To slow walnut diseases, UC seeks resistant rootstocks
ByDennis Pollock Five researchers have rolled up their sleeves to tackle disease resistance inwal- nuts, and two of them, both with the U.S. AgriculturalResearchService, spokeduring this year’sCaliforniaWalnutConference. The twoareDanielKluepfel,workingon crown gall, and Greg Browne, working on Phytophthora. Bothscientists arebasedat theUniversity of California, Davis. Others researching disease resistance screening include Andreas Westphal, a UCRiverside nematologist at the Kearney Agricultural Research and Extension Center in Parlier; JanineHasey, a field site coordinator and tree crop and environ- mental horticulture advisor emeritus with UC Cooperative Extension in Sutter and Yuba counties; andChuck Leslie, with the UC Davis Department of Plant Sciences, whoworks onplant propagation. Kluepfel talked of managing crown gall with disease-resistant rootstocks, saying the long-termgoal of researchis todevelop and deploy walnut rootstocks with resis- tance to the major soil-borne pathogens: crown gall, Phytophthora root and crown rot, and lesionnematodes. He said four possibly disease-resistant hybridshavebeenclonallypropagatedand arebeingexaminedinfieldtestsatmultiple
locations in the Central Valley. Additional hybridshavebeenclonallypropagatedand will be ready for testing in2021-22. The research is supported by the U.S. Department of Agriculture and the California Walnut Board. Nurseries co- operating with researchers include Sierra GoldNursery andDuarteNursery. Kluepfel said one objective is to gener- ate and identify a hybrid Juglans walnut germplasm that resists the key soilborne pathogens. The germplasm collection is housedat theNationalClonalGermplasm Laboratory inDavis. Once those are identified, researchers said they plan to propagate those and val- idate their efficacy in field trials—and then deliver the rootstocks to farmers. Kluepfel saidthereare11diverseJuglans species. Researchers are focusing on Juglans microcarpa, a black walnut, and generating hybrids with Juglans regia. He saida rating systemwas established, ranging from no symptoms to varying degrees of girdling up to greater than 50% of the stem. Plants are injected with Agrobacterium tumefaciens (crown gall), thenchecked in twomonths todetermine the degree of resistance. Browne andhis researchers are looking at two different species of Phytophthora.
Researchers from the University of California and U.S. Department of Agriculture have been working on ways to protect walnut orchards from diseases such as crown gall and Phytophthora.
They judge resistance based on survival duration, the percentage of crown length rotted and the percentage of root length rotted. Research continues on resistance to root-lesion and root-knot nematodes. Nematode research has special chal- lenges, because it must be done in the field. Unlike crowngall andPhytophthora research, it can’t be done in the green- house. Nematode sampling is done over two years. As part of the research, Browne and Kluepfel each develop “elite” clonal dis- ease-resistant rootstocks that are passed on to Sierra Gold Nursery, which gen- erates large numbers of each clone. The clones are then grown at four fieldplots in Tulare, Lake, Glenn and Sutter counties.
Commercial rootstocks are also grown at those sites for comparison. Browne talked of identifying and managing walnut diseases caused by Phytophthora. He said it’s now estimat- ed there are 300 species of the soilborne pathogen that affect thousands of plant hosts—up from 86 species identified in 2000 and 180 described by 2018. Phytophthorawas thecauseof Irishpotato famine in the 1840s. It is exacerbated with water satu- ration. Rivers and canals can spread Phytophthora, and it canalsobe spreadby infestednursery stockandsoilmovement. Once introduced,Phytophthora isvirtually
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May 12, 2021 Ag Alert 11
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