Phytophthora cinnamomi is a pathogen with a widespread global presence that poses a significant threat to ecosystems, particularly in Mediterranean-type climates where it severely impacts biodiversity. In New Zealand, P. cinnamomi is widely distributed and has been isolated from various exotic and indigenous ecosystems. With the growing impact of climate change and a shift towards warmer and wetter environmental conditions, P. cinnamomi infection, spread, and associated disease impacts are likely to increase. However, no research has been conducted to understand the population diversity of P. cinnamomi in New Zealand.
This research presents a preliminary study examining the genetic and phenotypic variation of 25 P. cinnamomi isolates obtained from five regions and 16 host plants across New Zealand. Of the 25 isolates, one was of the A1 mating type, 20 were A2, and four were sterile. To evaluate genetic diversity, whole-genome sequences (Illumina HiSeq 150 bp paired-end) were mapped onto a reference genome. Variant calling was conducted to obtain single nucleotide polymorphism (SNP) data. A distance tree was constructed using the SNP data to assess genetic diversity. The isolates clustered into four distinct groups with no apparent relationship to host or region. The largest clonal grouping consisted of 19 P. cinnamomi isolates of the A2 mating type while the second largest cluster comprised four A2 isolates. Two singleton isolates were clearly distinct from each of these groupings and from each other. One of these was the A1 isolate, while the other was a single A2 mating type isolate obtained in 1968, which was the only pre-1990s isolate in the study.
Phenotypic variation was assessed through growth temperature (minimum, maximum, and optimal) tested between 5 °C and 35 °C, differences in virulence measured by lesion lengths on detached leaves of Euphorbia wulfenii, and mating type identification through pairing with known A1 and A2 tester isolates sourced from the Vegetation Health Service, Western Australia. Colony growth rates varied between isolates across different temperatures. Two isolates were adapted to higher temperature ranges, while one isolate exhibited preference to lower temperature. Significant differences in lesion lengths revealed variation in virulence within the population.
These findings show that there is genetic and phenotypic variation within the P. cinnamomi population in New Zealand and provide insights into the species’ ecological adaptability and pathogenic potential. Further research will expand on this study to better understand the P. cinnamomi population across the country.