PREDICTION OF POPLAR LEAF RUST EPIDEMICS FROM A LEAF-DISK ASSAY

The reliability of a leaf-disk assay to assess resistance of Populus deltoides Bartr. to Melampsora medusae Thuem. f.sp. deltoidae, the causal agent of poplar leaf rust, was evaluated. Leaf disks of eight host genotypes were inoculated in a spore settling lower with 11 isolates of the pathogen in all possible combinations, and the latent period, infection probability, sporulation, and progeny/parent ratio were recorded. Correlations were established between these measures and measures derived from field epidemics, namely the relative area under the disease progress curve, the apparent rate of infection, the final disease severity, and the number of days before defoliation. Four genotypes were highly resistant to all 11 isolates tested with the leaf-disk assay and to the local inoculum in field tests. Three genotypes were highly susceptible in inoculation assays and were also susceptible in the field. One genotype was highly resistant in inoculation assays but had intermediate resistance in the field. Significant correlations (p < 0.05) were found between all leaf-disk and all field parameters.