Die-Back Tolerance in Eucalypt Species in Relation to Fertilization and Soil Populations of Phytophthora cinnamomi

Abstract

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.