Backcross Program

Our Maine Chapter of TACF is one of about a dozen state chapters conducting a back-cross breeding program coordinated by TACF’s research farms in Meadowview, VA, using local American chestnuts as the mother trees to ensure that blight-resistant trees coming out of our breeding will be most ideally adapted to our Maine growing conditions. Since 1997, we have been manually pollinating Maine mother trees with pollen sent from Meadowview from selected second-backcross trees. We plant the third-backcross seeds that we harvest from these crosses in our breeding orchards. When these trees are 5-6 years old, we challenge them with live blight fungus injections. Trees that have partial blight-resistance are bred with each other in the breeding orchard. We make these crosses in the breeding orchard for 2-3 years. The seeds from these crosses are then planted in seed orchards, and some of these will be blight-resistant. Our breeding program will begin producing blight-resistant Maine-adapted American chestnuts in about 2018

Breeding Program Update – October 2017 by Eric Evans, Maine Breeding Coordinator
The current focus of our breeding program is to harvest seeds (B3-F2) from the most blightresistant trees in our third-backcross (B3) orchards, and plant them in our seed orchards, which constitute the 5th generation of our 6-generation breeding program. After evaluation and selection for blight
resistance and American type, the seed orchards will produce seeds (B3F3 – the 6th generation) for chestnut test and restoration plantings in Maine’s forests, starting in about 2020. We have two parallel and separate programs, named for the 1st-backcross tree that was the source of blight resistance – Clapper and Graves (the breeders who made the original Chinese-American crosses). Each program has 20 breeding lines planted in our 3rd backcross orchards, and each is producing seeds for a system of 9 replicate seed orchards (called “blocks”). Each of the 9 “blocks” will eventually have one “plot” of 150 trees planted from each of the 20 breeding lines. The large numbers of breeding lines and replicate plots and blocks is an essential feature of our goal to restore chestnuts with enough genetic diversity to thrive and continue evolving in Maine’s forests.

The following table lists all of our breeding orchards, and shows the chronology of the development of our program. We made each “line” by pollinating a single native Maine chestnut “mother” tree with pollen sent from Meadowview from a single selected second-backcross tree. About 40 different Maine “mother” trees and 40 separate pollen-source trees in Meadowview, VA are represented in our back-cross orchards. In spring 2012 we planted our first crop of nuts from some of our back-cross orchards in our new seed orchards. In the next few years we will be filling out our seed orchards with nearly 60,000 chestnut seeds from our back-cross orchards. The “Graves” and “Clapper” designations in the table correspond to the two different Chinese great-great-grandparents used to start the breeding program. These two resistance sources have been kept separate in the back-cross orchard systems and will also be kept separate in the seed orchards.

Back-Cross Orchards

Graves source of blight-resistance

TownLandownerYear Planted# of Lines
ChinaClark Granger2002-20036
LovellLynn & Steve Korth20042
UnityAndy Reed2004-20125
MorrillDaleen & David Dutton2005-20063
BradleyPEF, US Forest Service20062
VeazieTown of Veazie20093
HopeAndy Rowe20112
HopeMary Bok20133
Graves Total 23

Clapper source of blight-resistance

TownLandownerYear Planted# of Lines
CamdenMerryspring Nature Center1999, 20026
UnionPhillip Groce19992
MonmouthHighmoor Farms, Univ. of Maine2002-20045
Hope
Harold Mosher2006-20123
Clapper Total 22

The next table lists the location and landowners of our seed orchards. The Graves and Clapper systems each will have nine replications of the basic seed orchard unit. Each orchard unit is about ¾ acre and has 150 trees from each of the 20-23 back-cross lines. See the breeding program update for more details and see our Orchards page for descriptions and photos of all our orchards.

Seed Orchards

Graves Lines

TownLandownerYear Planted# of Orchard Units
HartlandUniversity of Maine2012-20205
WinthropSmall Woodland Owners Association of Maine2013-20205

Clapper Lines

TownLandownerYear Planted# of Orchard Units
PhippsburgThe Nature Conservancy2012-20202
StetsonPenobscot County Conservation Association2012-20205
SearsportSmall Woodland Owners Association of Maine2013-20202

The following time-line chart describes the progress of our breeding program.

breeding_program_timeline_july_2012