WALNUT BLIGHT CONTROL USING XANTHOMONAS JUGLANDIS BUD POPULATION SAMPLING

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1 WALNUT BLIGHT CONTROL USING XANTHOMONAS JUGLANDIS BUD POPULATION SAMPLING Richard P. Buchner, Steven E. Lindow, James E. Adaskaveg, Parm Randhawa, Cyndi K. Gilles, and Renee Koutsoukis ABSTRACT Years and years of work have clearly identified copper products tank mixed with ethylene bisdithiocarbamate (EBDC) as the best available material choice for walnut blight control. Additional years of material evaluation have failed to identify a spray material equal to or better. In fact, very few, if any have even showed promise for advanced field testing. Our reduced spray programs based upon population sampling and weather based predictive models rely on an effective treatment for an Integrated Pest Management approach to managing walnut blight. Given the lack of alternative materials, EBDC chemistry is essential for commercial walnut blight control, particularly in Northern California where weather conditions often favor disease development. Given the lack of new chemistry for walnut blight control, the best use of resources is to improve our understanding of walnut blight biology and use that information for precision spray timing. Our research has emphasized improving walnut blight spray decisions based upon Xanthomonas campestris pv juglandis (Xc) occurrence in dormant bud samples. Buchner and Lindow worked with Parm Randhawa, president of California Seed and Plant Lab, to provide bud Xc population information to walnut growers. In, for the first time ever, walnut farmers could mail dormant bud samples to California Seed and Plant Lab for evaluation of initial disease inocolum. In addition, bud population history can be monitored to determine if populations are increasing or decreasing. Spray programs can be adjusted based on what the bacteria are doing. OBJECTIVES 1) Evaluate the effect of initial inoculum levels at start of growing season on disease incidence. 2) Continue to monitor commercial orchards to demonstrate the value of initial inoculum on disease expression. 3) Improve our extension efforts to help walnut growers use population information to make management decisions. PROCEDURES Thirty walnut orchards in Butte and Tehama counties were identified for bud population analysis. Growers randomly selected 200 dormant buds from each orchard and sent 100 buds to Steve Lindow at UC Berkeley and a second 100 bud sample to the California Seed and Plant Lab. From the 100 bud sample, 30 individual buds were plated on media to determine the percent of infested buds and the Xc population in those buds (Log cfu/g). In addition, spray programs California Walnut Board 279 Walnut Research Reports

2 were characterized for each location and walnut blight disease was visually rated by randomly inspecting 3000 to 4000 walnuts for blight symptoms. Figure 1 shows data for 2010 and, the others were added in and have one year of data. By sending samples to two independent labs we had an opportunity to compare analysis results. RESULTS AND DISCUSSION The most complete survey data set (Figure 1) includes information for 2010 and. Orchards 12, 13 and 14 were added in so only one year of information is available. A five half spray program resulted in very good disease control for both years. The percent of buds with pathogen appear to be slightly increasing with noticeable increase in Orchard 2 and Orchard 7. The two ½ sprays applied to Orchard 2 may be allowing bud population increase even though blight control looks good. Orchard 7 was the site for a walnut blight spray efficacy experiment with over-tree sprinklers and may have more variable Xc populations due to sprayed and unsprayed trees. California Walnut Board 280 Walnut Research Reports

3 Orchard 2010 Pathogen 2010 Population Log (cfu/g) avg % Blight Pathogen Population Log (cfu/g) avg. Spray Mix 8lbs. Nu-Cop 50DF, 2.4 lbs. Manzate Pro stick, 4 oz. Sylgard, 1 lb. Zinc Sulfate Spray Mix 6 lbs. Nu-Cop 50DF, 2.4 lbs. Manzate Pro stick, 4 oz. Sylgard, 5 lbs. Potassium Nitrate % Blight 1) Howard /12, 4/20, 5/2 5/12, 5/ ) Howard /12, 4/ ) Chandler /16, 4/23, 5/4 5/12, 5/ ) Hartley /14, 4/20, 5/2 5/11, 5/ ) Chandler /17, 4/21, 5/5 5/11, 5/25 0 6) Howard /12, 4/19, 5/4 5/10, 5/28 0 7) Chandler /16, 4/23, 5/5 5/9, 5/ ) Howard /14, 4/22, 5/6 5/12, 5/28 0 9) Howard /11, 4/21, 5/4 5/13, 5/ ) Chandler /16, 4/23, 5/5 511, 5/ ) Chandler /15, 4/22, 5/4 5/13, 5/ ) Howard /16, 4/20, 5/3 5/10, 5/ ) Chandler /19, 4/23, 5/5 5/12, 5/ ) Chandler /19, 4/23, 5/5 5/12, 5/24.02 Figure 1. Two years of walnut blight population survey information for 14 orchards primarily in Tehama County. The columns read left to right in chronological order starting with the 2010 blight population information listed as the percent walnut buds with pathogen and the average pathogen population in the total sample (logarithm of the colony forming units) followed by the resulting walnut blight in The next columns indicate the population information (same units as before), the spray program using half sprays (every other row alternating) by ground application and the percent blight resulting from that spray strategy. Dormant buds were sampled 3/15/11 and blight was visually rated 6/14/to 6/22/11 by randomly counting 3000 nuts per orchard. Fifteen Butte county orchards were added to the survey in (Figure 2). Blight incidence is somewhat high in several orchards but tends to track the initial population information. Low bud populations tend to follow lower final blight incidence and high bud populations tend toward higher blight incidence. Chandler #2 and Ashley #5 stand out as little or no initial inoculum and higher blight incidence. Early leafing Ashley variety is very blight susceptible and may have been a better host for the disease. California Walnut Board 281 Walnut Research Reports

4 Orchard Pathogen Population Log (cfu/g) Avg. 1) Hartley ) Chandler 0 0 3) Chandler ) Chandler ) Ashley 0 0 6) Howard ) Chandler ) Howard ) Howard ) Vina ) Howard ) Howard ) Tulare ) Vina ) Chandler /15(A) 4/15(G) 4/15(A) 3/31(A) NC+PS 4/12(G) 4/15(A) 4/13(G) B_PS 4/12(G) 4/16(A) 4/12(G) 4/6(A) 4/22(A) NC+PS 4/21(G) 4/21(G) 4/6(A) 4/18(G) 4/22(G) 4/16(G) 4/12(A) 4/15(G) 4/12(A) 4/20(G) Spray Schedule 5/5(A) 5/16(G) 5/5(G) 5/14(G) 5/5(G) 5/13(G) 4/28(G) 5/13(G) 4/21(A) 4/26(G) 5/11(G) 5/5(A) 5/14(G) 4/27(G) 5/11(G) NC+PS 4/27(G) 5/12(G) 4/21(A) 5/10(G) 4/27(G) 5/12(G) 4/27(G) 5/12(G) 4/21(G) 5/6(G) 4/22(A) 5/6(G) 4/28(G) 5/13(G) 6/2(A) 5/27(G) 5/24(G) 5/9(G) 5/21(G) 5/24(G) 5/20(G) 5/20(G) 5/20(A) 5/19(G) 5/21(A) 6/3 (G) 6/3(G) 5/20(A) 5/31(G) 6/3(G) 5/28(G) 5/28(G) 6/2(A) 5/30(G) 6/2(A) % Blight Figure 2. One year of walnut blight population survey information () for 15 orchards in Butte County. As in Table 1, the columns are listed in chronological order starting with the percent walnut buds with pathogen and the average pathogen population in the total sample (logarithm of the colony forming units) followed by the orchard spray program and the percent walnut blight resulting from that spray strategy. (G) or (A) indicates ground or air application. All ground applications were ½ sprays (every other row alternating) except the 5/16 spray on orchard #1. K=Kentan at 6.0 lbs/ac, NC=Nu-cop at 6.0 lbs/ac, PS=Pro-stick at 2.4 lbs/ac, KC=Kocide 2000 at 6.0 lbs/ac 2000 and B=Badge at 3, 4 or 5 lbs.ac. Buds were sampled 3/12/11 and blight was visually rated 6/23 to 6/27/11 by randomly counting 4000 nuts per orchard. Figure 3 represents four samples taken from a northern Tehama Vina orchard. Blight damage might be a little high but may be acceptable for that variety. Samples did not greatly differ suggesting four samples adequately represented the orchard. California Walnut Board 282 Walnut Research Reports

5 Orchard Location and variety pathogen Population Log (cfu/g) Spray Schedule 6 lbs/ac Kocide 2000 plus 2.4 lbs/ac Pro-stick % Blight 1) Upper, Vina /12, 4/26 & 5/ ) Lower South, Vina /12, 4/26 & 5/ ) Lower Middle, Vina /12, 4/26 & 5/ ) Lower North,Vina /12, 4/26 & 5/ Figure 3. walnut blight survey for a Vina variety walnut orchard in Northern Tehama County. Four samples were selected from the same orchard to evaluate variability between samples. As in figures 1 and 2, the columns are listed in chronological order starting with the percent walnut buds with pathogen and the average pathogen population in the total sample (logarithm of the colony forming units) followed by the orchard spray program and the percent walnut blight resulting from that spray strategy. Sprays were full coverage (every row) using ground application equipment. Dormant buds were sampled 2/18/11 and blight was visually rated 6/16/11 by counting 1000 walnuts per location. The last figure compares the bud population results between two labs (Figure 4). Growers randomly sampled their orchards and sent samples to each lab. Within variation, results are relatively comparable suggesting good accuracy between labs. Laboratory A Laboratory B Orchard/Variety Population Log Population Log pathogen (cfu/g) avg pathogen (cfu/g) avg 1) Howard ) Howard ) Chandler ) Hartley ) Chandler ) Howard ) Chandler ) Howard ) Howard ) Chandler ) Chandler ) Vina ) Hartley ) Chandler ) Howard ) Howard ) Chandler ) Howard ) Howard ) Chandler Figure 4. Comparison of two separate laboratories (A and B) for consistency in evaluating walnut blight populations in 20 survey orchards for. Listed are the percent walnut buds with pathogen and the average pathogen population (logarithm of the colony forming units) in the total sample. California Walnut Board 283 Walnut Research Reports

6 Blight History Following Artificial Inoculation In 2010, a group of seven walnut trees were artificially inoculated with a spray solution of X. juglandis about one week after first flowers were visible. X. juglandis agar plates cultured in the Lindow lab at UC Berkeley were the source of inoculum. Plates were gently scraped into one gallon of water and poured into 30 gallons of water in a spray tank with the agitator running. The final X. juglandis concentration was one million cfu per gallon. Trees were sprayed to run off by hand gun at 250 psi. Spray coverage was excellent. Walnut blight was counted 6/16/10 and the artificial inoculation resulted in percent blighted walnuts. A bud sample taken 12/1/10 and evaluated in the Lindow Lab indicated 73.3% infected buds with an average population of 2.92 cfu (Figure 5). No walnut blight control sprays were applied in 2010 to the artificially inoculated trees. 10 Tehama County Bud Samples Avg. log cfu = % infected # of samples More Log cfu/sample Figure 5. Incidence of populations of X. juglandis on Chandler walnut buds sampled following harvest on 12/1/10. Trees were sprayed once on 4/16/10 with X. juglandis solution of one million cfu per gallon. During the spring of, artificially inoculated trees received five grower applied walnut blight sprays by speed sprayer and three hand gun applied sprays (Figure 6). On 4/10, 4/17 and 4/23 the machine spray mix was 8 lbs. Nu-Cop 50 DF plus 2.4 lbs. Manzate Pro-stick plus 4 oz. Sylgard plus 1.0 lb. zinc sulfate. On 5/4 and 5/13 the spray mix was changed to 6 lbs. Nu-Cop 50 DF plus 2.4 lbs. Manzate Pro-stick plus 4 oz. Sylgard and 5 lbs. potassium nitrate. Hand gun sprays were applied 4/22, 5/5 and 5/13/11 using 4 lbs. Kocide 3000 plus 2.4 lbs. Manzate Pro-stick. In all, 8 total sprays were applied and the last spray on 5/13 was both grower and handgun applied to ensure good spray coverage. The goal was to reduce walnut blight bacteria with an aggressive spray program. California Walnut Board 284 Walnut Research Reports

7 1) Seven trees artificially inoculated 4/16/10 resulting in 61.41% blighted walnuts evaluated 6/16/10. 2) Composite bud sample 12/1/10 indicated 73.3% infected buds with an average log cfu of (Figure 5). 3) Eight walnut blight sprays applied: 4/10, 4/17, 4/22, 4/23, 5/4, 5/5, 5/13 (grower) and 5/13/11 (handgun) for blight suppression. 4) 1.46% blighted walnuts evaluated 6/21/11. Grower check trees with 0.0% blight. 5) Composite bud sample 11/28/11 indicated 30.0% buds infected with an average log cfu of 1.28 (Figure 7). Figure 6. Walnut blight and walnut blight bud population history for seven trees artificially inoculated on 4/16/10 and aggressively blight sprayed 4/10 to 5/13/11. The artificial inoculum applied on 4/16/10 resulted in 61.41% walnut blight with 73.3% of the buds infected with an average log cfu of Eight walnut blight sprays applied 4/10 to 5/13/11, reduced blight incidence to 1.46% blighted walnuts with 30.0% of the buds infected with an average log cfu of Grower control trees had 0.0% blighted walnuts (Figure 6). An aggressive spray program one year after artificial inoculum provided good blight control and bud population reduction but was not sufficient to reduce the bud population to zero. Following these trees for a second year will document the recovery time necessary to drive bud populations down. # of Samples Xcj Treated Walnut Buds Avg. log (cfu/bud) = % Infected Log (CFU/bud) Figure 7. Incidence of populations of X. juglandis on the seven Chandler walnut trees artificially inoculated on 4/16/10 and aggressively sprayed 4/10 to 5/13/11. Sample taken 11/28/11 and evaluated in the Lindow Lab. 5 More California Walnut Board 285 Walnut Research Reports