to the leaf gave rise to uredia. Uredium size was inversely related to the density of uredia, but uredia were consistently larger on ''Monon'' and ''Suwon 92'' than on ''Suwon 85'' and ''P6028''. The production of more urediospores per day per uredium on ''Monon'' and ''Suwon 92'', compared to ''Suwon 85'', was the result of larger uredia on these 2 cultivars. Low production of urediospores on ''P6028'' was due to less production per square millimeter of uredium and to smaller uredia. The effects of the resistance mechanisms in ''Suwon 85'' and ''P6028'' would be cumulative over the several infection cycles that occur during epidemic development in the field and should result in a much lower leaf rust severity on them compared to susceptible cultivars.