The Changing Peach Rootstock Picture

 

The NC-140 technical committee met virtually in early November 2020 and results from its fifth peach rootstock trial were just published. Since the next peach rootstock trial is being planned for 2023, there will be little new information on peach rootstocks for a few years. So, this seems like a good time to summarize the peach rootstock picture for the mid-Atlantic region.

 

For about 200 years, the North American peach industry has used peach seedling rootstocks. When I started my career at Rutgers University, Ernie Christ was the state tree fruit specialist, and Les Miller was the county agent in Camden County, and they were testing peach rootstocks in different areas of New Jersey. Of all the rootstocks they tested, Les preferred ‘Halford’ and Ernie preferred ‘Lovell’, but both were vigorous rootstocks and when I analyzed some of their data, the two rootstocks performed similarly.

 

During the four years that I was at Rutgers, I established a rootstock trial with 3 selections from Dr. Dick Layne at Harrow, Ontario, 3 selections from China, Rutgers Red Leaf, Lovell, Halford, Bailey, own-rooted, and the peach x plum hybrid ‘Citation’ and the cultivars were ‘Rio-Oso-Gem’ and ‘Cresthaven’. I also planted the first NC-140 peach rootstock trial which had ‘Citation’ and the peach x almond hybrid ‘GF 667’ and ‘Redhaven’ was the scion. The only thing I learned from these plantings before I left for Virginia was that ‘Redhaven’ on ’Citation’ went off-color in late summer which is a symptom of incompatibility, but ‘Rio-Oso-Gem’ and ‘Cresthaven’ looked normal. After nearly 40 years, ‘Halford’ and ‘Lovell’ are still important rootstocks, but this may change soon because there are some less vigorous rootstocks that may be better adapted to higher-density systems.

 

 

Until recently, nurseries offered dwarf peach trees budded on the western sandcherry (P. besseyi), native to the northern great plains, or the downy cherry (P. tomentosa) which is native to northern and western China. Both rootstocks produced small peach trees, but they were incompatible with some cultivars and after about 5 years the trees started to decline and eventually died. Many of the rootstocks tested during the past 35 years are hybrids involving at least 10 different Prunus species. Some of these species include P. domestica (European plum), P. americana (American plum), P. davidiana (Chinese wild peach), P. insititia (damson plum), P. incana (willow leaf cherry), P. cerasifera (cherry plum), P. dulcits (almond), P. tomentosa, P. salicina (Japanese plum), and P. mume (Japanese apricot). Some, but not all peach cultivars are compatible with many of these Prunus species and interspecific hybrids.

 

 

The dwarfing mechanisms of apple rootstocks have been studied for more than 60 years and is still not totally understood. Two dwarfing genes have been identified and dwarfing seems to be controlled by a combination of plant morphology and hormones. Dwarfing apple rootstocks have small xylem vessels that impede the flow of water, hormones, and mineral nutrients across the graft union. The xylem is the pipeline for organic and inorganic materials in water to move from the roots to the top of the tree. Among other things, the smaller vessels induce a mild water stress in the tree resulting in stomatal closure and reduced transpiration and photosynthesis. Many of the apple dwarfing rootstocks modify the partitioning of photosynthetic products (carbohydrates) from vegetative to reproductive growth, so the ratio of fruit-to-wood increases. Pomologists typically express this as ”yield efficiency” (kg fruit/cm2 trunk cross-sectional area). Precocity and high yield efficiency are probably associated with hormonal differences in the rootstocks. Unlike apple, most of the dwarfing peach rootstocks used in the 20th century produced smaller trees but did not have high yield efficiencies. This may be because the primary dwarfing mechanism for peach rootstocks seems to be low hydraulic conductance, caused by small xylem vessels. This results in reduced whole-tree photosynthesis and suppressed vegetative growth but may not alter carbohydrate partitioning and yield efficiency. However, some of the newer peach rootstocks seem to have higher yield efficiencies than the traditional peach rootstocks. Therefore, there may be opportunities to develop peach orchard systems that take advantage of increased productivity of dwarfing rootstocks.

 

 

The primary goal of the NC-140 multistate project is to rapidly evaluate the performance of new rootstocks grown in diverse conditions. Peach rootstock breeding programs around the world are selecting for ease of clonal propagation, graft compatibility, tolerance to heavy, saline and calcareous soils, iron chlorosis, waterlogging and drought, as well as pathogens such as bacterial canker, soil rot fungi, Armillaria, crown gall, plant parasitic nematodes, orchard replanting, and environmental factors such as winter cold. NC-140 has tested rootstocks from breeding programs in Canada, China, France, Italy, Russia, Spain, and public and private breeding programs in the U.S. Until about 20 years ago, we were searching for rootstocks with good survival and productivity. Inspired by the success of high-density apple orchard systems, peach researchers are now also looking for size-controlling rootstocks with high yield efficiency (yield/trunk size) that may be suitable for high-density orchard systems.

 

 

The first NC-140 peach rootstock trial was planted in 1984 at 16 sites, with 3 interspecific hybrids (peach x almond) from France, ‘Citation’, ‘Bailey’, ‘Siberian C’, ‘Nemaguard’, own-rooted ‘Redhaven’, and ‘Halford’ and ‘Lovell’ served as standards (Perry et al., 2000). After 7 years ‘GF 677’ (peach x almond hybrid) made the largest tree and had highest yield, but ‘Halford’, ‘Bailey’, and own-rooted trees also had high yields. In general, the more vigorous rootstocks with good tree survival also had high cumulative yields, but ‘GF 677’ had low yield efficiency.

 

The second NC-140 peach rootstock trial was planted in 1994 at 20 locations with 19 rootstocks (Reighard et al. (2004). This planting included peach rootstocks as well as some interspecific hybrids. Some of the peach rootstocks were as productive, but not more productive than ‘Lovell’. Similar to previous trials, rootstocks that produced the largest trees tended to have the highest cumulative yields. However, cumulative yield efficiency was highest on the lower yielding less vigorous trees on ‘Bailey’ and ‘Tennessee Natural 28-1’. Rootstock trials for apple and peach have usually been continued for 10 and 8 years, respectively. However, it became obvious from this trial that the relative vigor and productivity of peach is apparent after just 3 fruiting years when trees are mature. But more years may be required to evaluate tree survival.

 

The 2001 NC-140 peach rootstock trial was planted at 11 sites with 14 rootstocks (Reighard et al. 2011). This was the first trial to evaluate clonal-propagated interspecific Prunus rootstocks. Most of the rootstocks were interspecific hybrids and were compared to ‘Lovell’, ’Bailey’ and ‘Guardian®’. No rootstock had tree survival higher than Lovell and some of the hybrids produced trees only 20 to 50% as large as ‘Lovell’. Some of the hybrids produced many root suckers or had anchorage problems. Cumulative yields were highest on the peach seedlings, peach × almond, and ‘Cadaman®’ (peach x P. davidiana) rootstocks. Lowest cumulative yields were from the small trees on Jaspi, VSV-1 and K146-44 rootstocks. Cumulative yield efficiency was not consistently related to tree size. Rootstocks influenced dates of bloom and harvest, but not in a consistent manner across locations. ‘Guardian®’ is now widely planted in the southeast because trees survive and perform well on replant sites with a history of peach tree short life.

 

The 2002 rootstock trial compared 9 plum or interspecific hybrids to’ Lovell’ at 17 locations with either ‘Crestahaven’ or ‘Redhaven’ scions (Johnson et al., 2011). Some rootstocks were dwarfing, but had poor survival, produced excessive root suckers or small fruit. ‘Krymsk® 1’, a hybrid of P. tomentosa x P. cerasifera from Russia, showed promise as a dwarfing rootstock because it had trunks about 35% the size of Lovell with high yield efficiency and good fruit size. However ‘Krymsk® 1’ trees had high mortality at many sites.

 

The 2009 rootstock trial had 18 rootstocks at 13 locations with ‘Redhaven’ as the scion cultivar (Reighard et al., 2020). This trial compared several peach and interspecific hybrids to ‘Guardian®’ and ‘Lovell’. Tree survival was best for peach rootstocks. Non-peach species and hybrid rootstocks were susceptible to cold injury in the Midwest and bacterial canker in the southeast. Some Prunus hybrids produced larger trees than peach. The peach rootstocks, ‘Lovell’, ‘Guardian®’, KV010127, and hybrids ‘Atlas’ and ‘Viking’ had the highest cumulative yields. Plum hybrids and plum species had the lowest yields. Trees with the highest cumulative yield efficiencies were on the clonal peach rootstocks ‘Controller TM 7’, ‘Controller TM 8’, and the plums ‘Krymsk®1’ and P. americana. On high pH soils, almond x peach hybrids performed well by preventing iron deficiency.

 

In Pennsylvania the hybrids ‘Krymsk®1’ (Nanking cherry x myrobolan plum) and ‘Fortuna’ (plum x peach) had poor survival. Rootstocks with 100% survival at the 3 northeastern sites (NY, MA and PA) included ‘Lovell’, ‘Rootpac®R’, ‘’Controller TM 7 and ‘Controller TM 8’. ‘Guardian’, ‘Rootpac®R’ and the Controller rootstocks all had cumulative yield about 8 to 10% lower than Lovell, but ‘Controller TM 7’ and ‘Controller TM 8’ had cumulative yield efficiency about 17% higher than’ Lovell’.

 

 

Over the past 40 years many rootstocks have been evaluated for peach in North America and Europe. Although results vary with site, we have learned quite a bit about these rootstocks.

 

High-density peach plantings with most peach rootstocks have not been very successful. In the early years of an orchard, higher-density plantings have high yields because the orchard space filled quickly. But after 6 or 7 years, they are no more productive than low-density plantings. To maintain high yields in high-density plantings, canopy management to allow adequate light distribution throughout the tree is critical. Now that we have some promising dwarfing rootstocks, the next step is to determine how they perform in high-density systems and this is the primary objective of the NC-140 2023 trial. Dr. Jim Schupp is a little ahead of the curve because he is testing 5 rootstocks (‘Bailey’, ‘Guardian’ and’ Krymsk 85’, KV 10123 and’ Empyrean II’) at 3 spacings (5, 7.5 and 10’ in the row and 16’ between rows. Trees are trained to the Quad-V system.

 

This summer he presented results for the first 6 years at the virtual annual meeting of the American Society for Horticultural Science. He found that tree density had a greater effect on tree size than did rootstock. Tree size declined as trees are spaced more closely. ‘Bailey’ was most dwarfing rootstock, but the relative size of the trees on various rootstocks depended on the tree spacing. For example, trees on KV10123 were the largest at the low density but among the smallest at the high density. I would not expect this, and these results show why it is important to test promising rootstocks at different spacings and with different training systems. Cumulative yield per acre increased with increasing tree density. ‘Bailey’ was the best size-controlling rootstock and might be classified as semi-dwarf because it is about 10% dwarfing.

 

A quick survey of fruit tree nurseries indicates that peach trees on about 20 rootstocks are offered for sale. I expect several more will be added to the list within several years. Controller TM 7 and 8 are very exciting developments and Dr. Schupp has included them in his 2020 peach demonstration planting. Peach orchard production and efficiency will likely increase during the next 20 years as new size-controlling productive rootstocks that are better adapted to high-density training systems become commercially available.