Effect of Leaf Maturity and Shoot Age of Clones ofPopulusspecies on Susceptibility toMelampsora larici-populina

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Abstract
Leaves of varying maturity were detached from 4-33 day old shoots of clones of P. nigra ''Semi-evergreen'' (field susceptible) and P .times. euramericana [P. deltoides .times. P. nigra] ''65/27'' (field resistant) grown in a rust-free glasshouse. The detached leaves were inoculated with uniform loads of uredospores of M. larici-populina, and incubated in a controlled environment. Reaction type of the leaves and latent period (LP) for flecks and for uredia production were recorded. Within a shoot, LP and reaction type were related to leaf maturity, but for leaves of the same maturity, selected from shoots of varying age, the reaction type was determined by the age of the shoot. In comparisons between the clones or between leaves of different maturities or zones of shoot within clones, higher susceptibility was associated with shorter LP. The ecological and epidemiological significance of the results are discussed.