QUANTIFICATION OF DISEASE PROGRESS AND DEFOLIATION IN THE POPLAR LEAF RUST-EASTERN-COTTONWOOD PATHOSYSTEM

During three seasons, poplar leaf rust epidemics caused by Melampsora medusae f. sp. deltoidae were compared among families of eastern cottonwood, Populus deltoides, derived from natural stands along the Mississippi River valley from Cairo, IL, to Rosedale, MS, and grown in Carlisle County, KY. In 1988 and 1989, families of southern origin were more resistant than their northern counterparts at the test site with regard to the relative area under the disease progress curve (RAUDPC), the final disease severity (Yf), the rate parameter from the Richards model (Ra), and the number of days before defoliation (DLF). Cluster analyses using all variables grouped families into northern clusters of susceptible families and southern clusters of resistant families. Significant correlation coefficients were calculated between disease progress variables (RAUDPC, Yf and Ra) and defoliation variables (DLF). Because between 95 and 99% of the variation in RAUDPC was explained by one disease observation during the last week of September disease resistance was estimated, in 1990, in 40 families of eastern cottonwood by measuring disease severity and defoliation once in early October. A north-south gradient again was apparent, with families from the north showing more susceptibility than families from the south.