Virulence of Leptographium serpens on Longleaf Pine Seedlings Under Varying Soil Moisture Regimes

Longleaf pine (Pinus palustris Mill) was once the main southern pine species found throughout the southeastern United States, encompassing approximately 38 million hectares (6). After nearly complete destruction of the longleaf pine ecosystem, restoration efforts in recent years have caused resurgence in the planting of longleaf pine on many state and federal lands (14). Many factors have significantly contributed to this renewed interest, including the requirement of longleaf pine forest for several endangered and sensitive species, specifically the red-cockaded woodpecker (Picoides borealis Vieillot) (1). Unfortunately, decline and premature tree mortality has recently been observed in longleaf pine stands (19,20). The current rate of mature longleaf pine mortality may affect future restoration efforts. Studies concerning a similar decline and premature tree mortality in loblolly pine (Pinus taeda L.) have identified several possible contributing factors, including a group of root-inhabiting stain fungi in the genus Leptographium (3,5). Despite the common theory that longleaf pine is “resistant” to native insect and diseases (10,22), it is hypothesized that Leptographium spp. may be contributing to this decline and mortality of longleaf pine (19,20). Leptographium serpens (Goid.) Siemaszko has been associated with various pine diseases throughout the world (27), including root disease of two Pinus spp. in South Africa (28). In inoculation tests, L. serpens (formerly Verticiladiella alacris; 29) was found to cause 20-cm lesions on pine roots after 6 months (28). In contrast, Zhou et al. (30) found L. serpens to be nonpathogenic to Pinus branches in South Africa using a similar inoculation technique. Within the United States, it has been identified as a contributor to loblolly pine decline (5). In loblolly pine seedling inoculations, L. serpens was more pathogenic than both L. terebrantis and L. procerum (3), causing 30-mm sapwood lesions after 4 months. Many Leptographium spp. have been found associated with pine decline and mortality, usually with one or more contributing site or stand factors (5,12). These syndromes are most often referred to as decline diseases, characterized by many factors contributing to the prolonged death of the tree (15). It is possible that multiple inciting biotic and abiotic factors may be acting in conjunction with Leptographium spp. to cause the observed longleaf pine mortality. Inadequate soil moisture is often experienced by southern pines and imposes a significant stress on individual pine trees. Successive, extreme, and devastating droughts in recent years (2000 to 2007) along with the pathogen’s presence may contribute to the observed tree mortality. These studies seek to determine the virulence of L. serpens to longleaf pine seedlings grown in three soil moisture regimes.