Over the past year we’ve heard a lot about the metrics used to describe COVID-19 spread compared to other viruses. One such metric used to evaluate spread potential is the ‘reproductive rate’, which is the average number of secondary transmissions from one infected person.

The same metrics can be used to describe the spread potential of plant diseases.

In a recently published article on Xylella fastidiosa (1) in olives, authored by White et al. (2020), scientists have used mathematical models describing both spread and key determinants of this spread, based on observed field data of disease progression.

The authors estimated that each infected tree showing symptoms is able to infect around 19 other trees per year (95% credible range of 14-26).

The parameters used to calculate this reproductive rate are vital to consider, to ensure appropriate disease control measures are implemented both through policy and cultural behaviours.

Some of these spread factors include estimating:

  • The length of the symptomless period – noted to be important to estimate because visualisation of symptoms is currently key for policy implications in transporting olive trees, and for containment and control measures;
  • Field host to host transmission – currently only able to be extrapolated from lab studies, and important as a key determinant of the speed of spread locally and in the landscape; and
  • Time to death.

This valuable knowledge on predicting Xylella fastidiosa spread in olives is also applicable for predicting spread of the bacterium in grapevines and other plants, and this is work that could be done in the future.

And interestingly, there is significant commonality between the key spread-related factors of Xylella and those of grape phylloxera. As a result, this paper is a valuable resource for any future work undertaken to estimate grape phylloxera spread, as well as reviewing the appropriateness of current policies aimed at limiting this spread.


  1. https://bsppjournals.onlinelibrary.wiley.com/doi/full/10.1111/ppa.13238