Electrical resistances of combined bark and wood of 50 red maple (Acerrubrum L.) trees were strongly correlated with phloem width with an inverse correlation coefficient of −0.92. Diameter at 1.4 m aboveground was determined to be transitively correlated with electrical resistance through its strong correlation with phloem width. Little detectable relationship existed between electrical resistance and potassium concentration in phloem tissues. An electrical model is presented to illustrate the path of electrical current between electrodes in the tissues measured.

Greenhouse-grown elm seedlings inoculated with Ceratocystisulmi developed visible symptoms of Dutch elm disease within 14 to 23 days. Mean electrical resistance measurements were used to detect alterations within host xylem tissue up to 6 days prior to symptom expression in populations of trees. Criteria are given for prediction of the disease in individual trees from 2 to 12 days prior to symptom expression.