Tree Pathology Co-operative Programme, Forestry & Agricultural Biotechnology Institute (FABI), Dept. Microbiology & Plant Pathology, Univ. of Pretoria, Pretoria 0002, South Africa 2Dept. Plant Pathology, Univ. of Stellen bosch, Stellenbosch 7800, South Africa

Background and objectives
Sphaeropsis sapinea (Fr.) Dyko & Sutton is an economically important pathogen of pines in South Africa. The fungus causes various symptoms including dead top, shoot die-back, blue stain, collar rot and whorl canker. The most devastating disease caused by this fungus is post-hail associated die-back. Sphaeropsis sapinea has become notorious as a pine pathogen largely due to the impact of post-hail associated die-back in South Africa.

Four Pinus spp. are predominantly used in commercial plantations in South Africa. Of these, Pinus elliottii and Pinus taeda are relatively resistant and Pinus patula and Pinus radiata susceptible to infection by S. sapinea. Due to post-hail associated die-back the planting of P. radiata has been restricted to the winter rainfall areas, where hail is infrequent. Extensive die-back of susceptible pine species is common after hail damage and field observations indicate that tree age influences mortality associated with post-hail associated S. sapinea infections. Old trees show greater mortality and the most obvious difference between these and younger trees, is the presence of seed cones on old trees. Sphaeropsis sapinea is known to occur as latent infections of seed cones [1] and it spreads rapidly within trees once it is established in the pith. The aim of this study was to investigate the role of latent infections of seed cones in disease development on P. patula after hail.

Results and conclusions
Isolations from mature un-opened seed cones of four pine species indicated that S. sapinea is present as the dominant fungal inhabitant in all tissues screened [1]. Three age classes of P. patula seed cones were screened for the presence of S. sapinea in order to determine when infection occurs. Sphaeropsis sapinea was not recovered from first year cones. The fungus was recovered at low frequencies from second year cones and here, only from the outer ovuliferous scale tissue. During the third year of development when cone tissue becomes inactive, extensive colonization occurred in all tissues. Selected isolates obtained from third year cones were paired in all combinations on oatmeal agar (OMA) to determine the diversity of isolates within each cone. In all cones tested, the isolates represented a single vegetative compatibility group (VCG). This suggests that many infections may occur in the second year, but that most do not colonize tissue successfully. After a severe hailstorm, mortality rates were most severe in a stand of 25-year-old P. patula trees (92%) followed by a stand of 14-year-old trees (32%) and a 5-year-old stand (9%). Greater mortality in older trees could not be explained by infection of hail wounds. Isolations from diseased and healthy trees showed that S. sapinea could be recovered from darkly stained branch pith and attached seed cones of diseased trees. This was in contrast to the absence of the fungus in the pith of branches of healthy trees of the same age. Our results indicate that S. sapinea infects branch pith tissue directly from previously infected, attached seed cones, to cause rapid die-back and mortality of hail damaged trees.

1. Smith H, wingfield MJ, Crous PW, Coutinho TA. 1996. South African Journal of Botany 82, 86-88.