Resistant rootstocks for Phytophthora include Marianna 2624 and, to some degree, M-40 and Krymsk 86. Peach rootstocks, including Lovell and Nemaguard, and peach/almond hybrids are more likely to die from Phytophthora than plum rootstocks.  The only rootstock currently known to have tolerance or resistance to Armillaria (oak root fungus) is the Marianna 2624 plum.

While plum rootstock such as Marianna 2624 can solve many pathogen issues, including Phytophthora and oak root fungus, growers must contend with the rootstock’s incompatibility with preferred varieties, such as Nonpareil. Growers have had some success using trees where Nonpareil is grafted on top of an interstock (such as Padre almond or Havens 2B plum) that is compatible when grafted onto the Marianna 2624 plum rootstock.   But, Connell notes that these double-grafted trees are often less robust than traditional scion/rootstock combinations with a single graft. This practice may offer an option for filling in spots in orchards where oak root fungus is a selective problem.

Replant Disease

Connell says that virtually all rootstocks show some susceptibility where replant disease is a problem, and switching parentage does not make enough difference to justify sacrifices in horticultural benefits.

The bottom line is that growers must first look at the complexity of issues at the site when replanting almond orchards, and then find the rootstock and compatible variety that will thrive best under the given set of conditions.

“We have a lot of potential rootstocks out there but there is still not enough information about them to be able to recommend certain ones in many cases,” Connell says. “So it comes down to a more careful thought process about what is the limiting problem.  Are there rootstocks that can address the specific problem? And, which rootstocks do I avoid that could prevent my orchard from reaching its full potential?”

For more on the current Almond Board funded project “Field Evaluation of Almond Rootstocks,” go to AlmondBoard.com/ResearchReports. Select all the key categories (Annual Reports, Updates/Proceedings, Posters) and search for projects 08.HORT4.Duncan, 09.HORT4.Duncan and 10.HORT4.Duncan. For additional information on soilborne pests and diseases, go to AlmondBoard.com/farmpress16.