Influence of site factors and vascular conductivity on the development of procerum root disease
Procerum root disease (PRD) is a serious problem in eastern white pine (Pinus strobus L.) Christmas tree plantations in VIrginia. Procerum root disease is caused by Leptographium procerum (Kendr.) Wingf which is believed to be transmitted by the pales weevil Hylobius pales (Herbst). Symptoms of procerum root disease include reduced shoot elongation, reduced leaf conductance, low photosynthetic activity, low pre-dawn water potential and chlorosis of foliage. Resinous occlusion of the sapwood at the root collar is the likely cause of the suite of symptoms that resemble water stress. Increased incidence ofPRD has been associated with trees growing in poorly drained soils in low lying areas. Two studies were undertaken to explore the relationship of site factors and vascular conductivity of sapwood to the expression ofPRD symptoms in P. strobus. In the first study, plots were established in a variety of drainage classes in two Christmas tree plantations. Leaf conductance was monitored periodically in conjunction with measurements of soil factors to assess the role of abiotic factors on foliar symptom expression. At the termination of the field monitoring, trees were harvested and three vascular disease severity variables were measured: hydraulic conductivity of sapwood, percent basal occlusion and sapwood moisture content. These disease severity variables describe the permeability of sapwood to water and the relative hydration of the sapwood. Reduced leaf conductance was associated with reduced stem hydraulic conductivity, reduced sapwood moisture content and increased basal occlusion. Increased vascular disease severity and foliar symptom expression were associated with increased soil moisture content and several other factors that relate to soil moisture retention (percent slope, total porosity, textural class and bulk density). The second study was initiated to study the development of PRD in artificially inoculated P. strobus seedlings exposed to three soil water classes: droughty, optimum and saturated. Seedlings in the optimum soil water class exhibited the greatest biomass accumulation and shoot elongation, while seedlings in droughty showed the least. No negative effects of L. procerum inoculation and no symptoms ofPRD were observed eight months after inoculation regardless of the soil water class.