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How do pests and diseases affect the choice of planting Eucalyptus nitens or E. globulus?

Eucalyptus globulus is superior to E. nitens in many wood properties important for pulp and solid wood, and these superior properties can translate to a higher value per cubic metre of log. However, more than 80% of hardwood plantations on State forest are currently E.nitens. E. nitens is the only suitable species for cold sites, but we have recently reviewed climatic analyses and concluded that sites with a mean minimum temperature in the coolest month of >1.8°C are suitable for E. globulus as well as E. nitens.


Sites climatically suitable for E. globulus represent nearly 53% of the current State forest hardwood plantation estate, but E. globulus is planted on less than one-third of these sites.  The main reason for preferring E. nitens on warmer sites is the risk of severe defoliation of E. globulus by Mycosphaerella leaf disease, and the perception that
 E. globulus has a lower growth rate.  The best choice of species to plant on warmer sites can be determined by comparing the value and quantity of wood produced after accounting for the impacts of pests and diseases.


An analysis was done to identify the most important pests and diseases likely to affect the choice of planting
E. globulus or E. nitens on warmer sites. Four pests and diseases were identified as being the most important for species choice – Mycosphaerella leaf disease (MLD) and the weevil Gonipterus scutellatus, which were strongly associated with damage to E. globulus, and Phytophthora root rot (PRR) and drought death at or soon after transplanting, which were strongly associated with damage to E. nitens. These four pests and diseases were most prevalent in different climate regions. MLD and G. scutellatus tended to occur in similar climate regions, where the growth reductions they cause to      E. globulus would require significant, but realistic, price premiums for E. globulus pulp and peeler logs to provide higher financial returns than E. nitens. PRR and drought also tended to occur in similar climate regions, with impact of PRR on the growth of E. nitens being a major uncertainty affecting comparative financial returns.


This is the first analysis that compares financial returns from growing E. globulus and E. nitens in areas suitable for both species, after accounting for major pest and disease risks. It highlighted critical uncertainties, and suggested values for pricing parameters that would allow E. globulus to be more profitable than E. nitens.

 

Dr Tim Wardlaw


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