Backcross Breeding Orchards

Introducing Blight Resistance: The Backcross Breeding Orchards, Purpose and Implementation

The Backcross orchard is a basic unit of the TACF breeding program. This is the planting program that is the most rewarding and the one we need the most. It is a commitment of an acre of well-drained soil, and of 8 to 10 years of care. Any conscientious person can become a “chestnut steward” and help grow American chestnuts.

At a minimum, the orchard holds two “lines” consisting of two individual Kentucky pure American trees crossed with two separate hybrid chestnuts that are carrying forward Chinese blight-resistance.
The hardest thing for many new members of TACF is mastering the breeding chart, but if you do plant a breeding orchard, you will sooner or later spout off words like “B3F1” with confidence.

 Several basic facts can anchor your understanding:

  1. Chestnuts are self-infertile, and this includes the fact that full “sibs” — grown from nuts with the same Mother Tree (the tree with the burs) and same Father tree (the pollen parent) — are relatively infertile and poorly productive.  Nuts from closely related trees sprout poorly, if at all, and grow poorly, often with high mortality.  This is “in-breeding depression:”.  Therefore, in a Back cross orchard, the lines have to have separate Kentucky parents, and separate hybrid parents.  Searching out enough Kentucky Americans to get enough unrelated lines is important in creating a diverse and healthy population.
  2. The backcross generation (B1, B2, B3, B4) refers to how diluted the Chinese contribution is.  By the B3 level, when the trees are 15/16th American and 1/16th Chinese (or on average 94% American), most hybrid trees show no detectable Chinese traits.  If you send samples to a School of Forestry for identification, they should say they are pure American, based on physical characteristics.  But in a large group, there will still be trees that express some detectable Chinese heritage (besides blight-resistance).  They may tend to form multiple stems  (with low crotch formation rather than a single dominant trunk) or leaf out early.   In the selection process, when choosing the most blight resistant trees, it is important to made observations that help chose against other Chinese traits.
  3. The F1 part of designations like B3F1 refers to the initial cross between different species that makes a hybrid tree:  in the TACF program, the first cross was between a Chinese and American tree.  Every progeny tree from this cross gets at least 1/2 of the Chinese level of blight resistance, and will be “moderately resistant.”   Every part of the breeding cycle thereafter is designed to recover this level of resistance  and then transmit it to the next generation, while diluting out all other Chinese traits, until the final tree looks and acts like a pure American chestnut with blight resistance.  The F2 level occurs when two hybrid lines, both selected for blight resistance, are intercrossed — so that the resulting nuts and seedlings can inherit blight resistance from BOTH parents.  In the intercross, the overall level or % of “American-ness” stays the same, but the blight-resistance is concentrated.  Theoretically, a tree that has recovered full Chinese resistance, can now give all of its progeny at least moderate resistance (just like the Chinese F1 cross.)    Note that F1 crosses can occur between other species, like Japanese chestnut x American chestnut.  Recent studies show that the hardiness of Chinese trees makes them a good match for American trees and our climate range, and fortuitously, the decision to use Chinese trees as the resistance source was a good initial strategy.
  4. Backcross orchards are temporary but take substantial time to mature.   No matter how beautiful the B3F1s may be growing they must be severely culled by some selection process (presently inoculation with blight) in order to select only those trees ( approximately one in eight) with moderate resistance.  Since the program is trying to move forward rapidly, trees are grown with irrigation and fertilization to get them big enough and strong enough to be inoculated by their 5th and 6th year.  The process is sometimes a bit arduous — you plant 100 trees, by year 5, you hope to have 80 left, and of those you will only keep 6-8, since some of the most blight resistant selections will be discarded because of other persistent Chinese traits.  Some of the science and genetic research is designed to expedite and improve the selection process. If selection could occur in the third year from a leaf sample, trees could be grown at closer spacing, and susceptibles eliminated much earlier (and smaller).  Orchard space would be conserved.  Nonetheless, getting nuts from the intercross of the two lines, still requires that trees reach enough maturity to generate large quantities of B3F2 nuts.  (You will understand this soon enough — remember, the B3 is the % American-ness; the F2 tells us that the most blight resistant trees were crossed so that progeny could receive blight resistance from both parents.)
  5. The easiest way to generate B3F2 nuts is to restrict each orchard to two lines, and allow for open pollination among the selections (after emasculating any trees with pollen that could interfere, like the Chinese controls).  The risk with two lines is that some catastrophe will wipe out one line, or that there won’t be any trees with moderate resistance in the line, and then you end up with an isolated line, where controlled hand pollinations will be needed.  But when you have three lines (a back-up line), with open pollination, you do not have a defined cross, since any nut could be the result of either of two fathers.
  6. Blight isn’t the only chestnut disease or pest.  These orchards are monocultures, and therefore very likely to attract other chestnut pests.  The biggest problem recently has been Asian Ambrosia beetle, which attacks many nurseries with young native plants.  In addition, Oriental Chestnut Gall Wasp is now spreading rapidly into large chestnut holdings.   In the South, Phytopthora cinnamoni causes “ink disease” which kills the entire tree and spread in soil and water.   The important thing is to realize that you are managing these breeding trees to get them to a reproductive stage, and therefore certain disease treatments are reasonable, even though you don’t expect them to be used in forest restoration.  The Extension departments at UKY and other Schools of Forestry are actively studying other pests and pathogens and helping with management.  TACF is developing a program to add phytophthora resistnce to the breeding program, since it is lethal disease.

In general, two line orchards are a nice manageable size, less than an acre.  Because they create defined crosses, most of the Kentucky Back cross orchards managed by Volunteer Landowners will be two line orchards.

Every Kentucky Orchard has its own page below, but the standard information collected will be the same:  A map (Google Earth makes that easy); directions to the site, Site Analysis, H1 (Hybrid 1) and H2 (Hybrid 2) lines, and Chinese, American, and F1 (50% Chinese/50% American) controls and collection of monitoring data. TACF has a standardized data collection format, which helps the Regional Science Coordinators compare notes, to discover problems and solutions.   Slide shows and picture albums will be used to illustrate growth and certain problems.


This chart above summarizes the current working hypothesis about  breeding Chinese blight resistance into American chestnuts.  Three genes are assumed to control blight resistance, and each gene must be homozygous (that is, a copy of the gene is inherited from both parents)  in order to have full resistance.

The nuts planted in the Meades Landing Orchard to the right are from two separate lines of breeding.  The first hybrid group (H1) was planted in three rows in 2008, and the second hybrid cross  (H2) was planted in 2009. in two rows in between.  The difference in growth between the two years can be seen in the top picture on the left.

H1 is a cross of JB216 as the mother tree (a B2 tree with 87.5% American) with pollen from KY Hart1 (each Kentucky trees is named for its county and numbered).  This is written as JB216 x KYHart1. The result is a B3F1 tree that is now 94% American,  with 50% Kentucky heritage.  The second line results from GL239 x KYFlem1.  Only 55 nuts were created and this line has had more variability and mortality.

KY-ML08 June 20121

Nuts and Bolts of Back Cross Orchards

When you plant a Back Cross American Chestnut Orchard, you open a door and step over a threshold to become a participant in the TACF program. At some level, you are now a citizen scientist and a plant breeder. You are now personally safeguarding at least two Kentucky American chestnut sources and incorporating their contribution to the future of the species. No successful orchard works if there isn’t an onsite chestnut steward and manager for the project. This is a person who looks at the orchard, at least weekly, who walks into the rows to see what is going on, and observes. The person who realizes that there hasn’t been rain for 2 weeks. That person doesn’t need to be a trained professional at all, but they need to look often enough to recognize changes and address any problems early. No matter how carefully the Kentucky chapter explains growing chestnuts, every orchard almost immediately presents some new challenge to solve. A chestnut orchard is not something you do for the KY-TACF chapter; a chestnut orchard is your own work and commitment for the future of the species through implementing a part of the American Chestnut Foundation program.

This is a summary of the practical steps in establishing and managing a Back Cross Orchard, based on personal experience and opinions by Anne Bobigian, who has grown chestnut in Massachusetts and in Kentucky. Other people have different ways to the same end, and you will develop methods that work best for you. It is important to realize than everyone has to learn to grow chestnut. Before the blight, no one needed to deliberately plant chestnut. They conserved chestnuts and expected the forest to restore and perpetuate itself. Everyone in TACF who grows chestnut learns by trial and error, and whether you are planting your first trees or have been at it for 10 years, new challenges will come up. For example, in 2012, in Kentucky, a new widespread pest arrived — the Oriental Gall Wasp. All these are problems to manage. And you will meet wonderful people from our Universities, who are helping us surmount these new challenges.

Growers are patient and persistent. They notice things about and around their trees. They solve problems. They ask themselves — what will it take to make this orchard work? The work is rewarding – and the more hand work is done, the closer you are to the trees are they get established and grow. Few people get to sit in the shade of a chestnut tree, even a young one, and by 5 years, in a good orchard, you can take out a chair and read among the trees. By 7 years, your trees will have a few nuts — the first ones are too precious to eat, but soon enough, even after selection, your remaining trees will provide enough for Thanksgiving stuffing. And you will have restored a tradition taken for granted by Americans!

Two of the next pages will take two new orchards through the stages and steps of development.

Nuts & Bolts of Backcross Breeding part 1
Nuts & Bolts of Backcross Breeding part 2