Tolerance of Loblolly Pines to Fusiform Rust

Loblolly pines (Pinus taeda L.) thut were 8 to 17 yr old tolerated one to three fus[form rust (Cronartium quercuum [Berk.]Miyabe exShiraij sp. fusiforme) galls in theirstems. Families withfourormore galls in their stems lost 2.5% or more ofthe trees by age 17. In living trees with less thanf<jurstem galls, diameter growth was comparable to that of trees with no galls. Tolerance was indicated by the abilit?, of loblolly pines to maintain the rustfungus in stems that had dbh ‘s similar to asymptomatic trees on the same site. Inpluntutions, the number of galls in the stem was generally one to two per infected tree. This was ulso true,for mature trees (12 to 38 in. dbh) along the Nutchez Trace Parkway. These trees have been infected withfusiform rustfor nearly 100 yr. On the other hand, the presence offour or more stem galls seems to be a reliable indicator of mortality rather than tolerance. South. J. Applied For. 19 (2): 60-64. 0 ver the past 30 years, damage and death in loblolly (Pinus taeda L.) and slash (P. elliottii Engelm. var. elliottii) pines caused by the fusiform rust fungus (Cronartium quercuum (Berk.] Miyable ex Shirai f. sp. fusiforme) have increased dramatically. In response, foresters first tried to locate geographic sources of pines resistant to the disease (Wakeley and Bercaw 1965). As a result, millions of progeny of moderately resistant loblolly pines from Livingston Parish, Louisiana, were planted over large areas of the South (Wells 1985). Next, pathologists and tree breeders tried to produce strains of loblolly pine that were resistant to the disease as determined by gall formation (Powers and Kraus 1983, Sluder 1989). The USDA Forest Service established the Resistance Screening Center in Asheville, North Carolina, where newly germinated seedlings are exposed to high concentrations of rust spores (Laird and Phelps 1975). Pine families in which a low proportion of exposed seedlings become infected and subsequently develop rust galls are considered for use in rustresistant seed orchards, where seeds with a high degree of resistance to infection are produced (Powers and Kuhlman 1987). Overall, this genetic approach has reduced the number of slash and loblolly pines lost to rust in the areas of greatest hazard, but there have been problems associated with the wide genetic variation in the rust fungus (Powers and Matthews 1979). Some slash and loblolly pine families that survive inoculation tests become infected when exposed to different NOTE: We thank Tom Dell and Jim Gates of the USDA Forest Service for advice; and Richard Bryant, International Paper Company, and Marvin Zoerb, Union Camp Corporation. for help in planning observations and interpreting data. strains of the fungus in the field. Just as there are some geographic areas in which pines appear to be resistant, there are others in which the rust fungus is particularly virulent even among different galls in the same stand (Snow et al. 1975). As a result, if resistant slash and loblolly pines are planted over a large area (e.g., a county), the fungus strains in that area may eventually overcome the resistance. This problem would most likely develop in subsequent plantings. Part of the problem is that the genetic makeup of the fungus is so variable and adaptable. Furthermore, when pines are selected because they do not become infected, the less virulent fungus strains for which pines are resistant are not reproduced. The result may be some selection pressure for new virulent fungal strains. That it is important to maintain populations of less virulent strains has been demonstrated experimentally (Powers and Matthews 1979). If the objective of breeding is to prevent gall formation, forest managers must constantly breed new loblolly pines to avoid changes in the rust fungus, which is how crop plant breeders have controlled wheat rust. This may be the only practical approach if a gall on a tree bole usually kills the tree. Loblolly pines, however, often survive for very long periods with one or more galls on the tree bole. Also, infections in branches near the bole are sometimes prevented from entering the boles of loblolly pines. The data presented in this paper suggest a new strategy for managing rust in loblolly pines: instead of breeding pines to resist infection entirely, perhaps we should be breeding trees to minimize rust damage. This tolerance, or ability to maintain the rust fungus and produce good diameter growth, could further reduce damage to stem wood quality if galls remain small. Reprinted from the Southern Journal of Applied Forestry, Vol. 19, No. 2, May 1995 60 SJAF 19(2) 1995 To explore the possibility of using gall tolerance in the management of fusiform rust on loblolly pine, some questions need to be addressed: (1) Why do some loblolly pines with several rust galls in their boles live long and productive lives? (2) Do loblolly pine families vary in their tolerance to fusiform rust? (3) What types of rust-tolerance mechanisms exist in loblolly pines? To answer these questions, we collected data from progeny studies, commercial plantations, and natural stands of loblolly pine.