Table 5 Approaches used in resistant tree breeding programmes

Screening

- Inoculate following a standard method and monitor trees in TACF orchards

- USFS field testing of 12,000 TACF seedlings

- Inoculate following a standard method and monitor trees in ACCF orchards

- Also monitor external plantings using volunteer growers

- Screening at 22 sites, with susceptible trees as controls. Weevil numbers augmented and standardised between sites

- Investigate heritability of resistance and geographic differences in susceptibility

- Inoculate and screen trees in orchards

- > 50,000 hybrid seedlings raised and tested for DED-resistance and other desirable traits (e.g. growth form, colour)

- Field-collected disease-free material as the basis for initial screening trials

- Growth and mortality monitored after artificial inoculation

- Potential resistance validated in field trials

Tree breeding

- Interspecific hybridisation with Chinese chestnut

- Backcrossing, incorporating locally adapted material

- Considering potential for genetic engineering

- Heritability estimates of 0.2–0.5 (Steiner et al. 2017; Westbrook 2018)

- Intraspecific tree breeding, including grafts and crosses of survivors collected in the 1970s and between apparently resistance offspring

- Open-pollinated trees from many locations planted and crossed

- Intraspecific tree breeding, with material from Sitka spruce range

- Grafts and controlled crossing of putatively resistant trees

- Heritability of 0.5 (family) and 0.9 (individual) (Alfaro et al. 2013)

- Interspecific hybridisation between European and Asian elm species

- Including several provenances to maintain genetic variation

- Intraspecific tree breeding, including grafting and controlled crosses

-Incorporate several mechanisms to increase durability of resistance

- Heritability estimates of 0.2–0.6 depending on trait (Mahalovich 2010)

Production

- Local TACF state chapters plant centrally produced material in seed orchards

- 52,000 partially resistant seedlings planted in 2015 (TACF 2015)

- Main seed orchard owned and managed by ACCF

- Approx. 4000 nuts from open pollination supplied annually to volunteers providing $20 and agreeing to monitor performance

- Seed orchards owned by B.C. Ministry of Forests

- Collection from geographic ‘resistance zones’ identified in screening

- IPP seed orchards

- Commercial nurseries receiving IPP licence

- Several seed orchards, often building on previous screening and breeding

- Orchards include wild-collected material, stock from earlier orchards, and more advanced material from ongoing breeding

Deployment (Scale)

- Not yet taking place (but see ‘Screening’ and ‘Production’ for an indication of scale)

- 10, 20 or 100 putatively resistant chestnuts provided annually to volunteers for test planting

- Aim to plant around 4000 chestnuts annually

- B+ material (from zones with a high proportion of resistance) from 1990s

- Breeding programme seed (Class A) available

- > 400,000 trees planted from 2005 to 2011

- Available commercially from licenced nurseries

- Planting for conservation purposes being trialled

- Large area potentially of interest (to reverse historical losses)

- Seed available for planting, but deployment limited by land availability

Deployment (Strategy)

- Likely to target environmentally suitable sites and include silviculture to increase chance of success

- Likely to involve volunteers

- Aim to deploy a population of trees able to persist as a genetically diverse wild species, likely based on seed orchard seedlings

- Currently, nuts provided to volunteers contributing $20 to ACCF (material is open pollinated with only a proportion expected to have partial resistance)

- Will probably use seed orchard seedlings

- Recommend up to 30–50% stocking where weevil hazard is medium-high

- Uses seed orchard seedlings

- Scale and locations of operational deployment not clear

- Several clones available, but planting large numbers of a single clone is not recommended

- Seed orchard seedlings distributed to public and private forestry organisations

- Recommended for operational use in areas with medium-high WPBR hazard

- May be unsuitable in highest hazard zones, with more virulent rust strains

- Avoid in pure stands

Monitoring

- Field testing of ongoing trials by USFS

- Data from TACF breeding and production orchards

- Emphasise need to record field performance

- Volunteer growers provide annual performance data

- Primarily within long-term screening trials

- Some monitoring by organisations that plant IPP material

- Within field populations and breeding and production orchards

- Also includes susceptible trees as controls

Problems

- Land availability for seed orchards and deployment

-Mortality from other causes (e.g. Phytophthora) ranges from 12 to 70%

- Too soon to assess durability of resistance

- Resistance in American chestnut is partial, occurs at very low frequency, and has low heritability

- Initial problems gathering data on performance

- Resistance is partial, so not recommended for deployment at high densities in weevil hazard areas

- Some resistant cultivars withdrawn from sale due to susceptibility to other pests and pathogens

- Demand and confidence also potentially affected by losses of putatively resistant elms from other breeding programmes

- Mechanism of complete resistance has been overcome in some areas

- Partial resistance mechanism more durable, but some impacts occur

- Land availability for planting

Other strategies used

- Silviculture

- Targeted deployment to suitable sites

- Silviculture

- Targeted deployment to suitable sites

- Hypovirulent blight to reduce impacts

- Resistant trees viewed as the most effective approach for using Sitka spruce in pine weevil areas

- N/A

- Resistant trees viewed as a key part of strategies to recover species

- Silviculture to maintain regeneration

- Removal/pruning of affected trees