Corky Root Disease of Douglas-fir: Relation of Xiphinemabakeri Nematodes to Symptom Severity, and Observations on Seedling Tissue and Soil Nutrients

Abstract

Four-month-old Douglas-fir Pseudotsugamenziesii Mirb. (Franco) seedlings were collected from a coastal British Columbia forest nursery, and the number of Xiphinemabakeri nematodes per cubic millimeter of root volume was related to the severity of several corky root disease symptoms. The strongest relationships existed between nematode root populations and seedling epicotyl length, and the number of first-order lateral roots over 5 mm long. Diseased and healthy seedlings were analyzed for tissue nutrients. Healthy seedlings contained significantly more nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), and chlorophyll, and significantly less calcium (Ca) than diseased plants. The nutrient content of soil samples collected at 0–4, 4–8, 8–12, and 12–16 cm depths from areas with or without diseased seedlings was determined. The N, Ca, Mg, and conductance levels of soils from the healthy seedling area were significantly greater than those from the disease-infested area, but no other pertinent differences, relating to disease occurrence, were found in the nutrient content of samples from the different depths, and no relevant disease class - sampling depth interactions were detected. Several factors, which help explain the recent increased incidence of corky root disease, are discussed.