Comparative soil water conditions for oomycete infections on Quercus suber

Abstract: The root rot caused by the soilborne oomycetes Phytophthora cinnamomi and Pythium spiculum is the main cause of mortality for cork oaks in southern Spain. How both pathogens are able to infect oak roots at the same plot or even adjacent trees is an important point for control. One year-old seedlings were infected and submitted to different soil water regimes: continuous flooding, periodical flooding (2 days per week) and standard watering. Different inocula (water suspensions of resistant spores) were prepared: chlamydospores of P. cinnamomi, oospores of Py. spiculum and a mix of both pathogens. Eight plants (replicates) per water regime and inoculum were transferred to plastic pots, each one containing 3 l of infested or uninfested (control) fertilized peat. Uninfested plants were exposed at the same hydric conditions than infested ones. All the plants were incubated in an acclimatized greenhouse and severity of foliar symptoms was weekly evaluated on a 0-4 scale (0 = 0-10% of symptomatic leaves, 4 = total wilt). Root symptoms were assessed at the end of each treatment following a similar 0-4 scale. Phytophthora cinnamomi appeared as a pathogen highly virulent to cork oak, causing disease symptoms at any soil water content tested, from total flooding (100%) to 38% of soil water content. In contrast, Py. spiculum acted as cork oak pathogen only in unflooded soils and only when there was not competition with P. cinnamomi.