Development of an In Vitro Pathogenicity Test for Phytophthora cinnamomi for Screening Quercus suber Genotypes

Dieback of Quercus is a serious environmental and economic problem in the Iberian Peninsula and the Mediterranean basin, and the main causal factor is infection by the oomycete Phytophthora cinnamomi Rands. Reliable and rapid methods for evaluating host susceptibility are needed to support breeding and selection programs. This study developed and evaluated an in vitro pathogenicity assay for screening cork oak (Quercus suber L.) genotypes using both mycelial and zoospore inoculation methods. The effects of inoculum type and dose, symptom development, pathogen establishment, and disease progression were assessed, and the ability of the assay to discriminate among host genotypes was evaluated. Both inoculation methods consistently induced infection, producing characteristic root necrosis followed by aerial symptoms and plant death. Root necrosis became visible within two days after zoospore inoculation, and successful pathogen re-isolation from surface-sterilized roots confirmed tissue colonization within 24 h. Observations suggest that survival time and root necrosis length are useful indicators of disease progression. Genotype-related differences were detected in time-based variables, namely time to first symptoms and time to complete necrosis in the mycelial-suspension assay and survival time in the zoospore assay, whereas early root necrosis and growth variables did not differ significantly among the tested genotypes. Overall, these results support the use of the proposed in vitro protocols as short-term approaches for the preliminary assessment of genotype responses to P. cinnamomi, complementing greenhouse and field evaluations.