Basal stem injection of Dicamba herbicide (20% 3,6-dichloro-2 methoxybenzoic acid) into the outer sapwood was tested as a technique for predisposing conveniently placed groups of Pinus radio/a trees to attack by the European tree-killing wood wasp Sirex noctilio. The trials included injection of Dicamba during spring and summer, injection of de-ionised water during spring and untreated controls, and were undertaken in unthinned plantations of two ages, 8 and 15 years.Sirex noctilio was attracted almost exclusively to the Dicamba-treated trees, which provided suitable breeding habitats for the pest and its introduced natural enemies, both wasp parasitoids and nematode parasites. Spring injection gave more satisfactory results than the summer injection. The trees in the older stand were more susceptible to attack, as were the trees in the smaller diameter classes in both stands. Infested treated trees contained an average of 275 wasps irrespective of tree age. Dicamba injections did not induce major attacks on any nearby untreated trees.The practical importance of these findings is that S. noctilio can now be readily detected, its populations periodically monitored, and control improved in plantations, by attracting the pest, and its lethal wasp parastoids, to strategically placed and widely distributed groups of Dicamba-treated 'trap' trees. Inoculation of the trees with the nematode parasite, De/aden us siricidicola, which infests S. noctilio larvae, will cause the infested females which subsequently emerge to be sterile and, as a result, the population is expected to fall.

Detached, six-week old cotyledons of silvertop (Euca/ypllls sieberi F.Muell.) floated over samples of eucalypt forest soil were used for baiting P. cinnamomi. The fungus could be identified either directly on the cotyledons or by plating cotyledon pieces on antibiotic agar. The method proved sensitive, reliable and easy to use, especially for broad scale surveys.

The behaviour of Phytophthora cinnamomi Rands was examined over a 2 year period on newly cleared and replanted eucalypt sites in the coastal forests of eastern Gippsland, rated as of low, moderate and high die-back hazard in relation to the drainage and disease characteristics of the original forest cover and surrounding trees. The fungus behaved like a saprophytic survivor. Maximum population levels, assessed as a 'population density index' (PDI), occurred within the soil influenced directly by the root mass. The PDI in adjacent bare soil was very low. The PDI depended, inter alia, on fungal pathogenesis, soil temperature and soil moisture. Mean maximum PDI values recorded respectively for the high, moderate and low hazard sites were in the ratio of about 8/4/1. This ratio was directly related to the depth of the clay pan beneath the surface. There was a marked seasonal variation in PDI, most pronounced on the high hazard site and least on the low hazard site. The minimum and maximum values were recorded in June (winter) and December-March (summer) respectively. Moderately heavy rainfall had little effect on the PDI in well-drained soils, even during midsummer when soil temperatures were optimum for fungal populations to increase. Fire affected PDI values only temporarily. PDI values in the surface soil were greater than at a depth of 75 cm, and the distribution of the fungus through the soil profile was influenced by soil texture. Before canopy closure occurred, PDI values were greater under eucalypt species tolerant to root rot than under sensitive species. Canopy closure reduced the seasonal fluctuation in soil temperature and, on sites where tree growth was vigorous, the reduction in PDI was striking. Addition of fertilizers to the soil had no direct effect on the PDI, the reduction observed being an indirect effect produced possibly by accelerated growth, increased transpiration and rapid canopy closure. These results suggest that root rot-sensitive eucalypt species growing on coastal sites with impeded drainage will prove vulnerable to die-back, and that any form of logging activity that reduces the amount of green cover on infected sites will aggravate root rot if the fungus is present. Consequently it will not be possible to manage these stands by conventional methods. On steeper, well-drained sites the disease should not prove to be a serious hazard except in exceptionally wet summer and autumn periods.

The distribution and symptoms of die-back in the mixed-species eucalypt hardwood forests of eastern Victoria are described. The disease was recognizedin 1952 in small patches of forest situated in the flat, badly drained sandy coastal soils and has spread rapidly. This spread was associated with above-average rainfall during spring, summer, and autumn. Disease symptoms were similar to that caused by drought, and were observed best during dry periods following summer rain. The lupin bait of Chee and Newhook was used to isolate Phytophthora cinna- momi from the diseased areas, and the field symptoms seen on susceptible eucalypts of the subgenus Renantherae were similar to those observed on young saplings inoculated with P. cinnamomi in the greenhouse. Species of the subgenus Macrantherae observed in the forests and tested in the greenhouse are tolerant to the disease. Estimates of population density indices of the fungus gave highest values in areas where the disease was most active. The fungus was not isolated from adjacent healthy forests. Improvements in the baiting and identification of P. cinnamomi with use of excised lupin radicles floated over a small sample of soil in a petri dish are described. Several variants of the fungus with differing growth rates were isolated.

The die-back tolerance of 16 fertilized (17/9/7 nitrogen-phosphorus-potassium) and unfertilized eucalypt and two conifer species was tested in the coastal forests of east Gippsland on sites rated as of "high", "moderate", and "low" hazard on the basis of previous damage, internal soil drainage, and infection by P. cinnamomi. Measurements were made of the population density index (PDI) of P. cinnamomi, of soil moisture, and of soil temperature. Supporting greenhouse and laboratory experiments are also reported. The high hazard site showed the most uniform infection and the greatest PDI, the low hazard site the least uniformity in the infection pattern. During the first year's growth, five renantherous eucalypt species showed considerable sensitivity to root rot and die-back; the intensity of the disease and the number of deaths were directly proportional to the hazard rating of the sites. The 11 species of Macrantherae tested were very tolerant to die-back. The disease was aggravated by temporary waterlogging during a 7 day period, but waterlogging did not cause die-back. The disease first appeared in the plots when soil temperatures rose above 15°C. Greenhouse tests showed that P. cinnamomi was most virulent at 22°, and visible injury became evident between 15 and 18°. Fertilizers produced striking growth responses during the first year in both subgenera of eucalypts on the low hazard site, with only minor differences between the two groups. Similar responses were seen only on the Macrantherae on the moderate and high hazard sites. The growth of the surviving renantherous eucalypts was uneven, and fertilizers greatly increased their sensitivity to die-back disease. The response to fertilization in both subgenera was directly related to the disease hazard of the site and the intensity of infection by P. cinnamomi. Differences in response to fertilization between the Macrantherae and the Renantherae were directly proportional to the population density and distribution of P. cinnamomi in the soils. The initial tolerance to die-back of the two conifers, Pinus radiata and P. Elliotii, was similar to that of the most resistant macrantherous eucalypts tested.