In vitro assays of Phytophthora agathidicida on kauri leaves suggest variability in pathogen virulence and host response
Keywords:kauri dieback, kauri, Phytophthora, asymptomatic infection
Phytophthora agathidicida is an aggressive soil-borne oomycete pathogen that kills New Zealand kauri trees (Agathis australis). When artificially inoculated, P. agathidicida causes lesions on leaves as well as roots, providing a non-invasive method for virulence screening. However, little is known about the extents to which the pathogen varies in virulence and kauri trees vary in disease susceptibility. Three isolates of P. agathidicida grown in culture were inoculated onto detached leaves from six kauri trees. Visible disease lesions were measured and the extent of asymptomatic leaf colonisation determined by culturing. All six trees were susceptible to P. agathidicida, but one showed higher susceptibility than the others. The pathogen also showed variability in virulence among isolates. Asymptomatic colonisation of leaf tissue was also found, suggesting a latent or biotrophic phase for the pathogen. Although further work is needed, the variability of both pathogen virulence and host susceptibility have important implications for management of kauri dieback. Furthermore, asymptomatic colonisation of kauri tissues suggests that P. agathidicida could be present outside of regions with visible disease symptoms.
Crone M 2012. Persistence of <i>Phytophthora cinnamomi</i> in nature: Biotrophic growth and presence of stromata, oospores and chlamydospores in annual and herbaceous perennial plant species. PhD thesis, Murdoch University.
Horner IJ, Hough EG 2014. Pathogenicity of four <i>Phytophthora</i> species on kauri: in vitro and glasshouse trials. New Zealand Plant Protection 67: 54â€“59.
HÃ¼berli D, Lutzy B, Voss B, Calver M, Ormsby M, Garbelotto M 2008. Susceptibility of New Zealand flora to <i>Phytophthora ramorum</i> and pathogen sporulation potential: an approach based on the precautionary principle. Australasian Plant Pathology 37: 615â€“625.
Ireland KB, HÃ¼berli D, Dell B, Smith IW, Rizzo DM, Hardy G 2012. Potential susceptibility of Australian native plant species to branch dieback and bole canker diseases caused by <i>Phytophthora ramorum</i>. Plant Pathology 61: 234â€“246.
Jeffers S, Martin S 1986. Comparison of two media selective for <i>Phytophthora</i> and <i>Pythium</i> species. Plant Disease 70: 1038â€“1043.
Tahi GM, KÃ©bÃ© BI, N'Goran JAK, SangarÃ© A, Mondeil F, Cilas C, Eskes AB 2006. Expected selection efficiency for resistance to cacao pod rot (<i>Phytophthora palmivora</i>) comparing leaf disc inoculations with field observations. Euphytica 149: 35â€“44.
Waipara NW, Hill S, Hill LMW, Hough EG, Horner IJ 2013. Surveillance methods to determine tree health, distribution of kauri dieback disease and associated pathogens. New Zealand Plant Protection 66: 235â€“241.
Weir BS, Paderes EP, Anand N, Uchida JY, Pennycook SR, Bellgard SE, Beever RE 2015. A taxonomic revision of <i>Phytophthora</i> Clade 5 including two new species, <i>Phytophthora agathidicida</i> and <i>P. cocois</i>. Phytotaxa 205: 21â€“38.