Comparison of the Wheat Brown and Yellow Rusts for Monocyclic Sporulation and Infection Processes, and their Polycyclic Consequences

Sporulation capacity and infection efficiency of wheat brown and yellow rusts were measured daily in favourable controlled conditions. Monocyclic sporulation capacity for a single lesion of yellow rust was 9 times greater than for an isolated lesion of brown rust, and 40 times greater than for a lesion of brown rust at medium infection density. Infection efficiency fluctuated and reached about 40 % for brown rust but remained under 5% for yellow rust. For both fungi, sporulation capacity and infection efficiency compensated for each other, but their product, the daily multiplication factor, was greater for yellow rust than for brown rust. Progeny/parent ratio was 3 times greater for yellow rust. Effect of daily multiplication factor variations on epidemic progress was simulated using a simple matrix model. Increase in number of lesions was faster in brown rust than in yellow rust because of a latent period shorter by 2 days. Semi‐systemic growth of yellow rust fungus reduced, however, the difference between both fungi when sporulating surface was calculated.