The development of an assay for resistance to Phytophthora cambivora in almond rootstocks using shoots excised from tissue cultures

The pathogenicity of Phytophthora cambivora to Mission almond seedlings was confirmed by twig inoculation in situ. An excised twig assay using four isolates of P. cambivora showed almond cvs Mission and Chellaston to be susceptible, whereas peach cv. Nemaguard, which is commonly used as a rootstock for almond, was resistant. P. cambivora isolates 179 and 108, the most and least aggressive, respectively, were selected for further studies. The excised twig assay was modified for screening micropropagated shoots in vitro for response to P. cambivora. Defoliated shoots were placed upright in agar medium colonized with the fungus, and development of necrosis was assessed over a period of 2-3 days. Shoots of Chellaston developed extensive necrosis and those of Nemaguard developed little necrosis, whereas shoots of hybrid selections from peach x almond crosses were generally intermediate in response. This method permits rapid and efficient screening of large amounts of material, and would be a useful adjunct to conventional almond breeding programmes.