New aspects about the epidemiology of oomycetes causing Quercus suber root rot in Spain

Abstract: Phytophthora cinnamomi and Pythium spiculum are the main pathogens causing root rot on Quercus suber in the Iberian Peninsula, although the environmental conditions favoring root infections by each pathogen seem to be different. The present work analysed the competition level between both species under two dry soil conditions: moderate drought (51% ≥ θs ≥ 38%) and extreme drought (30% ≥ θs ≥ 20%) in comparison with previous results obtained in wet soil conditions. Cork oak seedlings were inoculated with resting spores (chlamydospores of Py. spiculum and/or oospores of P. cinnamomi) from both pathogens alone or mixed. Eight plants (replicates) per water regime and inoculum were grown in infested or uninfested (control) fertilized peat and incubated in an acclimatised greenhouse. Severity of foliar symptoms (yellowing, wilting and/or defoliation) was weekly evaluated, and the percentage of root necrosis assessed at the end of the experiments. Phytophthora cinnamomi and Py. spiculum showed a similar moderate virulence at moderate drought in comparison with control plants, which also showed some root damage not associated with oomycete infections. In contrast, only Py. spiculum caused a significant root necrosis in extreme drought, even considering that control plants also showed a high level of root damage in absence of pathogens. These results suggest that Py. spiculum could be better adapted than P. cinnamomi to terrestrial habitats, becoming less dependent on wet soil conditions. In conclusion, P. cinnamomi and Py. spiculum are able to cause root rot on Q. suber in a wide range of soil humidity. Then, independently of variations in soil water content, oaks growing in infested soils would be in high risk of disease.