Methyl Bromide Alternatives, Focus on Rootstocks

We now have two consecutive years of yield data following soil fumigation that provided differing volumes of sterilized soil. Tarped methyl bromide at 400 lb/ac provided soil sterilization throughout the surface six feet of soil. Telone II applied in 11 foot wide strips at 50 gallons per acre provided sterilized soil to 4.5 feet deep across half the field surface area. Nine years later the yield from Chandler scion on Paradox rootstock was reduced by 25% unless the Paradox roots also possessed a mechanism for at least some nematode resistance. In addition, yields following methyl bromide were also beginning to decline unless the rootstock possessed some nematode resistance. These yield data conclude our effort to provide quantification of the importance of nematode resistance relative to soil fumigation. Yields from our VX211 and AX1 trial were also collected for the second time in 2010. Yields were almost doubled when VX211 was the rootstock, whether nematodes were inoculated or not. Highest inoculum levels provided the opportunity to also transfer nematode biocontrol agents (Bent et.al.). Our 2009 report indicated that J. cathayensis trees located at Wolfskill seldom provided seeds having resistance to Pratylenchus vulnus or Meloidogyne incognita. The exception was J. cathayensis #21 which also provided extra vigor. However, we are able to report that 60% of the J. cathayensis trees imported from China by Gale McGranahan appear resistant to P. vulnus and M. incognita. This source has provided a mother-lode of resistant material but it is also notable that the trees from China appear to be relatively low in vigor. One more year of tree evaluation is needed. We have also identified a second source of nematode resistance and it emanates from J. microcarpa. Our selection of RX032 in 1999 as a rootstock of interest has been difficult to assess because of propagation difficulties. We did evaluate trees from hard wood cuttings four years ago. At that time they appeared tolerant of nematode presence but did host P. vulnus. More recently Wes Hackett and Chuck Leslie have had success with tissue culturing of RX032 and their first five trees all appear resistant to P. vulnus after one year of study. We also now have an abundance of UZ229 trees which are growing very well, an attractive feature when compared to our earlier observations of their root development from hardwood cuttings. In 2008 we initiated a study to determine tolerance of eight Juglans sources, 48 trees of each, to the rejection and root lesion components of the replant problem. Clonal VX211, UZ229 and young NCB seedlings grew 12 to 22% better when planted to fumigated soil. Older 11⁄4 inch diameter NCB grew poorly whether planted to fumigated soil or not. Own-rooted Chandler and Serr trees of various size plus seedlings of J. cathayensis and clones of RX1 appeared tolerant of the rejection component. In fall 2010 we initiated a trial to treat replant sites with: 1) a new formulation of Telone, 2) 2,3dithiobutane and 3) steam sterilization utilizing a new portable steaming unit. California Walnut Board 259 Walnut Research Reports 2010 OBJECTIVES 1) Maintain the Rio Oso trial for yield data, rates of nematode return and tree growth with and without Movento applications. 2) Remove up to half the trees (6 reps) from the VX211 and AX1 trial at Kearney Ag Center and graft over to Chandler for yield data. 3) Obtain for further nematode and field evaluations the clones of RX032 and UZ229 (Wes Hackett). 4) Continue nematode evaluations of various Juglans species from the USDA Davis Repository. 5) Explore at our KAC trial site the value of Garlon + one year, Vapam drench and/or non-hosts as methods for reducing soil populations of P. vulnus, Mesocriconema xenoplax and Meloidogyne spp. down to various soil depths. PROCEDURES Objective 1. The Rio Oso site is now 8-leaf and involves approximately 8 acres with every other tree being either NX or DN paradox. These rows have been planted over soil that received either methyl bromide to the 6-foot depth, Telone II at 50 gallons per acre stripped with delivery to the 4.5-foot depth or a broadcast application of Metam sodium delivered to the 4-foot depth. Objective 2. Within 48 macroplots (1/100 acre each) at KAC we now have 4 reps each of VX211 and AX1 in each without nematodes plus 12 reps with nematodes. In fall 2006 we cut the trunks within 1/3 of these macroplots and applied Garlon. The remaining 4 or 8 reps will be grafted to Chandler for purposes of yield collection. Nematode sampling of the soil and trunk measurements will continue twice yearly. Objective 3. Selection RX032 is a Juglans microcarpa that we selected because it performed well in our earliest nematode screens but it has not rooted well to this date. We have also observed it to support high populations of Ten-lined June Beetle. UZ229 is of interest because it is now the largest of three saved UZ trees. These three trees were originally selected because three out of ten appeared to keep P. vulnus away from their roots, very similar to what was observed with NX rootstock. RX032 is of interest for its host status against nematodes but also for its potential growth habit against the rejection component. Objective 4. Evaluation of more exotic selections of Juglans from the National repository initiated in 2005. We evaluated varying numbers of J. regia, J. microcarpa, J. major, J. ailantifolia, J. mandshirica, J. cathayensis and J. nigra. Our focus is on J. cathayensis. In 2010 we will receive numerous first-leaf trees of J. cathayensis collected during an expedition by Gale McGranahan. Objective 5. Fifty J. cathayensis will be planted into a walnut replant setting at Kearney in 2010 for evaluation of their tolerance to the rejection component of the replant problem. California Walnut Board 260 Walnut Research Reports 2010 RESULTS AND DISCUSSION The value of pre-infection resistance The two sources identified in 1999 to have pre-infection resistance mechanisms were NX and UZ seedlings. Three to four of the 10 seedlings examined from these two sources were unique within the Paradox Diversity Study, but we did not know the value of this type of resistance mechanism within an orchard setting. This mechanism has been reported in the literature but what is its worth? An 8-acre soil fumigation experiment at Rio Oso, CA provided an opportunity to compare NX Paradox seedlings with seedlings of DN Paradox seedlings. Yields were collected in 2009 and 2010 and those findings parallel trunk circumferences from earlier years. 2009 yield (lb of hulled nuts/tree) of Chandler on NX vs DN seedlings 0 10 20 30 40 50 60 70 80 MB broadcast Strip Telone Vapam broadcast NX paradox DN paradox a a a