During the past 100 years there has been of lot of rootstock information published and I cited more than 600 papers in my review. I was surprised to learn how much recent work has been done to identify the genes controlling rootstock characteristics because I don't normally read journals that publish molecular work.
Differential Gene Expression
Research performed at Penn State used 'Gala' on M.7 and M.9T337 to compare gene expression patterns in the scion and showed that gene expression differed depending on the rootstock. This differential gene expression likely plays important roles in tree stature, stress tolerance, photosynthetic activity, fire blight resistance, and other aspects of tree growth. There is even evidence that RNA can move across a graft union. Some of this work is being performed by the USDA, but most of the research is being done in New Zealand and China. Thus far, scientists have identified two primary genes that control rootstock vigor and two genes that control precocity. I always thought that these two characteristics were related, but they are controlled by different genes that are located near each other on the same chromosomes, and that is why the two characteristics are usually inherited together.
Differences in gene expression between M.9 (left) and M.7 (right) can lead to vastly different tree statures, amongst many other aspects of tree growth. Photos by Rob Crassweller and Mike Basedow.
Resistance to Fire Blight
Two sources of fire blight resistance have been identified. There seem to be two main types of resistance in apple rootstocks. One type involves more than one gene similar to that found in Malus robusta 5, where green tissues and flowers are not affected by the bacterium. A second type of resistance occurs in Budagovsky 9 (B.9) rootstock where the green tissues are severely affected, but two-year-old and older wood is resistant. When B.9 was first brought to the US, fire blight susceptibility was evaluated by injecting inoculum into young leaves of rapidly growing stool bed shoots, and results indicated that B.9 was no more resistant than M.9 and M.26, so we were surprised when older trees growing in the field were not killed by fire blight. The amazing thing is that B.9 also is able to transmit some resistance to the scion variety. Later it was learned that the level of resistance inherited from Robusta 5 varies for different strains of fire blight. Robusta 5 was heavily used in the Geneva breeding program because it is cold hardy and resistant to fire blight and woolly apple aphid. The inheritance of the B.9 type of resistance is currently not known.
Wooly Apple Aphid Resistance and Enhanced Nutrient Uptake
There are also two types of woolly apple aphid resistance. One type comes from 'Northern Spy,' which was used as a parent in the 1920s in the Malling Merton breeding program to develop the MM (MM.106, MM.111) rootstock series. The other type of resistance comes from Robusta 5 and was used in the Geneva program. In the future breeders will probably try to combine these two types of resistance to make it more unlikely for aphids to overcome resistance. In 2013 the first report was published showing that there are genetic factors affecting nutrient concentrations in leaves and fruit, so breeders should soon be able to select rootstocks with enhanced ability to take up various minerals from the soil and transport them to leaves and fruit.
G.41 (left) and MM.111 (right) are both resistant to wooly apple aphid however their mechanisms of resistance differ. Future breeding efforts can combine these to reduce the risk of the aphid overcoming the resistance. Photos by Rob Crassweller and Mike Basedow.
At a recent NC-140 meeting, held at Penn State, we learned about a new large project, funded by the National Institute for Food and Agriculture (NIFA) called "Root2Fruit" that is designed to better understand how rootstocks affect disease resistance and nutrient uptake. This is a collaborative effort among plant breeders, plant physiologists, pomologists, sociologists and plant pathologists at several universities and the USDA. Hopefully they will be able to identify some of the factors involved in replant problems and bitter pit development in 'Honeycrisp'.
Latent Viruses and Orchard Decline
Another issue that was emphasized at the NC-140 meeting is the importance of using virus-free scion material on the Geneva rootstocks. During the 1950s researchers began learning that some viruses were latent. Latent viruses usually produce no obvious symptoms in most varieties, but some varieties or rootstocks may be sensitive. For example, Virginia Crab, once commonly used as a rootstock, is sensitive to apple stem pitting virus and apple stem grooving virus. MM.106 is sensitive to tomato ringspot virus, when certain varieties, such as 'Delicious' are budded onto it. The hypersensitive reaction is known as "brown line necrosis" and kills the tree. When it became apparent that virus-infected scion wood and rootstocks were common throughout the apple industry, researchers at the East Malling and Long Ashton research stations in England developed a program to produce virus-free rootstocks.
In 1969, the first stool bed shoots free of chat wood, rubbery wood and apple mosaic were made available and they were designated with the letter A (for example M.9A). In 1973 the EMLA (East Malling Long Ashton) clones (for example M.9 EMLA), that were free of all known viruses were released. Although trees infected with latent viruses did not show obvious symptoms, tests in England showed that trees on M.9 EMLA had yields 40% higher than non-EMLA trees. When I was a grad student at Vermont, we had an experiment where 'Empire' trees on Robusta 5 rootstock were infected with 4 different latent viruses individually or in combination. Growth and yield were not significantly affected by virus after 6 years, but yield and most aspects of growth were about 10% lower for infected trees.
Some of the Geneva rootstocks are sensitive to latent viruses. We have known for several years that G.16 is hypersensitive to at least one virus. Infected trees may die in the nursery or during the first year in the orchard. Recently we have learned that some of the other Geneva rootstocks, especially G.935, are also sensitive to latent virus and trees that grew normally in the nursery decline in vigor and die several years after planting in the orchard. So it is important to get assurance from nurseries that they are using virus-free bud wood for trees on Geneva rootstocks.
Apple Replant Disorder
There has also been quite a bit of work on apple replant disorder. We usually recommend that upon the removal of an orchard, growers spend 3 to 5 years to renovate the soil. I have seen orchards replanted within a month or two of removing old trees. Sometimes the new trees seem to grow well, but other times the trees grow very poorly for a year or two. Poor growth may be due to a complex of factors consisting of the buildup of pathogen inoculum and herbicide residue, pH and nutrient imbalance, and soil compaction. Fumigation is often used to eliminate pathogens, insects and weed seeds, but the practice is expensive, provides only temporary disease control, eliminates beneficial soil organisms, and does not alleviate abiotic factors that can negatively affect growth of young trees. Symptoms of the disorder are often obvious within a few months after planting and may include stunted shoot growth, low root biomass, root tip necrosis, nutrient deficiencies, and water stress. Although abiotic factors (especially soil compaction) may be involved, a number of studies identified biotic factors as the primary cause of poor growth.
Researchers have identified several species of Phytophthora and Pythium that infect apple roots in commercial orchards, and each orchard seems to have a different complex of species. The problem is complicated by the fact that different rootstocks vary in their relative resistance to different species of these pathogens. Results from rootstock trials and observations by commercial growers indicate that some rootstocks, especially some of the Geneva rootstocks, perform better in replant situations than the widely grown Malling rootstocks.
At Cornell University, Isutsa and Merwin (2000) compared the growth of 941 rootstock genotypes in soil collected from replant sites that was fumigated or not fumigated and found that G.65, CG.6210 and G.30 were tolerant to replant disease. Based on field trials in New York G.935 and G.202 had good tolerance to replant disease. In a replant study in Washington State, G.11 and G.30 were more tolerant to lesion nematode than M.7, M.9, M.26, MM.106 and MM.111. Trees on M.26, MM.106 and MM.111 were more susceptible to Pythium spp. than trees on B.9 and rootstocks in the Geneva series. In replant trials in North Carolina, trees on G.30 and G.210 performed better in replant soils than trees on M.26 and M.7.
Bitter Pit and Zonal Chlorosis
This year, with the drought and high summer temperatures, bitter pit was a severe problem on 'Honeycrisp' across the state. 'Honeycrisp' is the scion variety for a couple of the NC-140 multi-location rootstock trials and some cooperators have collected data on zonal chlorosis (leaf yellowing with symptoms similar to potato leaf hopper damage), leaf and fruit nutrient concentrations, and bitter pit. There seems to be some differences in bitter pit on different rootstocks, but another year or two will be needed to determine if rootstock effects are consistent. Over three years in the 2010 rootstock trial the percentage of the canopy showing zonal chlorosis varied from 10 to 68%, but severity of symptoms was not very consistent from year to year or from site to site. Trees on PiAu 9-90, from Germany, tended to have severe chlorosis. Rootstocks that tended to have little chlorosis (less than 45% averaged over all locations in all years) included B.10, B.7-20-21 and B.70-20-20. The commonly planted Malling and Geneva rootstocks tended to be intermediate.
Future Rootstock Breeding and Evaluation Efforts
Apple rootstock research began at East Malling in 1912 when Ronald Wellington and Robert Hatton collected stool bed shoots of dwarfing apple rootstocks from European nurseries. The rootstocks were classified by vegetative and reproductive characteristics, such as shoot length, leaf shape, and blossom color, as well as fruit size, shape, and color. The rootstocks were initially designated as Type I - Type IX and were later designated as (East Malling) EMI - EMIX. In the 1970s the nomenclature for EM rootstocks changed to M and Arabic numbers replaced Roman numerals (M.1 - M.9). Over the years apple rootstock breeding programs were developed in many countries and some remain active. Rootstock breeders are combining traditional breeding and evaluation methods with modern genomics techniques, and rootstock development should be accelerated.
In the near future, our current rootstocks will likely be replaced by dwarfing rootstocks that are highly productive and have enhanced tolerance to cold, diseases, soil salinity, high soil pH, and drought. Some may influence fruit quality by preferentially taking up certain soil nutrients. Members of the NC-140 project will be the primary evaluators of new apple rootstocks for North America and the project will continue to depend on federal, state, and industry funding.
NC-140 plantings will continue to be the primary means of evaluating orchard performance of new rootstocks from around the world. Photo by Rob Crassweller.