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PhD research furthers knowledge on red needle cast

Scion is the leading provider of forest-related knowledge in New Zealand
Formerly known as the Forest Research Institute, Scion has been a leader in research relating to forest health for over 50 years. The Rotorua-based Crown Research Institute continues to provide science that will protect all forests from damage caused by insect pests, pathogens and weeds. The information presented below arises from these research activities.

From Forest Health News 294, November 2019.

You must have heard many times about Phytophthora pluvialis, a pathogen that infects needles of Pinus radiata (radiata pine) and Pseudotsuga menziesii (Douglas fir) in New Zealand (NZ) and the US Pacific Northwest (PNW). But, I invite you to read on and discover a few more things about this amazing pathogen. Given the meaning of Phytophthora (plant destroyer), you may picture dead trees at best (better said, at worst). And you might be right in a few cases. However, Phytophthora pluvialis has become notorious for causing red needle cast (RNC), a disease typically affecting the lower crown of radiata pine and Douglas fir from early autumn to late winter in NZ, but rarely, if ever, causing mortality. During late spring and summer, symptomatic needles cast and regrowth starts, leading to green but thin canopies.

During my PhD research, performed at Scion from May 2015 to December 2018, I increased knowledge on (1) the epidemiology of P. pluvialis in NZ and Oregon forests, and (2) the physiological impact of P. pluvialis infection on the host.

Phytophthora pluvialis is a polycyclic pathogen, with several infection cycles per year. I characterised the epidemics of P. pluvialis on radiata pine by fitting data from detached-needle assays and on-plant inoculations for different radiata pine genotypes in an epidemiological model1. I showed that for a single cycle of infection in susceptible plant material, P. pluvialis starts sporulating four days after inoculation. At the plant level, the polycyclic infection peaks at 22 days after inoculation. When P. pluvialis attempts infection of radiata pine resistant genotypes, it succeeds in creating infection, but it produces mycelium and sporangia at a lower rate. Further, P. pluvialis dies at a rate 2.5 greater in resistant than in susceptible genotypes. The epidemiological model used here could be further extended to explore the RNC epidemics at the forest level. For example, it could be used to test the impact of spatial heterogeneities of susceptible and resistant trees for disease control.

Mireia sampling Douglas fir in coastal Oregon.

Another study showed that some susceptible genotypes are able to tolerate defoliation, continuing to grow and store carbon for future stress events2. I artificially defoliated 72 radiata pine trees from two susceptible genotypes, mimicking RNC in the field. Defoliated grafts of one of the genotypes grew and stored carbon as much as the control trees, whereas the other genotype saw a 32% reduction of its diameter growth. Specific leaf area was different between genotypes. These results are important for RNC resistance breeding, as they may indicate a functional trait that may be a marker for tolerance, rather than resistance to infection itself.

When I started my PhD, little research had been done on P. pluvialis infection of Douglas fir. This track was full of surprises. When I attempted to characterize P. pluvialis epidemiology in Douglas fir, I was confronted by another pathogen3, the ever-present ascomycete Nothophaeocryptopus gaeumannii, cause of Swiss needle cast.

I carried out a thorough survey of the abundance of both P. pluvialis and N. gaeumannii across NZ and the PNW4. Both pathogens were more abundant in the host’s exotic environment of NZ, in contrast to its endemic range of the PNW. The relative abundance of the two pathogens was negatively correlated in the PNW, where both pathogens are presumed to have coexisted for longer. Greater P. pluvialis abundances were observed under higher winter relative abundances. The different range of winter temperatures between Oregon and NZ also suggested that, in Oregon, (with colder winter temperatures and lower P. pluvialis abundance) winter can be the limiting season for P. pluvialis development, while summer is observed to be the limiting period in NZ.

This research has demonstrated the need for an integral approach in needle disease control programmes. Not only is it important to consider multiple pathogenic infections, but also the impact on the physiology of the host.

I very much enjoyed undertaking my PhD and learned a lot in the process, with the help of my supervisors Nari Williams, Martin Bader, Sebastian Leuzinger and Peter Scott, and colleagues and friends from Scion’s Forest Protection team.

I continue to work on the epidemiology and physiological impact of forest disease in my current postdoc position in the Department of Forest Mycology and Plant Pathology at SLU (Swedish University of Agricultural Sciences). Here I am part of two different projects. In the first, I analyse samples from spore traps for detection of invasive species. In the second, I investigate the interaction between drought and pathogen infection, specifically, the root rot pathogen Heterobasidion anosum. I hope to be contributing to the two fields and to continue on-going collaborations with the Pathology group at Scion.

Mireia Gómez-Gallego
Swedish University of Agricultural Sciences
mireia.gomez@slu.se

  1. Gómez-Gallego M, Gommers R, Bader MK-F, Williams NM (2019). Modelling the key drivers of an aerial Phytophthora foliar disease epidemic, from the needles to the whole plant. PLoS ONE 14(5): e0216161.
  2. Gómez-Gallego M, Williams NM, Leuzinger S, Scott PM, Bader MK-F (submitted). A two-year defoliation experiment in Pinus radiata grafts: does the lower canopy matter?
  3. Gómez-Gallego M, Bader MK-F, Scott PM, Leuzinger S, Williams NM (2017). Phytophthora pluvialis studies on Douglas-fir require Swiss needle cast suppression. Plant Disease 101: 1259-1262.
  4. Gómez-Gallego M, LeBoldus J, Bader MK-F, Hansen E, Donaldson L, Williams NM (2019). Contrasting the pathogen loads in co-existing populations of Phytophthora pluvialis and Nothophaeocryptopus gaeumannii in Douglas-fir plantations in New Zealand and the US Pacific Northwest. Phytopathology.

This information is intended for general interest only. It is not intended to be a substitute for specific specialist advice on any matter and should not be relied on for that purpose. Scion will not be liable for any direct, indirect, incidental, special, consequential or exemplary damages, loss of profits, or any other intangible losses that result from using the information provided on this site.
(Scion is the trading name of the New Zealand Forest Research Institute Limited.)

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