It may be true for many almond growers that your next orchard could live longer and be more productive while reducing input costs for fertilizers, soil amendments and nematicides. The secret is in choosing the correct rootstock to meet the physical, chemical and biological challenges inherent to your soil. If your roots are healthier, your orchard will require fewer corrective actions to maximize the performance of your trees. Some rootstocks are better at extracting nutrients from the soil or excluding salts like sodium or chloride. Even if you have no significant soil or disease issues, you might consider adding just a little more vigor or hope for better anchorage.

Beginning many years ago, stone fruit industries (including almond) relied heavily on peach rootstocks, partially because the seedlings are generally easy to grow at the nursery and are uniform and vigorous. Seeds from Lovell canning peach orchards were often used, but with the elimination of Lovell as a commercial variety, some nurseries turned to the use of Halford peach as a rootstock. In 1959, the USDA released a peach seedling called Nemaguard, which was preferable to Lovell in most cases because of its resistance to root knot nematode, increased vigor and reduced susceptibility to crown gall. Nemaguard quickly grew to be the most widely planted rootstock for almond and other stone fruit growers in the San Joaquin Valley. Many Sacramento Valley growers continued to use Lovell because of its perceived better tolerance of “heavy” soil where root knot nematode was not a problem.

Today, most nurseries offer several rootstock choices to better meet the unique challenges of each new orchard. These include peach rootstocks like Nemaguard, Lovell, Guardian, Empyrean 1 and Cadaman; hybrids of peach and almond like Hansen 536, Nickels, Brights 5, Cornerstone and various selections of Titan hybrid; plum hybrids like Krymsk 86 (peach x plum) and Rootpac R (almond x plum) and complex hybrids like Viking and Atlas, which are part peach, almond, plum and apricot.

So how does a grower know which rootstock may be best for their new orchard? UCCE has conducted multiple field trials in commercial orchards throughout the Central Valley, comparing most commercially available rootstocks under various soil and climatic conditions. Clearly, there are better options than Nemaguard for many orchards throughout California.

Rootstock Options for Almond

Peach
Nemaguard performs best in sandy loam soils with good drainage. Trees are moderately vigorous, resistant to most root knot nematode species and generally don’t show much crown gall. However, Nemaguard has several flaws. Nemaguard is susceptible to high sodium, chloride and boron. It is also inefficient at extracting zinc from the soil. It is susceptible to chlorosis (yellowing) and stunting in soils with even moderate levels of lime or a pH much over 7.0. Despite its name, Nemaguard is susceptible to ring nematodes which can reduce vigor and are associated with bacterial canker of young trees.

Some growers may choose Lovell peach rootstock over Nemaguard in heavier soil. However, the difference in wet soil tolerance between Lovell and Nemaguard is relatively small. Lovell is perhaps even more salt-sensitive than Nemaguard and is susceptible to root knot nematode and crown gall.

If you like all the good things about Nemaguard but will be planting a second-generation orchard in sandy loam soils with the potential for ring nematode, you may consider Guardian peach rootstock. It was developed by Clemson University in South Carolina and is used by stone fruit growers in the Southeast U.S. to help with peach tree short life, a disease similar to bacterial canker. It has vigor and root knot nematode resistance similar to Nemaguard, but it has better tolerance to ring nematode, and thus also bacterial canker. Guardian is susceptible to the same salt, soil chemistry and disease challenges as Nemaguard. Think of Guardian as being very similar to Nemaguard but with better ring nematode tolerance.

Plum Hybrid
Krymsk 86, a cross of plum and peach, has replaced Lovell as the most commonly planted rootstock in the Sacramento Valley. This is due to its superior anchorage and better tolerance of heavy soil. Although it is half plum, it doesn’t have the root suckering problem of Marianna 26-24 and is compatible with Nonpareil. Krymsk 86 is susceptible to root knot, ring and root lesion nematodes as well as high sodium, chloride and boron. Krymsk 86 may not be the best choice for sandier soils on the floor of the San Joaquin Valley or in the saltier, higher pH soils along some areas of the San Joaquin Valley westside or Delta region. Krymsk 86 may be a good choice for Sierra foothill orchards where drainage problems exist, and nematodes and salt are not an issue. The Monterey variety can sometimes develop yellow, cupped leaves on Krymsk 86, but this is largely attributed to temporary, saturated soil conditions.

Complex Hybrid
Viking, a complex hybrid of peach, almond, apricot and plum from the Zaiger Genetics breeding program, does all the things expected from Nemaguard and much more. It is immune to root knot nematode just like Nemaguard, but is resistant to ring nematodes, and therefore bacterial canker, similar to Guardian. It is generally just a little more vigorous than Nemaguard and is more tolerant to sodium, chloride and moderately high soil pH and lime. Viking has excellent anchorage and has almost always outperformed Nemaguard in UC trials. Viking might be considered one of the all-around best suited rootstocks for sandy loam soils in the San Joaquin Valley and should strongly be considered as a replacement for Nemaguard. Although data is limited, Viking does not appear to offer better tolerance to soilborne diseases like Phytophthora or Armillaria (oak root fungus) than Nemaguard.

Peach/Almond Hybrid
Rootstocks that are hybrids of peach and almond are gaining popularity in the San Joaquin Valley, especially in areas with alkaline soils. One of the better-known options is Hansen, a product of the UC Davis breeding program. Many nurseries also sell their own proprietary peach/almond hybrid rootstocks.

There are subtle differences among peach/almond hybrids, but as a group they are very vigorous, resistant to root knot nematodes and tend to be the most tolerant rootstocks to alkaline and saline soils. They also exclude or sequester boron better than most other rootstocks. On the negative side, they tend to be fairly susceptible to wet soil and root diseases, including Phytophthora, crown gall and Armillaria. Most are also highly susceptible to ring nematode and therefore bacterial canker. Because of the increased vigor, hull split and harvest can be delayed by at least 10 days compared to Nemaguard in some soils, although this can be mitigated somewhat with pre hull split deficit irrigation. This should be considered if late-harvested varieties like Monterey or Fritz will be planted, especially in high rainfall areas.

Additional Niche Options

There are other less widely planted rootstocks that may work well under some conditions. Rootpac R is from Spain and is a cross of almond and plum. The niche for Rootpac R seems to be in heavy soil where chloride is a problem, such as the San Joaquin Valley westside. Rootpac R has not performed well in lighter-textured soil where it tends to produce a small tree that is also sensitive to sodium and ring nematodes.

Atlas is another rootstock of interest. It is a complex hybrid with genetics similar to Viking, but the horticultural characteristics are very different. Atlas is more susceptible to ring nematode, sodium and high pH soils than Viking, but it is a little more tolerant of chloride than Nemaguard with better yield efficiency.

Empyrean 1, a hybrid of two peach species, has high vigor similar to peach/almond hybrid rootstocks but without many of the disease problems. It also appears to be somewhat tolerant of salt, alkaline soils and ring nematode. This rootstock should be considered by growers in ring nematode sites who want high vigor. Anchorage may be of concern in high wind areas.

Do the Homework First

Before choosing your rootstock, it is important to sample the soil profile in the new orchard site. Soil should be sampled at 12- or 18-inch increments to a minimum of three feet deep, preferably deeper. Test for nematodes and do a complete soil chemistry analysis, paying particular attention to total salts, sodium, chloride, boron, lime and pH. Take a look on Google Earth to identify potential problem areas where previous trees underperformed and sample them separately. Once you know the challenges your new orchard will face, make an informed decision on the best rootstock for a more profitable orchard.

It can feel like a big commitment to plant a whole orchard on an alternative rootstock. If growers are unsure about making a change, they might consider planting a bundle of one or two alternative rootstocks within the new orchard to see how they compare to the other trees side by side. In a few years, you may be kicking yourself that you did not plant the whole orchard on the alternative rootstock. Or you might congratulate yourself that you stayed with your old, comfortable pair of shoes.

For questions on this article, contact Roger Duncan at raduncan@ucdavis.edu.