Studies on the etiology of the leaf blotch disease of Eucalyptus spp. caused by Mycosphaerella nubilosa (Cke) Hansf
A detailed study of the leaf blotch disease, caused by Mycosphaerella nubilosa (Cke.) Hansf. on Eucalyptus regnans F. Muell (Mountain Ash) and E. delegatensis R.T. Baker (Syn. E. gigantea Hook. F., Alpine Ash) was made. The taxonomy of the fungus was studied(illustrations provided) and a previously unidentified imperfect state (Colletogloeum nubilosum) was described along with the spermagonial state (Asteromella). The histology and mode of infection process by ascospores and conidia was examined using Scanning Electron Microscopy, and wax and epoxy resin sections. The development of the fungus within the host was studied up to the stage of mature pseudothecia. The fungus was isolated from diseased tissue and Koch's postulates were carried out to prove pathogenicity. Detailed development of the leaf blotch and twig canker symptoms was followed in the field and in glasshouse conditions. The effect of substrate and environmental conditions on growth and sporulation of the fungus was studied. Controlled environment chambers and glasshouses were used. Several day/night temperature combinations (12/6, 18/12, 24/18 and 30/24C) as well as several light levels (50, 75, 150, 200, 300 and 650 ?E) were investigated. The optimum temperature for infection was 24/18C and least infection occurred at 12/6C. The optimum light level for infection was 300 to 650 ?E while the slowest infection was obtained at 50 and 75 ?E. Under constant temperature conditions infection was most severe at 18C for both E. delegatensis and E. regnans. Wetness periods under sprinklers of one to seven days were compared with similar incubation periods in polythene bags. One-day incubation produced very little infection compared with 2, 4 or 7 days. Incubation in polythene bags gave better results than sprinklers. Experiments showed that leaf and stem infection was not affected by nutrient levels. Leaf expansion rates for both the eucalypt species were followed for twelve weeks to determine the optimum leaf size for infection. Fungicide trials in the field and in-vitro suggested that several chemotherapeutants may be effective in controlling the disease. Diseased leaves from field trials were assessed for ascospore discharge to determine fungal viability. From this study it was shown that diseased (inoculated)"plants lost significantly more leaves than controls. If conditions were optimum for infection, tips of inoculated plants were killed, leading to stem distortion, branching and reduced growth. In general, the fungus causes severe damage on susceptible Eucalyptus spp. in sapling stages.