Determining the extent and causes of abnormal vertical growth (MC15011)

What was it all about? 

This project looked at the current impact of abnormal vertical growth (AVG) on the macadamia industry, as well as potential causes of the development and spread of the disorder.

Some top-level findings of the research include that:

• AVG is responsible for the loss of over 2000 tonnes of in-shell production annually
  
  
• AVG has become more widespread – there has been a five-fold increase in the number of trees with symptoms since 2003, with approximately 200,000 trees/900 hectares having been affected
  
  
• Severe AVG symptoms have been observed in varieties previously thought to be tolerant or only moderately susceptible to the disorder, with varieties of Hawaiian origin more susceptible than those developed in Australia, and a higher incidence in trees planted in soils prone to severe moisture stress
  
  
• The extreme seasonal fluctuation in flowering that AVG causes results in yield losses of between 30 and 88 per cent, depending on tree variety
  
  
• Management of AVG in orchards can cost up to $2500 per hectare annually
  
  
• Estimates of return-on-investment suggest that if affected trees aren’t treated, it’s more cost-effective to remove and re-plant (though there is a risk of AVG reoccurring in young trees).

Regarding the development of AVG, though initial observations suggested a role of geminivirus in the progression of the disorder, molecular analysis revealed no clear evidence of an association between the two. There remains, however, scope for further work to understand the role of bacteria and fungi found in samples.