Pathogenicity of Oomycete Species to Different Prunus Hybrid Rootstocks

Diseases caused by soilborne oomycetes are a limiting factor for the cultivation of Prunus spp., which makes the choice of a suitable rootstock a key factor. The objective of this study was to evaluate the pathogenicity of 12 oomycete species belonging to the genera Globisporangium, Phytophthora (Ph.), and Phytopythium (Pp.) to three Prunus hybrid rootstocks: 'Garnem', 'GF-677', and 'Rootpac-40'. These three rootstocks are widely used to grow stone fruit and almond in the Mediterranean Basin. Pathogenicity tests were conducted using 15 oomycete isolates and 1-year-old rootstock seedlings. Ninety days after inoculation, disease symptoms were evaluated on a severity scale, and the area under the disease progression curve and the survival probability of the inoculated seedlings were calculated. Moreover, root dry weight was recorded. All the isolates included in the pathogenicity tests were pathogenic on the rootstock seedlings and were reisolated from root lesions. Large differences in virulence were detected among the different oomycete species and isolates of Ph. niederhauserii for each rootstock. Phytophthora multivora and Pp. helicoides were generally the most virulent species. The results of the present research offer substantial contribution to increase our knowledge about the pathogenicity of several oomycete species that are frequently isolated in Prunus orchards and the potential risks that they pose for Prunus spp. crops.