Experimental minimum threshold for Phytophthora cinnamomi root disease expression on Quercus suber

Abstract

Quercus suber seedlings were potted in soils infested with increasing concentrations of Phytophthora cinnamomi chlamydospores and submitted to weekly flooding for 3 months to favour root infections. Increasing quantities of chlamydospores led to an exponential increase in their ability to germinate. Root symptoms (necrosis and/or absence of feeder roots) were significantly more severe than those recorded in uninfested soil only for plants potted in soils infested with 61 cfu g^-1 or more. Although generated using potting mix, this minimum threshold represents a tool for checking the potential infectivity of infested soils or to assess the effectiveness of some control methods to reduce soil inoculum. However, a low level of root infection was recorded even at 3 cfu g^-1. Therefore, long-term disease risk may be present whenever the pathogen is detectable in oak forest soils.