WALNUT BLIGHT MANAGEMENT USING XANTHOMONAS ARBORICOLA PV JUGLANDIS DORMANT BUD POPULATION SAMPLING Richard P. Buchner, Steven E. Lindow, James E. Adaskaveg, Cyndi K. Gilles, and Renee Koutsoukis ABSTRACT Walnut blight orchard surveys have been underway for four years in Tehama County and three years in Butte County. In total, thirty walnut orchards are being monitored using dormant bud population assays for Xanthomonas arboricola pv juglandis (Xaj), individual orchard spray programs and the subsequent amount of walnut blight visually evaluated in early June. For the Tehama County orchards, bud populations are relatively low and effective spray programs are managing walnut blight damage. For Butte County orchards, walnut blight damage was high in 2012, however, lower blight disease pressure and effective spray programs in 2013 dramatically reduced walnut blight damage. Comparisons of the percent dormant buds infested with pathogen to the Xaj populations within buds revealed a very good relationship suggesting the percent infested buds can be used to indicate disease potential. For 2012 and 2013 the percent buds with pathogen did a good job of predicting disease risk. Typically, walnut orchards with walnut blight damage in the previous year are at risk of damage in the current year. Dormant bud population information appears to be a better predictor of disease risk. Relationships are surprisingly good particularly when spray programs and weather are uncontrolled variables. Walnut blight transects for the number of blighted walnuts per tree is beginning to show how much tree to tree variation exists within orchards. This information is valuable for determining how many trees are necessary for a representative sample. INTRODUCTION Depending upon weather conditions, pathogen population size and walnut variety, walnut blight caused by the bacterium Xanthomonas pv juglandis (Xaj) can cause significant crop loss. In Northern California USA, we have measured over 50% crop damage on Chandler walnuts when overwintering pathogen populations in dormant buds were high and spring rainfall favored disease. Conversely, we have measured little to no disease on Chandler walnuts with low dormant bud population levels even when wet spring weather favored disease. Over the years we have defined a relationship between initial inoculum in dormant buds and subsequent disease severity and have successfully used that relationship to predict disease. Three things are necessary for a disease to occur: presence of pathogen, susceptible host, and favorable weather. Plant Pathologists refer to this as the disease triangle. If we can reduce or eliminate any leg of the triangle we can greatly reduce disease. We have some control over the host (late leafing varieties), no control over the weather, but we can monitor predicted weather conditions, the pathogen and use bud population levels to make spray decisions to reduce the effective population size of the pathogen. Walnut blight epidemics proceed in either a monocyclic or polycyclic manner. In a monocyclic disease, infections that happened during the spring do not lead to inoculum that can spread and cause secondary infections. Infrequent rainfall during the spring can be one reason why monocyclic progress of the disease can occur, as is California Walnut Board 307 Walnut Research Reports 2013
commonly the case in California. In contrast, if infections during the spring lead to productions of inoculum that can spread to and infect new tissue, the disease is polycyclic (analogous to compounded interest on money). Frequent spring rainfall would tend to favor such disease epidemics. Why walnut blight bacteria are sometimes monocyclic and sometimes polycyclic is not always clear and both have been observed in our blight experiments although monocyclic disease progress is most common. For Chandler walnuts in our experiments we have measured very low pathogen populations and very little blight damage suggesting a monocyclic growth pattern. Although the search continues for additional spray materials effective for Xaj control, the most effective spray mix currently available in California are copper compounds tank mixed with a coordination product of zinc ion and manganese ethylenebisdithiocarbamate (EBDC) The EBDC usually applied in California is Manzate ProStick fungicide (United Phosphorus, Inc) US EPA # 70506-234. Applications are usually applied at 40% prayer stage and a second application 7-10 days later (Buchner, et al. 2010). Subsequent application decisions may be more challenging and disease risk or potential for blight damage is a useful tool. A Section 3 registration has been issued for California so there is no longer a Section 18. OBJECTIVES 1) Evaluate how many bud samples are necessary to accurately represent pathogen abundance in an orchard. Since cost per sample is substantial and accuracy is critical, improving sampling techniques will help the economics and improve spray decisions based upon an integrated blight management approach. 2) Monitor pathogen populations in commercial orchards and utilize those data to make walnut blight spray decisions. Bud sampling would be the basis for an integrated blight management strategy. The goal is to establish comparisons of efficacy of reduced spray programs to standard spray programs in relation to estimates of early season pathogen abundance. 3) Use monitoring data to develop a walnut blight bud population history which would indicate if populations are increasing or decreasing in orchards differing in disease control strategies. 4) Support commercial labs that wish to offer bud population evaluations to walnut growers. 5) Improve our extension efforts to help walnut growers use population information to make management decisions. PROCEDURES Dormant buds in thirty walnut orchards in Tehama and Butte Counties in California (Ca) were sampled to evaluate pathogen population size. One hundred buds per orchard were sampled by growers and evaluated by the Lindow lab at UC Berkeley and The California Seed and Plant Laboratory in Elverta, California. The microbiological analysis indicated the percent of buds with detectable pathogen and the pathogen population size in each bud. An average pathogen abundance could then be calculated (logarithm of the colony forming units, cfu). Spray programs for each orchard were compiled and blight damage was visually rated by counting 3000 to 4000 walnuts per orchard for blight symptoms. The data set includes a four year history for the Tehama Ca. orchards and a three year history for the Butte Ca. orchards. California Walnut Board 308 Walnut Research Reports 2013
A single row assessment of walnut blight was made in a transect of a Tehama County Vina variety orchard with a 2013 disease incidence of 1.93% to 2.34% blighted walnuts. A single row of 65 trees oriented east to west were rated for number of walnuts dropped as a result of walnut blight infection. Individual trees were evaluated 7/13/12 by estimating the number of blighted walnuts on the ground. A 0 rating represented no blighted walnuts; L (low) represented less than 30 dropped walnuts; M (medium) represented 30 to 100 dropped walnuts and H (high) indicates more than 100 dropped walnuts per tree. Composite bud samples were collected 11/13/12 by collecting 5 dormant buds from each tree for each rating category. In total, four dormant bud samples were evaluated by the Lindow lab at UC Berkeley and a frequency histogram of pathogen population size in individual buds was determined for each of the four disease severity classes. For 2013, the same transect trees were evaluated 6/21/13 by counting the total number of blighted walnuts on the ground. RESULTS AND DISCUSSION Buchner and Lindow have shown walnut blight damage is strongly related to the presence and magnitude of Xaj overwintering under dormant bud scales (Buchner et al. 2012). These Xaj populations serve as the initial inoculum for walnut blight infections. As buds open in the spring and young shoots emerge, walnut blight bacteria can move onto walnut flowers and developing nuts, presumably by rain splash, causing infection. Occasionally, under conditions particularly favorable for disease such as the occurrence of frequent rainfall, additional secondary infections may result from inoculum formed by earlier infections of the walnuts in the spring. The disease cycle is complete when bacteria again over-winter in the dormant buds (Figure 1). Figure 2 illustrates 2013 walnut blight analysis for an early leafing walnut orchard (Vina variety) in Tehama County. The three sampling locations give an indication of variation occurring within the orchard suggesting that many trees need to be sampled to get an accurate representation of infested dormant buds. A poor spray program in 2012 allowed 2.42 to 6.49 percent blight damage to occur. Low 2013 rainfall (Figure 3) and an improved spray program reduced blight damage to 1.85 to 2.34 percent blighted walnuts for 2013. It has always been assumed that while the severity of disease is quite variable between different orchards (probably as a result of different spray histories that have led to different abundances of pathogen in the buds) that the variation in pathogen abundance and hence disease is relatively small within an orchard. We explored this assumption by examining differential nut drop due to walnut blight infection from tree to tree in the same orchard. All trees received the same walnut blight control spray program. Results suggest substantial tree to tree variation in the amount of disease in 2012 and 2013 (Figure 4). Of the 65 sampled trees, 13 trees had no dropped walnuts (no disease), 28 trees had less than 30 dropped walnuts (low disease), 15 trees had between 30 and 100 dropped walnuts (moderate disease) and 9 trees had over 100 dropped walnuts (heavy disease). For 2013, of the same 64 sampled trees, only one tree had no blighted walnuts, 26 trees had less than 30 dropped blighted walnuts, 24 had between 30-100 blighted walnuts and 13 trees had over 100 blighted walnuts. The trees are in a single row, with tree #69 being on the west end and tree #1 on the east end. Trees 56 to 63 represent a cluster of trees with relatively more blighted walnuts on the ground compared to the others. For the rest of the row, the disease appeared to be more random, varying between none, low and medium with a possible gradient California Walnut Board 309 Walnut Research Reports 2013
with increasing damage to the west. Except for orchard variation, the survey trees were relatively similar so there may be a legacy effect of disease from one year to the next. The blight pattern suggests it is very important for growers to take their dormant bud samples for pathogen assessment in the winter from widely scattered locations throughout the orchard; sampling of several buds from a given tree would not provide a good average estimate of pathogen abundance. The current suggestion of one bud from each of 100 trees appears to be an acceptable sampling strategy. For fourteen walnut orchards in Tehama County walnut blight suppression has been very good with the disease damage below 0.5% (Figure 5). When you look at the disease history (Figure 6) one orchard is trending upward but also has 53 percent of the dormant buds infested with pathogen. Walnut blight history for fifteen orchards in Butte County is shown in Figure 7. When you look at the disease history (Figure 8) improved spray programs and weather that was not conducive to disease occurrence contributed to better disease control in 2013. It appears as though the relative amount of blight in different orchards is constant but the absolute amount varies from year to year based on weather and rigor of the spray program. When we looked at the percent of the buds infested with pathogen compared to the amount of pathogen in the bud (Figure 9) we found a very strong relationship. As the number of buds with pathogen increases so did the pathogen population in the infested buds. It should be recognized that there is a large range in population sizes of Xaj in a collection of buds in a given orchard; many buds are free of the pathogen while others can harbor a wide range, including some higher than 10 6 cells/bud. The strong linear relationship between mean bud population (including those free of the pathogen) and the incidence of infestation suggests that the population size of Xaj in those buds having any pathogen population is independent of the likelihood of being infested. That is, populations in infested buds are no higher in orchards having a high frequency of infestation than in those orchards where most buds are free of the pathogen. This relationship thus suggests that disease prediction can be most easily linked to measures of how many buds are infested without a need to estimate the numbers of cells in each bud. This could greatly reduce the cost for assessing Xaj bud populations. It would make sense that the amount of walnut blight disease in the previous year would influence the amount of disease in the current year, particularly when conditions favor increasing pathogen in the dormant buds. To investigate that relationship we compared percent blight damage between 2011 and 2012 and again for 2012 and 2013 (Figures 10, 11). Unfortunately the correlation is somewhat variable probably because of the effects of weather and efficacy of the spray program. 2013 was a low disease pressure year and spray programs were more efficacious so damage was lower and the historical relationship poorer. The predictive ability of percent buds with pathogen are shown for 2012 (Figure 12) and 2013 (Figure 13) for the thirty walnut orchards in the blight survey. As the percent of the buds infested with pathogen increases, the likelihood of blight damage also increases. It should be noted that the slope of this relationship is a measure of the infection efficiency of the pathogen. Infection efficiency in this context can be defined as the incidence of disease resulting from a given frequency of infestation of walnut buds. Comparison of Figure 12 and Figure 13 reveal that the infection efficiency was higher in 2012 than in 2013, especially for the Butte County Ca. California Walnut Board 310 Walnut Research Reports 2013
orchards. In addition, the infection efficiency of orchards receiving different management schemes (managed by different growers) differed within a given year. For example, the infection efficiency, and thus overall disease severity at a given frequency of infestation with Xaj was higher in the Butte County Ca. orchards than in the Tehama County Ca. orchards in 2012 (Figure 8). Thus while pathogen populations in overwintering buds drive epidemics of walnut blight disease, the efficiency of this inoculum in leading to infections can be influenced by management schemes such as appropriate spray programs. It is also clear, however, that prophylactic application of bactericides are needed less than those orchards in which the initial inoculum in overwintering buds are low, since disease is relatively unlikely even in the absence of aggressive management programs. Thus, assessment of disease risk by assessing Xaj populations prior to bud break is a valuable management tool that can reduce the economic and environmental costs of disease control. Additional walnut blight information is available at: cetehama@ucanr.edu walnutresearch@ucanr.edu LITERATURE CITED Buchner, R.P., Lindow, S.E., Adaskaveg, J.E., Gilles, C.K. and Koutsoukis, R. 2012 Walnut Blight Control Using Integrated Pest Management Techniques. Walnut Research Reports, California Marketing Board. pp. 329-344. (http://walnutresearch@ucdavis.edu). Buchner, R.P., Lindow, S.E., Olson, W.H., Adaskaveg, J.E., Gilles, C.K. and Koutsoukis, R. 2010 Spray Timing and Materials for Walnut Blight (Xanthomonas campestris pv juglandis) Control in Northern California, USA. Proceedings of the Sixth International Walnut Symposium. ActaHorticulture number 861, pp. 457-463. California Walnut Board 311 Walnut Research Reports 2013
Figure 1. Walnut Blight Disease Cycle. California Walnut Board 312 Walnut Research Reports 2013
Orchard Location and Variety % Buds with pathogen 2013 Spray Schedule 2013 % Blight 2) Lower South, Vina 3) Lower Middle, Vina 4) Lower North, Vina 2011 2012 2013 16 30 40 3/26 1 4/3 2 4/12 3 4/26 4 5/7 4 2.34 23 10 0 3/26 1 4/3 2 4/12 3 4/26 4 5/7 4 1.85 6 16 10 3/26 1 4/3 2 4/12 3 4/26 4 5/7 4 1.93 Figure 2. Walnut blight population survey information for a Vina variety walnut orchard in Northern Tehama County. 2013 dormant buds infested, the 2013 spray program and the resulting blight damage are listed. Dormant buds were sampled 3/12/13 and blight was visually rated 6/12/13 by counting 1,000 walnuts per location. Application rates: 1 BadgeX2 @ 6 lbs/ac, ProStick @ 2.4 lbs/ac, Syl-Coat @ 0.03gal/ac 2 BadgeX2 @ 4 lbs/ac, ProStick @ 2.4 lbs/ac, R-11 @ 12 oz/ac 3 BadgeX2 @ 6 lbs/ac, ProStick @ 2.4 lbs/ac, Syl-Coat @ 4.8 oz/ac 4 BadgeX2 @ 4 lbs/ac, ProStick @ 2.4 lbs/ac, Syl-Coat @ 3.2 oz/ac Date Rainfall (in) Max Temp ( F) Min Temp ( F) 3/19.06 66 44 3/20.03 63 50 3/31.37 66 50 4/4.23 62 53 4/7.06 68 47 4/8.04 67 40 5/16.08 72 57 5/27.10 69 54 6/10.01 70 57 6/24.04 70 60 6/25.45 75 60 1.47 Figure 3. 2013 Rainfall and maximum/minimum temperature for the Gerber (CIMIS #8) weather station in Tehama County. California Walnut Board 313 Walnut Research Reports 2013
Tree 2012 2013 Blighted 69 L L 13 68 M M 57 67 L M 63 66 L M 100 65 M H 104 64 L M 39 63 H M 63 62 H H 119 61 H H 146 60 H H 132 59 H H 199 58 H M 96 57 H H 142 56 H H 121 55 M H 147 54 M H 239 53 L L 21 52 L Replant Replant 51 M H 204 50 0 L 8 49 M M 47 48 L M 31 47 M M 64 46 M M 42 45 M L 22 44 L M 39 43 M M 65 42 H M 92 41 Replant Replant Replant 40 L L 9 39 0 L 7 38 M H 108 37 L M 59 36 L L 16 35 L M 88 34 M M 70 33 L M 41 32 L M 86 31 M M 61 30 M M 50 29 L L 14 28 L M 37 27 L L 25 26 L L 24 25 Replant Replant Replant 24 L M 78 23 M H 123 22 L H 122 21 L M 60 20 0 L 3 19 L M 58 18 L L 15 17 L L 9 16 Replant Replant Replant 15 0 L 14 14 0 L 10 13 0 L 2 12 L L 10 11 0 L 3 10 L L 5 9 0 L 4 8 0 L 5 7 0 L 4 6 0 L 5 5 L 0 0 4 L L 3 3 Replant Replant Replant 2 0 L 12 1 0 L 1 Blight data - 7/13/2012 Walnuts were visually rated on the ground under each tree. Tree Total Total 0 = No dropped nuts 13 65 20.00% L = Less than 30 dropped nuts 28 65 43.08% M = 30-100 dropped nuts 15 65 23.08% H = Over 100 dropped nuts 9 65 13.85% Replants 4 Blight data - 6/21/2013 Walnuts were counted on the ground under each tree. Total Tree Total % \ 0 = No dropped nuts 1 64 1.56% L = Less than 30 dropped nuts 26 64 40.63% M = 30-100 dropped nuts 24 64 37.50% H = Over 100 dropped nuts 13 64 20.31% Replants 5 % Figure 4. Walnut blight transect for a Vina orchard in Tehama County. Blight drop under each tree was visually rated 7/13/2012. 0 represents no blighted walnuts on the ground. L is less than 30 dropped walnuts. M represents 30 to 100 dropped walnuts. H indicates more than 100 dropped walnuts per tree. For 2013, the actual number of blighted walnuts per tree were counted 6/21/13. California Walnut Board 314 Walnut Research Reports 2013
Orchard % Buds with Pathogen 2011 2012 2013 2013 Spray Mix 8lbs. Nu-Cop 50DF, 2.4 lbs. Manzate Pro-stick, 4 oz. Sylcoat, 1 lb. Zinc Sulfate 2013 Spray Mix 6 lbs. Nu-Cop 50DF, 2.4 lbs. Manzate Pro-stick, 4 oz. Sylcoat, 5 lbs. Potassium Nitrate 2013 % Blight 1) Howard 13 6 20 4/1, 4/6, 4/11 4/18, 5/3, 5/9.13 2) Howard 70 10 13 4/1, 4/6, 4/11 4/18, 5/3, 5/9.10 3) Chandler 20 30 30 4/6, 4/11, 4/17 5/2, 5/8.39 4) Hartley 3 0 0 4/2, 4/6, 4/10 4/17, 5/2, 5/9 0 5) Chandler 3 10 0 4/7, 4/9, 4/17 4/30, 5/7 0 6) Howard 10 10 0 4/3, 4/8, 4/15 5/1, 5/8 0 7) Chandler 93 0 53 4/7, 4/11, 4/17 5/3, 5/9 2.7 8) Howard 3 13 30 4/2, 4/10, 4/18 5/4, 5/11.03 9) Howard 0 0 0 4/2, 4/10, 4/16 4/27, 5/8.03 10) Chandler 0 0 0 4/2, 4/12 5/5*, 5/13*.03 11) Chandler 10 0 0 4/5, 4/10, 4/18 5/2*,5/9* 0 12) Howard 6 0 4/3, 4/7, 4/12 4/18, 5/2, 5/10.09 13) Chandler 10 0 4/6, 4/12, 4/18 4/26*, 5/8*.36 14) Chandler 30 13 4/5, 4/12, 4/18 5/1*, 5/9*.03 Figure 5. 2013 walnut blight population survey information for fourteen orchards in Tehama County. The percent dormant buds with pathogen, 2013 spray program and the resulting blight damage are listed left to right. All sprays were half sprays (every other row alternating) by ground application. Spray dates followed by (*) included 14.5 oz. pristine with the blight spray. Dormant buds were collected 3/28/13 and blight was visually rated 6/7/13 by visually counting 3000 walnuts per orchard. California Walnut Board 315 Walnut Research Reports 2013
Figure 6. Walnut Blight damage for 14 walnut orchards in Tehama County from 2010 to 2013. Blight damage is increasing in one orchard with a history of high initial inoculum. California Walnut Board 316 Walnut Research Reports 2013
Orchard % Buds with Pathogen 2013 Spray Schedule 2013 % Blight 2011 2012 2013 1) Hartley 16 0 16 2) Chandler 0 46 76 3) Chandler 73 43 60 4) Chandler 56 33 40 5) Ashley 0 70 30 6) Howard 20 16 3 7) Chandler 46 30 86 8) Howard 40 6 13 9) Howard 16 10) Vina 60 43 56 11) Howard 53 20 0 12) Howard 40 23 3 13) Tulare 0 0 0 14) Vina 3 3 20 15) Chandler 83 50 56 4/5 A N,M,S 4/2 GH 4/3 GH 4/1 GH 3/19 GH 3/27 GH 4/5 A N,M,S 3/26 GH 3/27 GH 3/25 GH 3/25 GH 3/25 GH 3/27 GH 3/26 GH 4/1 GH 4/12 G 4/10 GH 4/11 GH 4/6 GH 3/25 GH 4/4 GH 4/11 GH 4/3 GH 4/4 GH 4/2 GH 4/1 GH 4/1 GH 4/3 GH 4/2 GH 4/5 GH 5/9 G N,M,S,Z 4/19 GH 4/18 GH 4/15 GH 4/1 G 4/12 GH 4/20 GH 4/10 GH 4/12 GH 4/10 G 4/6 GH 4/6 GH 4/13 GH 4/10 GH 4/15 GH 5/6 A N,M,S,Z,H 5/7 A N,M,S,Z,H 5/8 G N,M,S,Z 4/9 G 4/17 GH 5/7 A N,M,S,Z,H 4/19 GH 4/18 GH 4/21 A N,M,S,PC 4/15 GH 4/15 GH 4/20 GH 4/19 GH 5/7 A N,M,S,Z 5/24 G N,M,S 4/20 A N,M,S,PC 5/6 GH N,M,S,Z,H 5/25 G N,M,S 5/6 G 5/4 G N,M,S,Z,H 5/8 G N,M,Z,S 5/7 G 5/7 G 5/10 G N,M,S,Z 5/13 G N,M,S,Z 5/24 G B,M,S 5/8 G N,M,S,Z 5/13 GH N,M,S,Z 5/24 G N,M,S 5/24 G N,M,S.73 1.48 4.35 1.99 1.44.17 4.85.43.29 1.43.16.74.05.29 1.25 Figure 7. 2012 Walnut blight population survey information for 15 orchards in Butte County. 2013 percent dormant buds infested with pathogen, 2013 spray program and the associate blight damage are listed left to right. Dormant buds were sampled 3/8/13 prior to bud break and blight was visually rated 6/10/13 by randomly counting 4000 nuts per orchard. A= aerial application, G = ground application every row, GH = ground half spray every other row alternating. M = Manzate pro stick @ 2.4 lbs/ac N = Nucop DF @ 8 lbs/ac F = Freeway @ 4 oz/ac Z = Zinc Sulfate 36% @ 1-2 lbs/ac L = Lime @ 2-4 lbs/ac P = Pencap @ 6 pts/ac H = Hatchet @ 2 qts/ac B = Badge2X @ 4 lbs/ac California Walnut Board 317 Walnut Research Reports 2013
Figure 8. Walnut Blight damage for 15 walnut orchards in Butte County from 2011 to 2013. Improved spray programs plus a low disease pressure year resulted in much better disease control in 2013. California Walnut Board 318 Walnut Research Reports 2013
Figure 9. Relationship of % buds with pathogen and the associated bud population. Very strong correlation of incidence of bud infestation with average population size of pathogen disease prediction can be made based on more easily measured incidence of infestation rather than average population size. The lines drawn represent the linear regressions Y=0.0308X + 0.055 (R 2 =0.967) (2012); and Y=0.0398x + 0.067 (R 2 =0.626) (2013). California Walnut Board 319 Walnut Research Reports 2013
% Blight 2012 20 18 16 14 12 10 8 6 4 2 0 Tehama Co. Butte Co. 0 1 2 3 4 5 6 % Blight 2011 Figure 10. Relationship of previous year percent blight damage to current year blight damage for 2011 and 2012. The incidence of blight in one year is somewhat predictive of disease incidence in the following year when considered over many orchards. 6 5 Tehama Co. Butte Co. % Blight 2013 4 3 2 1 0 0 5 10 15 20 % Blight 2012 Figure 11. Relationship of previous year percent blight damage to current year blight damage for 2012 and 2013. The incidence of blight in one year is somewhat predictive of disease incidence in the following year when considered over many orchards. California Walnut Board 320 Walnut Research Reports 2013
. Figure 12. Relationship between the percent of buds infested with pathogen and the incidence of walnut blight damage for 30 orchards in Butte and Tehama counties in 2012. The lines drawn represent the linear regression Y=0.218x 0.28 (R 2 =0.56); and Y=0.001x + 0.14 (R 2 =0.631) for orchards in Butte and Tehama counties, respectively. Figure 13. Relationship between the percent of buds infested with pathogen and the incidence of walnut blight damage for 30 orchards in Butte and Tehama counties in 2013. The lines drawn represent the linear regressions Y=0.039x + 0.067 (R 2 =0.626); and Y=0.032x 0.096 (R 2 =0.58) for orchards in Butte and Tehama counties, respectively. California Walnut Board 321 Walnut Research Reports 2013