Abstract:
A new and distinct variety of walnut rootstock denominated ‘RX1’ is described. This new variety, ‘RX1’, can be propagated through micropropagation. It has excellent survivability in the nursery and orchard. The new variety also has reduced susceptibility to damage from  Phytophthora citricola  in greenhouse screens and in the field compared to other available walnut rootstocks.

Description:
FIELD OF THE INVENTION  
       [0001]     Botanical/commercial classification:  Juglans microcarpa×Juglans regia /new walnut rootstock. Varietal denomination: ‘RX1’.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a new and distinct clonal rootstock for English walnut ( Juglans regia ) that has been denominated varietally as ‘RX1’, and more particularly to such a walnut rootstock that has reduced susceptibility to cankering by  Phytophthora  ( Phytophthora citricola ), and that further is easily clonally propagated by micropropagation.  
         [0003]     It has long been recognized that  Phytophthora  root and crown rots are some of the most serious diseases of walnut worldwide. In California,  Phytophthora citricola  and  P. cinnamomi  are recognized as the most virulent species of the fungus, but  P. citricola  is more widespread. The rootstock of the present invention, ‘RX1’, has been identified as being more resistant to  P. citricola  than other available clonal walnut ( Juglans ) rootstocks.  
       SUMMARY OF THE INVENTION  
       [0004]     It was found that the walnut rootstock ‘RX1’ of the present invention exhibits the following combination of characteristics: 
        a) can be propagated through micropropagation;     b) has excellent survivability in the nursery and orchard; and     c) has reduced susceptibility to damage from  Phytophthora citricola  in greenhouse screens and in the field compared to other available walnut rootstocks.        
 
       BRIEF DESCRIPTION OF THE TABLES  
       [0008]     Table 1 shows comparative nursery performance of ‘RX1’ and other rootstock clones grown in Stanislaus County, Calif. in 2004.  
         [0009]     Table 2 shows comparative nursery performance of ‘RX1’ and other rootstock clones grown at in Butte County, Calif. in 2004.  
         [0010]     Table 3 shows field performance of ‘RX1’ and other clonal and seedling rootstocks in non-infested soil and soil infested with  Phytophthora citricola.   
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  shows relative susceptibility of ‘RX1’ and two other potential rootstock clones to  Phytophthora citricola,  and the effect of pre-inoculation chilling on disease severity, 2003 greenhouse screen.  
         [0012]      FIG. 2  shows relative susceptibility of ‘RX1’ and two other potential rootstock clones to  Phytophthora citricola,  data combined for plants subjected to pre-inoculation chilling and non-chilled plants, 2003 greenhouse screen.  
         [0013]      FIG. 3  shows relative susceptibility of ‘RX1’ and six other potential rootstock clones to  Phytophthora citricola,  2003 greenhouse screen.  
         [0014]      FIG. 4  shows relative susceptibility of 10 hybrid walnut clones to  Phytophthora citricola,  2004 greenhouse screen.  
         [0015]      FIG. 5  shows relative susceptibility of 17 hybrid walnut clones and Northern California black walnut to  Phytophthora citricola,  2006 greenhouse screen.  
         [0016]      FIG. 6  shows grafted ‘RX1’ in a new orchard.  
         [0017]      FIG. 7  shows ‘RX1’ in  Phytophthora  field trial.  
         [0018]      FIG. 8  shows grafted ‘RX1’ in replant situation.  
         [0019]      FIG. 9  shows visual rating of tree growth and condition of clonal and seedling test trees at a Jenny Lind, Calif. field site in 2006.  
         [0020]      FIG. 10  shows percent mortality for UC clonal selections and other rootstocks at a Jenny Lind, Calif. field site.  
         [0021]      FIG. 11  shows percent mortality for UC clonal selections and other rootstocks at a Linden, Calif. field site.  
         [0022]      FIG. 12  shows bark and new leaves of three-year old ‘RX1’ tree.  
         [0023]      FIG. 13  shows greenhouse grown ‘RX1’ tree about 6 months old.  
         [0024]      FIG. 14  shows upper side of leaf of ‘RX1’.  
         [0025]      FIG. 15  shows lower side of leaf of ‘RX1’.  
         [0026]      FIG. 16  shows flower of ‘RX1’. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     The new rootstock, ‘RX1’ was selected as part of the “Paradox Diversity Study” (PDS) which was initiated in 1996 to study the genetic diversity of commercial walnut rootstocks. The hybrid of  J. hindsii×J. regia,  commonly known as Paradox, is the most frequently planted rootstock for English walnut in California. The study included approximately 300-500 seed (depending on the predicted percent Paradox), from 37 black walnut sources of Paradox supplied by California walnut nurseries, and 7 controlled crosses and open-pollinated controls from several different walnut species including Texas black,  Juglans microcarpa.  Seed or seedlings were distributed to cooperating researchers for tests of response to nematodes ( Pratylenchus vulnus ),  Phytophthora  (seed supplied), crown gall ( Agrobacterium tumefaciens ) and the orchard environment (field trials). The study was repeated in 1997.  
         [0028]     In fall of 1997, seed from a  Juglans microcarpa  designated as DJUG 29.11 in location B6-3 at the National Germplasm Repository, Davis and growing at the Wolfskill orchards in Winters, Calif. was tested against  Phytophthora citricola.  From results of the previous year, about 50% germination and about 50% hybrids with  J. regia  from this tree was expected. In fact in 1998, germination was better (70%) but percent hybrids were very low (5%). Due to lack of sufficient seedlings for screening, a representative of the  J. microcarpa×J. regia  hybrid family was micropropagated. The seedling chosen (98-RX-SD8) later became ‘RX1’. It was introduced into culture in summer 1998 using the standard micropropagation protocol. In fall 1998, the cultures were transferred to a nursery for further multiplication and rooting.  
         [0029]     In summer 2001, a replicated trial in the greenhouse to determine the relative susceptibility of ‘RX1’ to  Phytophthora citricola  was initiated. The clone appeared to have resistance to the pathogen in preliminary tests. In September, 2001, the clone was evaluated for further production of plants. Between 2001 and 2005, ‘RX1’ was multiplied, rooted and acclimatized for trials for response to  Phytophthora citricola  and for additional field trials. During summers 2002, 2003, 2004, and 2006, plants were transplanted into appropriate containers, grown to appropriate size for screening, grown on appropriate inocula, and subjected to repeated greenhouse experiments to evaluate resistance of ‘RX1’ and other selected clones to  P. citricola .  Several modifications in propagation and pre-inoculation treatments were made including induction of dormancy of plants and treatment with hormones. ‘RX1’ was consistently at least moderately resistant to the pathogen ( FIGS. 1-4 ).  
         [0030]     In August 2006 a screen for  P. citricola  response was conducted with plants of ‘RX1’ that had been through cycles of dormancy which tended to equalize growth and kept them small enough to facilitate mass screening. The cycles included dormancy induced by storage at 6C for 3-5 months (2004), growth in the greenhouse for one year (2005) and natural dormancy in a lath house followed by growth in the greenhouse (2006). The screen for resistance was initiated in August 2006 by transplanting individual plants from one-liter pots to two-liter pots filled with UC potting mix soil that was either artificially infested with  P. citricola  (45 ml of  P. citricola -infested V8 juice-oat-vermiculite substrate per liter of soil) or treated as a control (45 ml sterile substrate per liter of soil)). There were 5 replicate plants planted in non-infested soil and 10-20 replicate plants in infested soil, evenly distributed in a split-plot design (main plots were inoculum treatments, subplots were rootstock) among 5 blocks. Every two weeks after transplanting, the soil in each pot was flooded for 48 hours. Three months after transplanting, the root and crown systems were washed free from soil and evaluated visually for incidence and severity of crown and root rot. Among the 17 clonal hybrids evaluated in the screen, ‘RX1’ was one of the hybrids most resistant to  P. citricola  ( FIG. 5 ).  
         [0031]     During the propagation of plants for  Phytophthora  testing, plants were also being propagated for field trials. These were grown at two nurseries in 2004. ‘RX1’ was one of the smallest plants at both nurseries (Tables 1 and 2), but produced between 70% and 75% graftable rootstocks as determined by the nursery. These were either grafted in place with ‘Chandler’ (U.S. Plant Pat. No. 4,388) or distributed for grafted field trials for replant situations or  Phytophthora  field screening in 2005. The ‘Chandler’-grafted ‘RX1’ (n=80) was planted in a new orchard with another promising clone AZ2 (n=80) and seedling Paradox ( J. hindsii×J. regia ) provided by the nursery. AZ2 turned out to be a weak clone that could not be transplanted bare root, and survival was very poor after transplanting. Nearly all the ‘RX1’ survived and were indistinguishable from the seedling Paradox ( FIG. 6 ).  
         [0032]     For the  Phytopthora  field trial, 30 each of 11 different genotypes including ‘RX1’ were planted in May, 2005 in Armstrong Tract, Department of Plant Pathology on the University of California campus and were artificially inoculated with  Phytophthora citricola.  A randomized block split plot design was used. For each rootstock clone, there were six four-tree plots to be infested and six single tree plots to serve as uninoculated controls. Northern California black ( J. hindsii ) and wingnut ( Pterocarya stenoptera ) were included as susceptible and resistant controls, respectively. In January 2006, 100 ml of a V8 juice-oat mixture infested with  P. citricola  was mixed into the upper 5 cm of soil around the trunk of each tree. A sterile mixture was applied to the uninoculated controls.  
         [0033]     The block artificially inoculated with  Phytophthora  was assessed for growth in trunk circumference and development of crown rot as indicated by trunk cankers extending up from the soil surface in November 2006. Sixty-two percent of the susceptible controls were rotted or dead. ‘RX1’ was one of the smaller clones (Table 3), but it was thriving ( FIG. 7 ) and not affected by the inoculation (Table 3).  
         [0034]     Preliminary results from grafted field trials suggest that ‘RX1’ is a survivor in spite of the challenge of being in replant sites ( FIGS. 8-11 ).  
       BOTANICAL DESCRIPTION OF THE PLANT  
       [0035]     This description is based on a 6-month old greenhouse grown clone of ‘RX1’ produced through micropropagation, a 3-yearold ‘RX1’ in the  Phytophthora  field screen and a 2-year old RX1 growing in the Pomology field at University of California, Davis. Data for the botanical description were collected in spring, 2007.  
         [0036]     The Munsell Color Charts for Plant Tissues (1977. GretagMacbeth, New Windsor, N.Y.) is used in the identification of color. Also, common color terms are to be accorded their ordinary dictionary significance. 
    Botanical classification:  Juglans microcarpa×Juglans regia.       Female parent:  Juglans microcarpa       Male parent:  Juglans regia ‘RX 1’ differs from its female parent by having fewer leaflets/leaf, broader leaflets and more vigor. ‘RX1’ differs from its male parent by having more leaflets/leaf and narrower leaflets.     Plant: The growth habit of the tree is illustrated in  FIG. 7 . This 3-year old tree is approximately 3.05 meters tall. Bark of two-year old wood is dark brown (2.5Y 5/2). Bark color of one-year old wood is lighter and redder (7.5YR 5/4) ( FIG. 12 ). Lenticels, about 48 in one square cm, are buff-colored (7.5YR 8/2). The six month old, greenhouse-grown tree is about 45 cm tall with a stem diameter of about 0.8 cm ( FIG. 13 ). The stem is green (5GY 5/10) with scattered lenticels (2.5Y 8/4) more dense towards the base and about 0.5 mm long.     Foliage: The leaves are pinnately compound and alternate. The slightly pubescent new spring foliage ( FIG. 12 ) has reddish new leaves (10R 5/8) and green older leaves (5GY 5/6). There are 13-15 leaflets. The six-month old greenhouse-grown tree has fewer leaflets (9-11). Leaves are 30 cm long and 28-30 cm wide with petioles 5-8 cm long. Leaflets are 12-14 cm long and 5-7 cm wide, dark green on the upper surface (5GY 5/10) ( FIG. 14 ) and slightly lighter on the lower surface (5GY 7/4) ( FIG. 15 ).     Inflorescence: The flowers are small (2 mm×5 mm) and borne in two or three at the shoot tip ( FIG. 16 ). The stigma surface is red (5R 5/8) and the involucre is green (2.5GY 6/6) covered with sticky hairs.     Disease resistance and susceptibility: This rootstock is more resistant to  Phytophthora citricola  in greenhouse tests than other  Juglans  rootstocks.    
 
         [0044]     Usage: The new rootstock of the present invention provides walnut growers with a new clonally propagated rootstock. It can be easily micropropagated.  
                                                                                                         TABLE 1                           Clones grown in Stanislaus County, California in 2004                Plan-   Graftable   Graftable   Diameter (mm)            Clone   ted N   N   %   Mean   SD   Range   CV                    Nematodes                                   VX211   106   87   82   31   4.9   21-44   12.6       Phytophthora       AZ2   230   151   66   26   5   13-38   19.2       AZ3   49   24   49   25   6.7   11-37   26.8       NZ1   172   111   64   26   4.4   10-39   16.9       JX2   246   191   78   29   4.1   13-39   14.1       RX1   104   78   75   18   1.6   14-22   8.8       AX1   163   86   53   27   4.3   14-40   15.9       GZ1   108   83   77   26   5.4   13-40   20.8       Px1   247   154   62   26   4.6   12-40   17.7       AZ1   52   38   73   30   4.4   22-43   14.7       UX1   27   23   85   25   4   15-30   16       GZ2   47   38   81   26   4.5   15-33   17.3       Blackline       WIP3   158   66   42   26   5   12-35   19.2       WIP9   10   6   60   25   2.3   23-99   9.2       Control       UX022   71   59   83   23   3.7   14-29   16.1       English       Vina   14   10   71   18   3.7   13-24   20.5       Sunland   64   20   31   26   3.8   18-31   14.6       Totals   1868   1225   66   25                  
 
         [0045]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                   
               
               
                 Clones grown in Butte County, California in 2004. 
               
             
          
           
               
                   
                 Planted 
                 Graftable 
                 Graftable 
                 Diameter (mm) 
               
             
          
           
               
                 Clone 
                 N 
                 N 
                 % 
                 Mean 
                 SD 
                 Range 
                 CV 
               
               
                   
               
             
          
           
               
                 AX1 
                 120 
                 107 
                 89 
                 19 
                 4.6 
                 10-30 
                 26 
               
               
                 AZ2 
                 120 
                 102 
                 85 
                 21 
                 4.7 
                 10-31 
                 22 
               
               
                 RX1 
                 120 
                 84 
                 70 
                 19 
                 3.2 
                 10-27 
                 17 
               
               
                 Totals 
                 360 
                 293 
                 81 
                 20 
               
               
                   
               
             
          
         
       
     
         [0046]    
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                   
               
               
                 Field performance of clonal Paradoxt hybrids, Northern California 
               
               
                 black walnut, and Chinese wingnut rootstocks in non-infested 
               
               
                 soil and soil infested with  Phytophthora citricola , Davis. 
               
             
          
           
               
                   
                 Maternal background 
                   
                   
                 Percent of 
                 Incidence of 
                 Increase in 
               
               
                 Clone 
                 of hybrid 
                 Soil treatment 
                 Incidence of 
                 trunk circ. 
                 tree mortality 
                 trunk circ. 
               
               
                 (or species) 
                 (or species of standard) 
                 (January 2006) 
                 crown rot (%) 
                 Necrotic 
                 % 
                 (mm) 
               
               
                   
               
               
                 AX1 
                 
                   californica 
                 
                 Control 
                  0 c 
                  0 c 
                  0 c 
                 163 c 
               
               
                   
                   
                 
                   P. citricola 
                 
                  4 c 
                  1 c 
                  0 c 
                 146 cde 
               
               
                 AZ2 
                 
                   (major x hindsii)x nigra 
                 
                 Control 
                  0 c 
                  0 C 
                  0 C 
                 116 fg 
               
               
                   
                   
                 
                   P. citricola 
                 
                  0 c 
                  0 c 
                  0 c 
                 117 fg 
               
               
                 NZ1 
                 
                   (major x hindsii)x nigra 
                 
                 Control 
                  0 c 
                  0 c 
                  0 c 
                 116 fg 
               
               
                   
                   
                 
                   P. citricola 
                 
                  0 c 
                  0 c 
                  0 c 
                 130 def 
               
               
                 GZ1 
                 
                   hindsii 
                 
                 Control 
                  0 c 
                  0 c 
                  0 c 
                 157 cd 
               
               
                   
                   
                 
                   P. citricola 
                 
                  4 c 
                  1 c 
                  0 c 
                 150 cd 
               
               
                 JX2 
                 
                   hindsii 
                 
                 Control 
                  0 c 
                  0 c 
                  0 c 
                 166 bc 
               
               
                   
                   
                 
                   P. citricola 
                 
                  0 c 
                  0 c 
                  0 c 
                 135 def 
               
               
                 PX1 
                 
                   hindsii 
                 
                 Control 
                  0 c 
                  0 c 
                 0 c 
                 169 bc 
               
               
                   
                   
                 
                   P. citricola 
                 
                  8 c 
                  1 c 
                  0 c 
                 157 cd 
               
               
                 VX211 
                 
                   hindsii 
                 
                 Control 
                  0 c 
                  0 c 
                  0 c 
                 191 b 
               
               
                   
                   
                 
                   P. citricola 
                 
                  0 c 
                  0 c 
                  0 c 
                 147 cde 
               
               
                 RX1 
                 
                   microcarpa 
                 
                 Control 
                  0 c 
                  0 c 
                  0 c 
                 112 fg 
               
               
                   
                   
                 
                   P. citricola 
                 
                  0 c 
                  0 c 
                  0 c 
                 116 fg 
               
               
                 WIP3 
                 
                   hindsii x regia 
                 
                 Control 
                  0 c 
                  0 c 
                  0 c 
                 100 g 
               
               
                   
                   
                 
                   P. citricola 
                 
                  8 bc 
                  2 c 
                  0 c 
                 121 efg 
               
               
                 (NCB) 
                 ( J. hindsii ) 
                 Control 
                 16 b 
                 17 b 
                 17 b 
                 68 h 
               
               
                   
                   
                 
                   P. citricola 
                 
                 62 a 
                 59 a 
                 59 a 
                 57 h 
               
               
                 (Wingnut) 
                 ( Pt. stenopiera ) 
                 Control 
                 0 c 
                 0 b 
                 0 e 
                 226 a 
               
               
                   
                   
                 
                   P. citricola 
                 
                 0 c 
                 0 b 
                 0 c 
                 193 b 
               
               
                   
               
               
                     2 All trees were planted May 2005. The assements of crown rot and mortality were made Nov. 21, 2006. Means within a column and without letters in common are significantly different (Waller k ratio).