Location: Crops Pathology and Genetics Research
2013 Annual Report
There are ten walnut rootstock field plots established by cooperating University of California (UC) Farm Advisors, from Kings County in the southern San Joaquin Valley to the most northern in Tehama County in the Sacramento Valley, to coastal Contra Costa County in California. These trials have various micropropagated clonal and seedling rootstocks, both commercially available and experimental, planted to address various problems. Stakeholders need more information on the performance of these newer and experimental clonal rootstocks for disease resistance. Crown gall disease, caused by Agrobacterium tumefaciens, is rated in the top three major problems by the California Walnut industry. We are in the process of surveying all these rootstock plots for crown gall incidence and disease severity and plan to complete surveys by fall, 2013. These plots range in planting date from 2000 through 2011.
All the plots have seedling Paradox rootstock as a standard which is the most planted by growers but also the most susceptible to crown gall. Ten clonal walnut rootstocks that are being tested in various situations, i.e., fumigated and non-fumigated plots, areas with walnut blackline or Phytophthora diseases, etc., and will be surveyed for crown gall as part of this grant. The survey in Stanislaus County has already been completed and a paper submitted for publication. The recent survey was combined with a previous study for the paper. Clonal Paradox rootstocks had significantly lower crown gall incidence than the seedling walnut rootstocks. These results and those from the other surveys will be presented at grower meetings and through newsletters in 2013.
Thousand cankers disease (TCD) caused by the pathogen Geosmithia morbida, and vectored by walnut twig beetle, is thought to have been present in urban plantings and stands of black walnut, J. nigra, in the western U.S. for decades. TCD’s potential significance to commercial walnut product was first recognized in 2008, when it was confirmed in California urban and native trees of J. hindsii and in native stands of J. californica. In 2009, the disease was confirmed in commercial black walnut seed orchards and Paradox hybrid walnut rootstock in California. In the fall of 2012, we identified commercial English walnut on seedling Paradox rootstock orchards adjacent to riparian stands or roadside plantings of J. hindsii exhibiting TCD or English orchards with TCD in Solano, Sutter, Yuba, Tulare, Lake, Contra Costa, Kings, and Yolo counties in California. Seventeen orchards were surveyed for the presence of TCD and samples taken to confirm the disease. Surveying orchards for TCD is very time consuming and difficult since you’re looking for tiny pin-sized beetle holes and then have to determine if there is a canker caused by the disease underneath the hole. The majority of orchards had positive samples for TCD. However, we noted that in most cases, even with disease in the Paradox rootstock or the English scion, unless the tree is declining because of some other problem, tree vigor was not affected, which is an important finding.
We have revised the protocol for the 2013 surveys to overcome some of the difficulties we encountered in 2012 so not to overestimate the amount of TCD. In addition to conducting more surveys in the counties listed above, surveys will also be conducted in Stanislaus and San Joaquin counties. We hope to have a fairly good assessment by 2014 as to the status of TCD in our commercial walnuts throughout most of our growing areas.
Additionally, the following study is being conducted in Tulare County to assess the threat from firewood that is infested with walnut twig beetle (WTB): After trees and limbs are removed from walnut orchards, the wood is often either stacked in commercial yards for firewood sale, or in burn piles adjacent to individual orchards. These wood piles are a potential source of WTB and Geosmithia morbida inoculum; however, the length of time required for natural disinfestation of wood is unknown. In March, 2013, a study was initiated at the Lindcove Research and Extension Center (LREC) in Tulare County, California, to investigate the entomological ecology of firewood removed from a walnut twig beetle- infested orchard, with a specific goal of addressing the period of time and rate at which WTB emerges from firewood. WTB-infested logs were placed in six insect-emergence chambers at LREC and the mass, volume, and surface area placed in each chamber was recorded. Each chamber is equipped with data-loggers for capture of temperature and relative humidity data and attached to a refrigeration unit for the collection and preservation of insects between collections. Total insects and sex ratio of WTB emerging from each chamber are identified and recorded weekly. Insect emergence data is calculated as population of a species or group per unit surface area, mass, and volume of wood in each chamber. The project is expected to continue through winter 2014.