Comparative growth, morphology, and physiology of three Sclerotium species

Isolates of S. coffeicola, S. delphinii and S. rolfsii from diverse geographical areas were compared for differences in morphology (colony characteristics and sclerotial formation), growth response to different temperatures and media (radial growth, dry weight and sclerotial production), and ability to produce oxalic acid and pectinase enzymes. The best medium for colony growth of S. coffeicola and S. rolfsii was V8 agar and for S. delphinii it was PDA. Greatest sclerotial production in all three species was achieved on PDA at incubation temperatures of 20, 35, and 20 C, respectively. Under these conditions, the total sclerotia produced in was 7, 1043, and 47 per petri dish and the average sclerotial diam was 5.1, 0.6, and 3.5 mm for S. coffeicola, S. rolfsii and S. delphinii, respectively. The species all formed clamp connections on leading hyphae, and the number of nuclei in hyphal cells ranged fro 1 to 6, with an average of 2 per cell for each species. The basidial stage was induced only in S. rolfsii (Athelia rolfsii) on PDA containing charcoal and was not observed in the other two species. The sclerotia of all species were comprised of three layers: an outer rind, a middle cortex and an inner medulla. The cortical layer was 15-20 cell layers thick in S. coffeicola compared to 4-10 layers in the other two species. Histochemical staining of sclerotia showed that the concentration of carbohydrates and metachromatic granules was greatest in the medulla, followed by the cortex, and was mostly in the cytoplasm. Proteins were also distributed in both the cortex and medulla of S. rolfsii and S. delphinii, but were more concentrated in the medulla of S. coffeicola, and occurred in both the walls and cytoplasm. Phenolic compounds were observed in the rind cells of all sclerotia, with S. coffeicola containing the least amount, and were localized to the walls. All three species produced oxalic acid, pectinase and polyphenoloxidase in culture, with highest production in S. rolfsii, followed by S. delphinii and S. coffeicola. The results from this study show a close similarity among the three Sclerotium species for the parameters measured. The species can be differentiated by colony characteristics, such as morphology and size of sclerotia, by differences in histochemical staining and composition of sclerotia, and by growth responses to temperature.