Virulence of Phytophthora cinnamomi on three Mediterranean Quercus species

Abstract: Mediterranean oak forests include a high diversity of Quercus species with different
response against the soilborne pathogen Phytophthora cinnamomi (Pc). While Quercus ilex and Quercus suber are very sensitive to this pathogen, other species, like Quercus faginea, has been described as tolerant. Moreover, up to now, there are not field records of Quercus pyrenaica (Pyrenean oak) and Quercus coccifera (Kermes oak) affected by root disease. Both species, like Q. faginea, may be potential sources of resistance genes. To determine the degree of tolerance/sensitivity of Q. pyrenaica and Q. coccifera against Pc, inoculation tests on excised twigs were carried out on 10 trees per species, including others 10 trees of Q. ilex as controls. For each tree, 20 twigs (~ 20 cm length) were inoculated with two different isolates of Pc of known virulence, and additional 10 twigs (replicates) per tree remained uninoculated as controls. After 4 weeks of incubation at 22 °C, lesion length was measured on cambial surface after removing the outer bark to calculate the percentage of twig affected (PTA). For the most virulent isolate, Q. coccifera showed a PTA significantly lower than Q. ilex, which did not differ from Q. pyrenaica. However, according to PTA induced by the least virulent isolate, three groups can be differentiated: very sensitive (Q. ilex), sensitive (Q. pyrenaica) and little sensitive (Q. coccifera). These results open new opportunities of hybridization with more tolerant oak species that could be considered in future Q. ilex breeding programs.