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Downy mildew (Hyaloperonospora parasitica) pathotypes in Australia

Akeel Mohammed: School of Agriculture and Environment Faculty of Science The University of Western Australia

<div>Downy mildew disease, caused by the pathogen <em>Hyaloperonospora parasitica</em>, is endemic in <em>Brassica</em> growing regions worldwide. The disease particularly effects younger plants and can reduce crop productivity, even when plants are infected at a later stage of growth. Isolates of <em>H. parasitica</em> collected initially in 2006-2008 from Western Australia and subsequently in 2015-2016 across southern Australia were inoculated onto cotyledons of >25 diverse <em>Brassica</em> hosts under controlled environment conditions at 18⁰C day and 13⁰C night with a 12 h photoperiod. At 7 days post-inoculation, disease severity was assessed on a 0-9 scale, where 0 = no disease, 9 = heavy sporulation, cotyledon collapsed. Host responses ranged from no visible symptom or only a hypersensitive response, to systemic spread and abundant pathogen sporulation in highly susceptible genotypes. There were significant effects of isolates, host genotypes, as well as a significant interaction between both. A group of six <em>Brassica</em> genotypes was identified to be ideal for use as host differentials and then utilized to characterize eight different pathotypes of <em>H. parasitica </em>using an octal classification based on the virulence of the isolates across each host differential. This is the first classification of <em>H. parasitica</em> pathotypes for Australia. This new ability to define pathotypes provides a basis for monitoring changes in <em>H.</em><em> parasitica </em>populations over time. In particular, it will allow early warning of development of new pathotypes able to overcome any current host resistances deployed commercially in the oilseed and vegetable <em>Brassica </em>industries.</div>