Updated March, 2018. Originally published November, 2016.

Richard Buchner, UCCE Farm Advisor Emeritus, Tehama, Butte and Glenn Counties; Luke Milliron, UCCE Farm Advisor, Butte, Glenn, and Tehama Counties; Franz Niederholzer, UCCE Farm Advisor, Sutter, Yuba and Colusa Counties; Katherine Jarvis-Shean, UCCE Farm Advisor, Yolo, Solano and Sacramento Counties; and Dani Lightle, UCCE Farm Advisor, Glenn, Butte & Tehama Counties.

As soil treatment options become increasingly limited, more restrictive and less effective, the priority to identify a genetic solution to solve or reduce the replant issue is of increasing interest. One genetic solution is to find or develop rootstocks to help manage soil related problems (soil borne fungi / bacteria, nematodes and soil acidity and excess mineral accumulation). Also of interest are root and tree characteristics imparting canopy size control, good anchorage and little or no root suckering.

Recognizing the need for identifying additional rootstocks for California Prune production, University of California Farm Advisors and campus based faculty with funding from the California Dried Plum Board (CDPB) designed and planted 3 large rootstock experiments in 2011 to evaluate 29 prune rootstocks. One experiment is planted near Davis at Wolfskill on Yolo loam, a second in Yuba County on Kilga clay loam and a third in Butte county on Farwell clay adobe alternating with Nord loam. Both Butte and Yuba experienced extensive mortality and were significantly replanted in 2012. As we discuss rootstock attributes, note that trees are still too young for definitive conclusions. Preliminary rootstock data is published in the CDPB research reports. In addition, the Prune production manual (UC ANR # 3507) has a very good chapter describing the traditional rootstock choices.

The prune rootstocks being evaluated at the Butte & Yuba county trial sites, as well as their genetic background, are shown in the table below:

Preliminary observations:
Rootstocks in the three experiments have only been observed for six growing seasons, but so far, none of the rootstocks have shown rootstock/scion incompatibility. All rootstocks have been visually rated for vigor. Atlas, Viking, Myrobalan 29c and Lovell impart the greatest vigor with M-30, M-40 and Krymsk 86 close seconds.

Viking, Atlas, M-30, M-40, Lovell, Citation and Krymsk 86  imparted little to no tendency to sucker in the Butte experiment. M-30 and M-40 demonstrated a slightly greater tendency to sucker at the Yuba location.

Tree anchorage was measured as degrees of lean from vertical in 2015. Viking, Atlas, M-30, M-40, Krymsk 86, Lovell, Myrobalan seedling and Myrobalan 29c are  all anchored well. Krymsk 86 was the best anchored rootstock in the Yuba experiment. Viking, M-40, Rootpac-R and Marianna 2624 were well anchored at the Yuba County location, but not as well as Krymsk 86.

Nematode resistance is variable depending upon which species of nematode is involved. Scientific comparisons are limited but most of the prune rootstocks do not appear to impart a great deal of resistance to root lesion nematode (Pratylenchus vulnus). Intraspecific hybrids tend to impart some resistance to root knot nematode (Meloidoyne spp) while Krymsk 86 appears to be very susceptible. Lovell peach is susceptible to root knot nematode while plum is variable in susceptibility. Little information is currently available for ring and dagger nematodes.

Crown and root rot (Phytophthora spp.) experience is also limited but in general rootstocks with plum heritage tend to be more resistant while rootstocks with peach heritage tend to be more susceptible. Similarly, plum heritage imparts  more crown gall ( Agrobacterium tumefaciens) resistance compared to peach. Krymsk 86 appears to have resistance to oak root fungus ( Armillaria spp.) as does Marianna 2624.

Rootstock bloom timing in Butte County:
2016 and 2017 each had very different bloom conditions which were evaluated at the Butte County location. In 2016, bloom was early (March 10 full bloom for traditional rootstocks like Myrobalan 29C), with cold temperatures and rain for much of the early bloom period. In 2017, the traditional rootstocks reached full bloom on March 19 and bloom conditions were more favorable for bees. Consistent across both years was that trees on some rootstocks reached full bloom well after the traditional rootstocks. Citation, Krymsk 86, and Marianna 58 all reached full bloom at least two days after the traditional rootstocks in both years. Bloom will continue to be evaluated to fully assess bloom timing differences over time. A potential application of different bloom timing by rootstock might be to plant blocks on different rootstocks to spread out bloom timing and reduce the risk of a crop failure due to a bad weather event.

Cankers and tree loss at the Yuba County plot:
Bacterial canker (Pseudomonas syringae) as well as Cytospora canker have plagued the Yuba County plot, with notable gumming and tree loss in 2013 and 2017. Rootstocks with a high percentage of tree loss are Marianna 30 (40%), Myrobalan 29C (23%), and Myrobalan seedling (17%). Other rootstocks suffered minor losses such as Rootpac-R (two trees or 7%), and Marianna 2624, Marianna 40, and Marianna 58 all with one tree lost (3%). Half of the rootstocks have suffered no losses at this site (Lovell, Atlas, Viking, Citation, Krymsk 86, Krymsk 1, and HBOK50). To date, extensive gumming has not been documented in the Butte County or Wolfskill plots.

Trunk size and 2017 yield:
The 2017 season was the first commercial harvest for the trials. Fruit were not thinned at either site to demonstrate the fruiting capacity of the different rootstocks. In general, we have observed that a larger trunk, referred to as trunk cross sectional area (TCSA, in cm²), is correlated with more fruit per tree, and higher dry yield (pounds per tree), higher dry away ratio (dry weight: fresh weight), and smaller fruit size. The relationship between increasing TCSA and these yield results has been very direct at the Butte County site (see Table 2), and although there is a relationship at the Yuba County site it has not been as strong (see Table 3).

Across rootstocks, TCSA and yield were much higher in 2017 at the Butte plot compared to the Yuba plot, while dry away ratios were lower and fruit were larger at the Yuba site (data not shown). This tree size and yield disparity may be due in part to soil/water differences between the sites, particularly the saturated soil conditions at the Yuba site during the 2017 bloom. Despite these differences between sites, the relative performance of many of the rootstocks was the same. Among the smaller (TCSA and tree canopy) and lower yielding trees at both sites were Krymsk 1 and Marianna 58. Among the largest and highest yielding trees at both sites were Viking, Atlas and Myrobalan 29C. A critical question in choosing the right rootstock will be whether your objective is to plant larger, more vigorous trees or to plant smaller trees at a much higher density.

Cautionary note:
When evaluating rootstock trial results, it is important to consider the collective information from all sites to inform your new orchard planning. A singular focus on yield from a particular rootstock trial can miss important tree health information.  For example, Marianna 30 and Myrobalan 29C were top tier producers in the Butte County trial in 2017, but sensitive to canker at the Yuba County site, with 40 and 30% tree death, respectively.