Bob Curtis, Associate Director, Agricultural Affairs; Gabriele Ludwig, Associate Director, Environmental Affairs; Almond Board of California

This article is the second in a series on considerations for replanting an almond orchard. (For the first in the series, please see Almond replant strategy pays dividends down the road)

Among the earliest and most important decisions a grower will make when replanting an almond orchard is the selection of rootstocks and compatible varieties.

Rootstock selection, and corresponding decisions about compatible and desirable varieties, is a complicated process. The first step is to select a rootstock based on conditions at a specific site, including chemical and physical properties of the soil, nematode pressures, disease pathogens and problems with anchorage or drainage.

“The important thing is to think about what challenges you are facing on a particular site and select the rootstock that will best address that challenge,” says UC Farm Advisor Joe Connell in Butte County. 

From there, growers can make the important decisions about variety compatibility with those specific rootstocks.

In many cases, Connell said, the rootstock/scion decision is driven less by what will help solve a problem than it is by knowing which rootstocks to avoid that would aggravate a problem or that wouldn’t survive in a given situation.

The Almond Board of California (ABC) has funded rootstock research for several years, including field evaluation trials dating back to 1997. ABC is currently funding three new trials in Merced, Yolo and Butte counties, in addition to several ongoing trials. 

With the help of Farm Bill Block Grant funds recently awarded by CDFA, ABC recently set the course for more focused and long-term assessment and development of rootstocks to provide resistance to soilborne diseases and nematodes.

This research continues to provide valuable information in making selections to address specific replant issues. Here’s some of what we know:

Abiotic Factors

Proper rootstock selection can help overcome limiting physical or chemical characteristics at the site. Newer rootstocks, including Atlas, Viking or peach/almond hybrids such as Hansen, Nickels or Bright’s Hybrid can help overcome soil problems related to high salinity and high pH.

Peach/almond hybrids, however, are more susceptible to disease problems related to poor drainage such as Phytophthora root rot or crown rot. And, the peach/almond hybrids are more severely affected by ring nematodes and bacterial canker.

Plum rootstocks such as Marianna 2624 do well in heavier soils where wet feet and water logging are a problem.

Krymsk 86, a newer peach/plum hybrid that is compatible with Nonpareil, shows good anchorage in young trees, but there are still many unanswered questions about how well it will hold up long-term in the face of crown gall or wood rot problems that might topple trees as they get older.

Nematodes

In the San Joaquin Valley, rootstock selection will often be limited by nematodes; it is important that growers select a rootstock based on sampled nematode populations and specific pressures in the field.

Nemaguard is the traditional rootstock selection where root-knot nematode is a problem. Other options include Atlas, Viking or newer peach/almond hybrids with Nemaguard parentage, including Hansen and Nickels. Both Lovell and Krymsk 86 are very susceptible to root-knot nematodes.

Orchards facing the ring nematode/bacterial canker complex, which can be especially severe on sandy soils, are best planted to Nemaguard, Viking or Lovell. Avoid plum rootstocks, such as Marianna 2624, and all peach/almond hybrids.

High-vigor rootstocks, such as peach/almond hybrids, can help trees outgrow lesion nematode problems, particularly on medium-textured soils.

Aggressive Pathogens

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.