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.