Effect of Drought and Freezing Stresses on Susceptibility of Blue Spruce to Cytospora (Valsa) Canker¹

by Donald F. Schoeneweiss²

Stems of 5-year-old Colorado blue spruce wound-inoculated with a conidial isolate of Cytospora (Valsa) kunzei became predisposed when subjected to controlled drought stress. Typical bark cankers appeared on stems with plant water potentials below –20 bars, while no cankers formed on nonstressed stems or on stems subjected to freezing stress of –20 and –30°C. Although the pathogen was recovered from wood in both stressed and nonstressed stems, necrotic bark cankers formed only on drought-stressed plants. These results support the hypothesis that drought stress is the controlling factor predisposing spruce to Cytospora canker.

It has been suggested that ascospores serve as inoculum and that conidia are not infective. In this study, cankers did form from conidial infection provided the plants were under significant drought stress for predisposition. Freezing stress had no apparent effect on susceptibility. A selective culture medium was developed (30 g Difco PDA, 0.1 g chloramphenicol, 0.1 g streptomycin sulfate, and 0.3 g ethazol (Truban) in 1 liter H₂O acidified to pH 5.5 with dilute HCI) which should aid in isolating Cytospora from spruce stems.

• © 1983, International Society of Arboriculture. All rights reserved.