Luke Milliron, UCCE Orchard Advisor Butte, Glenn, Tehama
Franz Niederholzer, UCCE Orchard Advisor Colusa, Sutter-Yuba
Jaime Ott, UCCE Orchards Advisor Tehama, Butte, Glenn, Shasta
The Skinny: Your rootstock choice should guard against your greatest fear for your new orchard, whether it be blow over, production potential, nematodes, or a disease. The historical choice of a plum rootstock (Myrobalan seedling, Myrobalan 29C, Marianna 2624, Marianna 40) was great for tolerating wet ground and Phytophthora. However, these rootstocks suffer from varying levels of poor anchorage when young, suckering, and susceptibility to bacterial canker. The grower cost and headaches inflicted by these problems have led to plum rootstocks combining for less than a quarter of current nursery sales. Instead, 75-80% of nursery sales from the two major nurseries supplying trees to the industry are Krymsk 86 (Krymsk) rooted. Growers have chosen Krymsk primarily because of its legendary anchorage, however the rootstock has several flaws growers should be aware of before purchasing. Finally, although not widely planted, Viking proved to be an excellent rootstock in recent UC trials and should be considered. No rootstock is flawless. Growers should understand all the qualities of a rootstock ahead of the planting decision.

Photo 1. At the Butte rootstock trial site (no bacterial canker pressure) plum rooted trees were among the largest, highest yielding, and provided some of the lowest dry-aways (photo Luke Milliron).
Some Subtlety:
Plum Rootstocks: If you don’t have bacterial canker or prune brownline in your orchard, and if you don’t see blowovers early in the life of your orchard, plum rooted trees can be incredibly successful. These great conditions were experienced at a recent Butte County trial site where Myroblan 29C produced the highest yields among 13 rootstocks and provided one of the lowest dry-away ratios. Marianna 2624 and 40 also did well at the site. However, if you have a history of bacterial canker, or if you have ring nematode which is closely associated with the disease, steer clear of these rootstocks. At the rootstock trial site in Yuba County where bacterial canker disease was common, roughly half of the Myroblan 29C and one-third of Myroblan seedling died in the first ten years. Marianna 40 and 2624 fared better at the site with about 20% mortality but didn’t do as well as the rootstocks we will discuss in the next sections. If you don’t fear bacterial canker but you are concerned about the newly rediscovered prune brownline disease, Marianna 2624 is the only known rootstock resistant to the disease. Unwilling to manage the suckering associated with Marianna 2624 one grower is trialing Marianna 40 replants at his site with prune brownline – stay tuned. If you don’t fear either bacterial canker or prune brownline – remember that none of these rootstocks have excellent anchorage when compared to Krymsk 86 or Viking.
Peach Rootstocks: Lovell has been one of the classic rootstock options in the Sacramento Valley and was once the default rootstock for almond production in the region. The rootstock had high survival at the Yuba County rootstock trial site with high bacterial canker pressure. However, the trees produced mediocre yields in recent UC trials. Anchorage was historically considered good but is now considered mediocre when compared to Krymsk or Viking. Very little if any suckering. Infamous susceptibility to wet feet and Phytophthora, a potentially fatal flaw if you are planting on heavy ground.
The hybrids…
Plum x Almond: Rootpac R is a rootstock that has gained some traction in Sacramento Valley almond production but not in prune production. In the recent UC trials there were only enough trees to plant at one site – Yuba, where the tree had high survival against bacterial canker pressure and moderate vigor. However, despite producing nice trees, these trees produced below average yields. Susceptibility to prune brownline is unknown. The rootstock would have to be tested in more locations to better understand its strengths and weaknesses.
Plum x Peach: As previously noted, Krymsk is now the number one choice of rootstock for prune production largely due to its stellar anchorage and success in Sacramento Valley almond production. For an industry long plagued by orchards with poor anchorage and in certain spots – bacterial canker – this rootstock had perfect survival at the bacterial canker site. At this site it also had the highest yields after 10 years of the 14 rootstocks. The rootstock is susceptible to plant parasitic nematodes, salinity, and prune brownline. There are a growing number of almond varieties with compatibility issues with Krymsk and it remains to be seen if there will be any compatibility problems with the new prune varieties being developed at UC Davis. Despite having some important flaws – it remains to be seen if the rootstock’s current strength in prune sales will lose any ground in the coming years.

Photo 2. Viking in the foreground had perfect survival at the Yuba County rootstock trial site with high bacterial canker pressure (photo Luke Milliron).
Complex hybrid: Viking is a popular rootstock in almond production but very little is planted in prune. Viking had the second-best anchorage after Krymsk in the recent UC trials, it also had perfect survival at the site with high bacterial canker damage (photo 2). In almond trials and in our recent prune trials, Viking produced a slightly larger tree than Krymsk. At the Butte trial site Viking was slightly more productive than Krymsk, and the reverse was true at Yuba. Viking had zero suckers at either rootstock site in our evaluations. Our understanding from almond rootstock trials is that Viking is more tolerant of salts, high soil pH, crown gall, ring and rootknot nematode but more susceptible to wet feet and Phytophthora. Susceptibility to prune brownline is unknown.
Again, no rootstock is flawless. Understand all the qualities of a rootstock ahead of your planting decision. Your local farm advisor is a great resource to talk through your options with.
Leave a Reply