Needle Weights as a Measure of Infection by Uredinopsis mirabilis and Chrysomyxa weirii. D. S. Abrahamson, Forestry Technician, Department of Forestry, School of Natural Resources, University of Vermont, Burlington 05405. D. R. Bergdahl, Associate Professor, Department of Forestry, School of Natural Resources, University of Vermont, Burlington 05405. Plant Dis. 71:270-271. Accepted for publication 12 October 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0270.

Needle weights were used to determine disease intensity of Uredinopsis mirabilis and Chrysomyxa weirii on Abies balsamea and Picea pungens, respectively. Needles were removed from twig samples, infected and healthy needles were separated and counted, and then fresh and oven-dry weights determined. Percentage of needle infection determined by either fresh or oven-dry weight was highly correlated with infection as determined by needle counts. R² values correlating weight and count were 0.92 and 0.98 (U. mirabilis) and 0.93 and 0.97 (C. weirii) for fresh and oven-dry weights, respectively (P < 0.01).