Variation in susceptibility among macadamia genotypes and species to Phytophthora root decay caused by Phytophthora cinnamomi Olufemi A. Akinsanmi a, * , Gang Wang a, b , Jodi Neal d , Dougal Russell d , Andr  e Drenth a , Bruce Topp c a The University of Queensland, Centre for Plant Science, Queensland Alliance for Agriculture and Food Innovation, Brisbane, Qld 4001, Australia b The University of Queensland, School of Chemistry and Molecular Biosciences, Brisbane, Qld 4075, Australia c The University of Queensland, Centre for Plant Science, Queensland Alliance for Agriculture and Food Innovation, Maroochy Research Station, PO Box 5083, SCMC, Nambour, Qld 4560, Australia d Department of Agriculture, Fisheries and Forestry, Maroochy Research Station, PO Box 5083, SCMC, Nambour, Qld 4560, Australia article info Article history: Received 15 January 2016 Received in revised form 26 April 2016 Accepted 27 April 2016 Available online 1 June 2016 Keywords: Oomycetes Proteaceae Soil-borne pathogen abstract Phytophthora cinnamomi is a major pathogen of cultivated macadamia (Macadamia integrifolia, Macad- amia tetraphylla and their hybrids) worldwide. The susceptibility of the two non-edible Macadamia species (Macadamia ternifolia and Macadamia jansenii) to P . cinnamomi is not well-understood. Com- mercial macadamia trees are established on grafted seedling (seed propagation) or own-rooted cutting (vegetative propagation) rootstocks of hybrids of the cultivated species. There is little information to support the preferential use of rootstock propagated by either seedling or own-rooted cutting methods in macadamia. In this study we assessed roots of macadamia plants of the four species and their hybrids, derived from the two methods of propagation, for their susceptibility to P . cinnamomi infection. The roots of inoculated plant from which P . cinnamomi was recovered showed blackening symptoms. The non- cultivated species, M. ternifolia and M. jansenii and their hybrids were the most susceptible germplasm compared with M. tetraphylla and M. integrifolia. Of these two species, M. tetraphylla was less susceptible than M. integrifolia. Significant differences were observed among the accessions of their hybrids. A strong association (R 2 > 0.75) was recorded between symptomatic roots and disease severity. Root density reduced with increasing disease severity rating in both own-rooted cuttings (R 2 ¼ 0.65) and germinated seedlings (R 2 ¼ 0.55). P . cinnamomi severity data were not significantly (P > 0.05) different between the two methods of plant propagation. The significance of this study to macadamia breeding and selection of disease resistant rootstocks is discussed. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Four species of macadamia (Macadamia integrifolia, Macadamia tetraphylla, Macadamia ternifolia and Macadamia jansenii) originate in Eastern Australia and represent a significant resource for the macadamia industry, as potential sources of resistance to biotic and abiotic stress, and for improving yield and quality. The wild germplasm is vulnerable to extinction due to habitat loss and fragmentation (Pisanu et al., 2009; Neal et al., 2010; Powell et al., 2010). Under the Australian Environment Protection and Biodiver- sity Conservation Act, 1999, three species (M. integrifolia, M. tetraphylla and M. ternifolia) are listed as vulnerable while M. jan- senii is listed as endangered (Shapcott and Powell, 2011). M. terni- folia and M. jansenii do not produce edible nuts and mostly exist in the wild ecosystem (Hardner et al., 2009). M. integrifolia and hy- brids with M. tetraphylla constitute the current commercial mac- adamia production worldwide (Hardner et al., 2009). Genetic diversity that exists in the wild populations of Macadamia has not been explored for resistance to pathogens and pests, yield, quality or tolerance to abiotic stresses. Macadamia trees in commercial orchards are propagated on grafted rootstocks derived from either vegetatively propagated clonal cuttings or germinated open-pollinated seeds (Hardner et al., 2009). Most rootstocks are selected based on ease of germination, propagation and grafting rather than their resistance to biotic or abiotic stress. In Australia, nearly all the grafted trees in commercial * Corresponding author. E-mail address: uqoakins@uq.edu.au (O.A. Akinsanmi). Contents lists available at ScienceDirect Crop Protection journal homepage: www.elsevier.com/locate/cropro http://dx.doi.org/10.1016/j.cropro.2016.04.021 0261-2194/© 2016 Elsevier Ltd. All rights reserved. Crop Protection 87 (2016) 37e43