Submitted to: Walnut Research Conference
Publication Type: Research Technical Update
Publication Acceptance Date: 3/1/2010
Publication Date: 1/1/2011
Publication URL: http://walnutresearch.ucdavis.edu/2010/2010_225.pdf
Citation: Browne, G.T., Schmidt, L.S., Bhat, R., Leslie, C., Hackett, W., Beede, R., Hasey, J. 2011. Etiology and management of crown and root rots of walnut. Walnut Research Conference. pp.225-236. Interpretive Summary:
Technical Abstract: Our objectives for 2010/11 were to: 1) develop a quick, small-plant (0.4-liter-pot) method for evaluating resistance to Phytophthora cinnamomi and P. citricola; 2) use a conventional large-plant (2-liter-pot) method to evaluate resistance to these pathogens in the remaining inventory clonal rootstock selections in the large-plant format, and 3) conduct orchard surveys and diagnostics to assess losses due to Phytophthora and determine etiology of other crown and root diseases. For the quick resistance evaluations (Obj. 1), a protocol was developed to inoculate rootstock clones with Phytophthora 2 months after transplanting them from tissue cultures into 350-ml pots. The medium in each pot was inoculated with 25 ml of V8 juice-vermiculite-oat substrate. The substrate was either colonized by one of the pathogens or was sterile as a control. Soil flooding then was imposed on the plants for 24 to 48 h at weekly intervals. Six weeks after inoculation, the root systems were washed free from the potting medium and evaluated visually for incidence and severity of crown and root rot. Clones evaluated using the quick method included AX1 (a standard of Juglans californica × J. regia that, based on conventional large-plant evaluations, is highly susceptible to both pathogens), PX1 (medium-susceptible standard of J. hindsii × J. regia), RX1 (low-susceptibility standard of J. microcarpa × J. regia), W17 (highly susceptible standard of J. hindsii), and Ch.Wn.10.05 (highly resistant standard of Pterocarya stenoptera). The relative susceptibilities of these rootstocks in the small-plant evaluation method were as expected based on previous large-plant evaluations. In practice, a small-plant resistance evaluation cycle requires < 6 months, whereas a large-plant cycle requires > 1 year. We have adopted the small-plant protocol for future evaluations. Using the conventional method to evaluate resistance in the remaining inventory of large clonal plants (Obj. 2), several rootstock clones (i.e., DAR, J1Acont, UZ229, WIP4, WIP6) developed relatively low levels of crown rot (i.e., means of < 30% of crown length or circumference rotted) in soil infested with P. citricola, but these clones tended to have higher levels of root or crown rot than RX1 in soils infested with P. cinnamomi. In the same test, rootstocks of RR4cont, Vlach, VX211, WIP2, WIP3, Chandler, and Howard were intermediate to relatively high in susceptibility to one or both of the pathogens. Surveys of orchard decline problems with a soilborne component (Obj. 3) were conducted with Bob Beede and Janine Hasey. In Kings County, new incidence of tree decline and death due to a canker disease on Paradox rootstock was observed in 2010 as in several previous years (2007-2009 reports to CWB, Browne et al.). Also in 2010, Themis Michailides reported the disease in Fresno County, and we observed it in several orchards of Yuba and Sutter Counties. We now refer to the disease as lethal Paradox canker. Brennaria nigrifluens (the species known to cause shallow bark canker in English walnut) was isolated from some of the Paradox cankers in Yuba County, and the isolates were pathogenic in excised Paradox walnut shoots. Koch’s postulates must be completed before conclusions on etiology of the lethal Paradox canker are justified. Finally, many trees of cultivar Howard on Paradox rootstock in Sutter and Yuba Counties exhibited a decline problem (chlorosis, wilting associated with few feeder roots). Orchard observations suggested that there is a rootstock genetic component contributing to the problem.