How To Choose Almond Rootstocks

Trials in Stanislaus County indicate that Lovell may show signficantly more verticillium wilt symptoms than other rootstocks. This photo shows verticillium  wilt symptoms on second-leaf Nonpareil on Lovell. (Photo credit: Roger Duncan)
Trials in Stanislaus County indicate that Lovell may show signficantly more verticillium wilt symptoms than other rootstocks. This photo shows verticillium wilt symptoms on second-leaf Nonpareil on Lovell. (Photo credit: Roger Duncan)

Editor’s Note: Roger Duncan is a pomology farm advisor with University of California Cooperative Extension in Stanislaus County.

Like hamburgers, almond trees can be served your way. Growers have the option of special ordering their trees with the variety they want and the rootstock they need. Unfortunately, many almond growers just order “the usual” without studying the menu.

Nemaguard is vigorous, immune to rootknot nematode, compatible with all commercial almond varieties, doesn’t sucker much, has decent anchorage, and growers are familiar with it. However, there are many situations in which Nemaguard struggles or just may not be the best choice. As we continue to replant back into old orchard ground, it may be a good idea to rotate to a different rootstock to lessen the effects of replant problems.

Nemaguard is prone to lime-induced chlorosis (yellowing/iron deficiency) in alkaline soils. It is also susceptible to salt toxicity (sodium, chloride, and boron), asphyxia from saturated soils, Phytophthora root rot, oak root fungus, crown gall, and “heart rot” from various wood decay fungi. Despite the name “Nemaguard,” this rootstock is susceptible to ring and root lesion nematodes. The susceptibility to ring nematode leads to the risk of bacterial canker in second-generation orchards replanted into sandy soil. Lovell may be slightly better in clay loam soils, but it has all the other drawbacks of Nemaguard plus more crown gall, and is highly susceptible to rootknot nematode.

Check Out Hybrids
Over the past 20 years, several hybrid rootstocks have become commercially available in California and are gaining in popularity. These include peach x almond, peach x plum, and other more complex hybrid rootstocks. The best known peach x almond (PxA) hybrid rootstock in California is Hansen, a product of the University of California (UC) breeding program. Another UC PxA hybrid, Nickels, is superior to Hansen but is more difficult to find.

Commercial nurseries also sell their own proprietary PxA rootstocks. There are subtle differences among these PxA hybrids, but as a group they are very vigorous and are the most tolerant rootstocks to alkaline and saline soils. On the negative side, they tend to be very susceptible to root diseases, including Phytophthora, crown gall, and Armillaria (oak root fungus). Most are also highly susceptible to ring nematode and therefore bacterial canker.

In the Sacramento Valley, where soils are often heavier and annual rainfall totals can be three to six times what falls in the San Joaquin Valley, the almond industry is quickly converting to Krymsk 86, a peach x plum hybrid originally from Russia. Although widespread commercial experience with this rootstock is still limited, so far this rootstock has performed well in these conditions. Krymsk 86 appears to have superior anchorage compared to Nemaguard and Lovell. Krymsk 86 also appears to perform well in heavy soils and is as tolerant to Phytophthora as Marianna 2624. It may be less suitable for sandy soils where rootknot nematodes are present. Krymsk 86 is a little less vigorous than Nemaguard and may lend to slightly tighter spacing.

Viking is easily the best rootstock choice for sites prone to bacterial canker (shown here oozing through bark). (Photo credit: Roger Duncan)
Viking is easily the best rootstock choice for sites prone to bacterial canker (shown here oozing through bark). (Photo credit: Roger Duncan)

Viking And Atlas
Another rootstock gaining in popularity is Viking, a complex hybrid of peach, almond, plum, and apricot. Viking appears to have all the best attributes of Nemaguard but has fewer weaknesses. Like Nemaguard, Viking is well adapted to well-drained loam and sandy loam soil. Viking is slightly more vigorous than Nemaguard and is also immune to rootknot nematode. Viking rarely suckers, appears to have better anchorage than Nemaguard, and is apparently compatible with all commercial almond varieties. In addition, Viking is more tolerant of alkaline soils than Nemaguard and Lovell and is also tolerant of ring nematode. The largest problem we have noticed with Viking is its sensitivity to dehydration during handling at the nursery and at planting. This has occasionally resulted in new trees failing to push after planting.

We are currently monitoring other rootstocks in UC field trials to determine their potential fit in our production system. Empyrean 1 (also known as Barrier 1) has vigor similar to PxA hybrid rootstocks but without many of the disease problems. It also appears to be somewhat tolerant of alkaline soils and maybe ring nematode, but our experience is still very limited.

Atlas is another rootstock of interest. It is a complex hybrid similar to Viking, but the horticultural characteristics are different. Atlas is more susceptible to ring nematode and alkaline soils than Viking but it appears to have better yield efficiency than other rootstocks tested and appears to be more tolerant of alkaline soil than Nemaguard.

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