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ARS Home » Pacific West Area » Davis, California » Nat'l Clonal Germplasm Rep - Tree Fruit & Nut Crops & Grapes » Research » Research Project #442954

Research Project: Evaluation of Walnut Blight Resistance in USDA-NCGR Juglans regia Accessions from the Republic of Georgia

Location: Nat'l Clonal Germplasm Rep - Tree Fruit & Nut Crops & Grapes

Project Number: 2032-30100-001-002-S
Project Type: Non-Assistance Cooperative Agreement

Start Date: Sep 1, 2022
End Date: Aug 31, 2024

In 2021, for the first time in many years, we evaluated nut quality in samples from accessions in the Juglans C block, which contains much of the collection’s global diversity of J. regia, as well as all of the collection’s J. australis, which has been used as a rootstock in Argentina and Chile. The newest major addition to the NCGR Juglans collection consists of 562 trees from 91 accessions collected in the Republic of Georgia in 2014. This region experiences warm wet springs with high blight pressure, has long been considered a promising source of blight resistance alleles in walnut, and forms one of five distinct populations (along with Europe, Iran, China, and the Himalayas) in a global survey of genetic diversity in Juglans regia (Arab et al., in prep). The Georgian walnut trees in the collection are planted on very close spacing (< 6’ within row) due to land constraints, and long-term maintenance of walnut trees at this density is expected to become progressively more difficult. In summer 2021 the well supplying irrigation water to this block failed, requiring heroic efforts on the part of NCGR staff to keep these trees alive. The primary motivation for collecting these accessions from the Republic of Georgia was to screen them for blight tolerance, but this has not previously been done.

We propose to apply the “gold standard” of blight resistance screening, overhead sprinkler irrigation, to the 91 accessions (562 trees total) of Juglans regia from the Republic of Georgia in spring 2023. The close spacing and dense overlapping canopies of these trees are expected to help accelerate the spread of the disease. We will continue to hedge these trees, along with the rest of the Juglans trees in the collection, to control their height and promote nut development within reach of the ground. Irrigation lines will be suspended above the canopy, at a height of ~15’, using wooden scaffolding. Overhead irrigation using microsprinklers will be delivered weekly in 8-hour events, beginning when the terminal buds on the earliest-leafing trees begin to swell, and ending when the nuts on the latest-leafing trees are at least 5 cM, or tentatively for a 10-week period from mid-March through the end of May. Blight development on both shoots and nuts of individual trees will be monitored weekly, along with phenology (leafing and flowering dates), using methods developed previously in the Walnut Improvement Program (WIP). We will also perform weekly Unmanned Aerial Vehichle (UAV flights) (using Phantom 4 Professional with 20 mega pixel (MP) red, green, blue (RGB) camera and a Micasense RedEdge), monitor changes in normalized difference vegetation index (NDVI) as a function of canopy volume, and compare the data from these overhead-irrigated trees with data from a ground-irrigated, adjacent control block. Genotype data previously collected from these trees will be used along with blight resistance phenotypes to test for genome-wide associations, with and without the use of leafing date as a covariate. In summer 2023, the most promising sources of blight resistance in the Georgian collection will be propagated through June-budding for planting into a future replicated overhead-irrigation experiment, and for preservation in both the National Clonal Germplasm Repository (NCGR) and WIP. All blight phenotypes will be deposited in Germplasm Resources Information Network (GRIN) database.