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Varietal susceptibility to multiple Phytophthora species in macadamia

Olumide Jeff-Ego: The University of Queensland

<div>Phytophthora root rot caused by <em>Phytophthora cinnamomi</em> is an important disease of macadamia worldwide, while several other <em>Phytophthora</em> species are known to cause disease on macadamia. Limited information exists on varietal resistance to these pathogens in macadamia. Since traditional screening methods for host resistance to Phytophthora root rot in tree crops are time-consuming, we sought to identify tolerant germplasm to multiple <em>Phytophthora </em>spp. associated with macadamia using a rapid high-throughput bioassay. Using this new assay, we determined the response to infection by seven <em>Phytophthora </em>spp. in 470 macadamia genotypes, including 20 accessions of the four wild <em>Macadamia</em> spp., the main commercial cultivars, and segregating populations from the macadamia breeding program. Results revealed a wide spectrum of varietal susceptibility. ‘HAES 816’ was the most susceptible commercial cultivar and <em>M. ternifolia</em> was the most susceptible wild macadamia species. The results correlate with the observed disease severity to <em>P. cinnamomi</em> in the field and our early screening methods may be a useful tool in the breeding program, to increase the level of resistance in new cultivars.</div>