Annosus Root Disease Hazard Rating, Detection, and Management Strategies in the Southeastern United States

Annosus root disease (ARD), is the major root disease of pines in the southeastern United States where severely affected trees exhibit growth loss. Assessing the potential damage of ARD is essential for making effective disease control and management decisions. A soil hazard rating system developed to identify potential for tree mortality is described. The Annosus Sampling Procedure accurately estimates the severity of ARD within a stand. The GY-ANNOSUS computer program estimates growth loss caused by ARD. ARD management strategies are discussed. Annosus root disease (ARD), which is the major root disease of pines in the southeastern United States, is most severe in thinned plantations and stands. Thinning creates fresh stump surfaces and root wounds which are points of entry for Heterobasidion annosum Bref., the fungus that causes ARD. Severely affected trees grow at a slower rate and are more susceptible to bark beetle attack. Tree mortality tends to occur in localized areas, although the disease may occur throughout severely affected stands. ARD tends to be most severe on well-drained soils. Determining the amount of ARD present in a stand is essential for making effective disease control and management decisions. In the southeastern United States, symptoms of ARD are for the most part similar to those for any stressed pine tree. Severely affected trees may have thin, light green to yellow crowns, with 1 Presented at the Symposium on Research and Management of Annosus Root Disease in Western North America, April 18-21, 1989, Monterey, California. 2 Associate Professor of Plant Pathology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia. tufted branches and short needles. Windthrown trees may be present in affected stands, especially on deep, sandy soils. Most infected trees show no detectable symptoms. Signs of ARD are basidiocarps (often called conks) of the pathogen produced at the base of infected trees or stumps. Conks may be small and difficult to locate unless the duff layer is removed from around the tree or stump. Most infected trees, however, do not produce conks, which are the source of the airborne spores that infect freshly cut stump surfaces. After spore germination, the fungus grows down through the stump and roots, and infects standing trees through root contacts. Wounded roots close to the soil surface also may be infected by spores in the soil. Infected roots initially become resin-soaked and brownish-red. Later, these infected roots become a white, stringy mass of decayed tissue. The most critical factor in estimating disease incidence and severity within populations of plants is the accuracy of the survey and effectiveness of the detection methods. This is particularly true when working with a root disease of forest trees, because areas are extensive and samples are often large and difficult to collect. A cursory method of determining the incidence of H. annosum in pine stands was used in several major surveys to measure the incidence of infection by H. annosum (Powers and Verrall 1962; Powers and Hodges 1963). This method, based upon the observation of basidiocarps of the fungus at the base of infected trees, found that 2.8 percent of the loblolly pines examined within 120 plots from Virginia to Texas were found to be infected (Powers and Verrall 1962). A more direct sampling method was used by Ginns and Gillespie (1962) and later was tested extensively by Cordell and Stambaugh (1966). They used an increment borer to remove four core samples from the root collar of suspect trees and plated out the cores on culture media. This method was used for the determination of disease incidence in USDA Forest Service Gen. Tech. Rep. PSW-116 111 living trees in 12 plots ranging in size from 0.10 to 0.20 acres (0.04 to 0.08 hectare). A total of 392 trees were sampled of which 254 were white pine (Pinus strobus L.), 54 were loblolly pine (P. taeda L.), and 15 were red pine (P. resinosa Ait.). The increment core method of sampling indicated approximately an 11 percent increase in recovery of H. annosum over the use of pathogen signs alone. A similar study on 71-year-old Norway spruce (Picea abies (L.) Karst.) (Dimitri 1970) compared the isolation efficiency from core samples removed by the increment borer method to samples taken from trunk disks. All samples were taken at the 50 centimeter height on the stems of spruce. The disks were removed after felling. Results showed that where increment core sampling procedures indicated only 40 percent of the trees were infected, isolations from trunk disks showed that 85 percent of the 256 trees sampled were infected. Results of another study that involved the sampling of boraxor urea-treated stumps showed that culture plating of chips removed from individual roots did not significantly change the percent recovery of H. annosum over results obtained with the increment core technique (Artman and others 1969). However, isolation of the pathogen from the roots of check (untreated) stumps demonstrated a nearly threefold higher detection of the pathogen by the root chip isolation method than by the increment core isolation method. Alexander and Skelly (1973) compared the methods used for surveying the incidence of ARD in loblolly pine stands. A total of 79 trees were evaluated. The methods compared were: 1) to count conks, 2) to extract and culture increment core samples (two per tree) from the root collar zone, and 3) to remove 18 in. sections of two primary roots and make isolations from the samples. The results were, respectively, 2.5, 10.1, and 31.6 percent trees infected. In other studies, that involved the excavation and sampling of root systems, the average incidence of ARD was 80 percent. Assuming the 80 percent number to represent the actual level of infection, the lower percentages detected by the other methods show how little of the actual ARD was detected. Without doubt, the current estimates of the impact of ARD on pine forests in the southeastern United States are low. Tree mortality is the most visible result of annosus root disease, and for many years it was considered to be the only impact of ARD on southeastern pine forests. Morris (1970), for instance, described timber volume losses caused by this disease on several sites by measuring only dead trees. Growth losses sustained by infected living trees were not considered. Alexander and Skelly (1973) provided the first evidence of the potential impact of ARD on the radial growth of loblolly pine. Alexander and others (1975) demonstrated that loblolly pines infected by H. annosum produced 19 percent less radial growth during the preceding 5 years in thinned, coastal plain plantations. Alexander and others (1981) later reported that loblolly pines in southern pine beetle (SPB)-infested plots, with an average ARD incidence of 65 percent and an average ARD severity (percentage of roots colonized by H. annosum) of 23 percent, produced 6 percent less radial growth during the preceding 10 years than control plots with an average ARD incidence of 73 percent and an average ARD severity of 11 percent. Between SPB-infested trees (54 percent ARD severity) and SPB-uninfested trees (11 percent ARD severity), the SPB-infested trees grew 20 percent less for the preceding 10 yrs than the SPB-uninfested trees. Trees infected by H. annosum, regardless of SPB presence, produced 36 percent less radial growth than noninfected (healthy) trees during the preceding 10 years. Bradford and others (1978) excavated the root systems of 25 loblolly pine trees in 14 thinned plantations and measured their infection by H. annosum. The average ARD incidence was 85 percent and the average ARD severity was 30 percent. The majority of the trees showed no above-ground symptoms or signs. Trees infected by H. annosum produced 19 percent less radial growth (the same as reported by Alexander and others 1975) during the preceding 5 yrs than noninfected (healthy) trees. Three plots were on intermediate ARD hazard soils; the average ARD incidence level was 70 percent with a severity level of 33 percent. Froelich and others (1977) evaluated 65 slash pines for ARD in one thinned plantation and found a 20 percent reduction in radial growth of trees with greater than 50 percent of their root system colonized by H. annosum and a 40 percent reduction in height growth 6 years after thinning. SOIL HAZARD RATING SYSTEM Morris and Frazier (1966) developed a field hazard evaluation system based on soil characteristics which identified soils associated with high mortality 112 USDA Forest Service Gen. Tech. Rep. PSW-116 caused by ARD. Froelich and others (1966) identified the characteristics of healthy and high ARD plantations. In an examination of highand low-hazard disease sites Alexander and others (1975) measured 14 soil parameters at depths of 10 to 20 centimeters. Sand, porosity, organic matter, and field capacity had the highest correlation when compared with disease incidence. Because of the simplicity and ease of use, the soil ARD hazard system has been extensively used, and misused, in the southeast. Many have assumed that ARD is a problem only on high ARD hazard soils. This belief is unfounded because most southern pines can be significantly infected and show no outward sign of the disease. We now know that ARD can reduce growth without causing any outward signs of infection. Tree mortality and to some extent high disease incidence are associated with: (1) high sand content, (2) high percentage of noncapillary pores, (3) high soil bulk density, (4) low organic matter, (5) a deep A-horizon, (6) a low water table, and (7) dense grass or similar vegetation cover. The most important independent variables are sand content, organic matter, and water table depth (Alexander and others 1975). Low ARD caused mortality is related to (1) high clay content, (2) high percentage of capillary pores, (3) high organic matter, (4) ground cover, (5) a high water table.