Geographic Variation in Melampsora Rust Resistance in Eastern Cottonwood in the Lower Mississippi Valley

-Eastern cottonwood clones originating from 36 young natural stands along the Mississippi River from Memphis, Tennessee, to Baton Rouge, Louisiana, were evaluated in a nursery near Greenville, Mississippi for resi~tance to Melampsora rust. In general, the northern sources had more rust and were more variable in rust susceptibility than the southern sources. Eleven percent of the clones from the 6 northernmost sources and 0.5 percent of the clones from the 24 southernmost sources appeared to be highly resistant to rust. Clones with slightly less rust resistance wer.e about evenly distributed over the geographic range. Additional keywords: Populus deltoides, Melampsora medusae.